142 BIOLOGY
There is a wide diversity in living organisms in our biosphere. Now a
question that arises in our minds is: Are all living organisms made of the
same chemicals, i.e., elements and compounds? You have learnt in
chemistry how elemental analysis is performed. If we perform such an
analysis on a plant tissue, animal tissue or a microbial paste, we obtain a
list of elements like carbon, hydrogen, oxygen and several others and
their respective content per unit mass of a living tissue. If the same analysis
is performed on a piece of earth’s crust as an example of non-living matter,
we obtain a similar list. What are the differences between the two lists? In
absolute terms, no such differences could be made out. All the elements
present in a sample of earth’s crust are also present in a sample of living
tissue. However, a closer examination reveals that the relative abundance
of carbon and hydrogen with respect to other elements is higher in any
living organism than in earth’s crust (Table 9.1).
9.1 HOW TO ANALYSE CHEMICAL COMPOSITION?
We can continue asking in the same way, what type of organic compounds
are found in living organisms? How does one go about finding the answer?
To get an answer, one has to perform a chemical analysis. We can take any
living tissue (a vegetable or a piece of liver, etc.) and grind it in trichloroacetic
acid (Cl
3
CCOOH) using a mortar and a pestle. We obtain a thick slurry. If
we were to strain this through a cheesecloth or cotton we would obtain two
fractions. One is called the filtrate or more technically, the acid-soluble
pool, and the second, the retentate or the acid-insoluble fraction. Scientists
have found thousands of organic compounds in the acid-soluble pool.
B
IOMOLECULES
C
HAPTER
9
9.1 How to Analyse
Chemical
Composition?
9.2 Primary and
Secondary
Metabolites
9.3 Biomacromolecules
9.4 Proteins
9.5 Polysaccharides
9.6 Nucleic Acids
9.7 Structure of
Proteins
9.8 Nature of Bond
Linking Monomers
in a Polymer
9.9 Dynamic State of
Body Constituents
- Concept of
Metabolism
9.10 Metabolic Basis for
Living
9.11 The Living State
9.12 Enzymes
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In higher classes you will learn about how
to analyse a living tissue sample and identify a
particular organic compound. It will suffice to
say here that one extracts the compounds, then
subjects the extract to various separation
techniques till one has separated a compound
from all other compounds. In other words, one
isolates and purifies a compound. Analytical
techniques, when applied to the compound give
us an idea of the molecular formula and the
probable structure of the compound. All the
carbon compounds that we get from living
tissues can be called ‘biomolecules’. However,
living organisms have also got inorganic
elements and compounds in them. How do we
know this? A slightly different but destructive
experiment has to be done. One weighs a small
amount of a living tissue (say a leaf or liver and
this is called wet weight) and dry it. All the water,
evaporates. The remaining material gives dry
weight. Now if the tissue is fully burnt, all the
carbon compounds are oxidised to gaseous
form (CO
2
, water vapour) and are removed. What
is remaining is called ‘ash’. This ash contains
inorganic elements (like calcium, magnesium
etc). Inorganic compounds like sulphate,
phosphate, etc., are also seen in the acid-soluble
fraction. Therefore elemental analysis gives
elemental composition of living tissues in the
form of hydrogen, oxygen, chlorine, carbon etc.
while analysis for compounds gives an idea of
Element % Weight of
Earth’s crust Human body
Hydrogen (H) 0.14 0.5
Carbon (C) 0.03
18.5
Oxygen (O) 46.6 65.0
Nitrogen (N) very little 3.3
Sulphur (S) 0.03 0.3
Sodium (Na) 2.8 0.2
Calcium (Ca) 3.6 1.5
Magnesium (Mg) 2.1 0.1
Silicon (Si) 27.7 negligible
* Adapted from CNR Rao, Understanding Chemistry,
Universities Press, Hyderabad.
TABLE 9.1 A Comparison of Elements Present
in Non-living and Living Matter*
Component Formula
Sodium Na
+
Potassium K
+
Calcium Ca
++
Magnesium Mg
++
Water H
2
O
Compounds NaCl, CaCO
3
,
PO SO
4
3
4
2− −
,
TABLE 9.2 A List of Representative Inorganic
Constituents of Living Tissues
the kind of organic (Figure 9.1) and inorganic constituents (Table 9.2)
present in living tissues. From a chemistry point of view, one can identify
functional groups like aldehydes, ketones, aromatic compounds, etc. But
from a biological point of view, we shall classify them into amino acids,
nucleotide bases, fatty acids etc.
Amino acids are organic compounds containing an amino group and
an acidic group as substituents on the same carbon i.e., the α-carbon.
Hence, they are called α-amino acids. They are substituted methanes. There
are four substituent groups occupying the four valency positions. These
are hydrogen, carboxyl group, amino group and a variable group
designated as R group. Based on the nature of R group there are many
amino acids. However, those which occur in proteins are only of twenty
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144 BIOLOGY
types. The R group in these proteinaceous amino acids could be a hydrogen
(the amino acid is called glycine), a methyl group (alanine), hydroxy methyl
(serine), etc. Three of the twenty are shown in Figure 9.1.
The chemical and physical properties of amino acids are essentially
of the amino, carboxyl and the R functional groups. Based on number of
amino and carboxyl groups, there are acidic (e.g., glutamic acid), basic
(lysine) and neutral (valine) amino acids. Similarly, there are aromatic
amino acids (tyrosine, phenylalanine, tryptophan). A particular property
of amino acids is the ionizable nature of –NH
2
and –COOH groups. Hence
in solutions of different pH, the structure of amino acids changes.
B is called zwitterionic form.
Lipids are generally water insoluble. They could be simple fatty acids.
A fatty acid has a carboxyl group attached to an R group. The R group
could be a methyl (–CH
3
), or ethyl (–C
2
H
5
) or higher number of –CH
2
groups (1 carbon to 19 carbons). For example, palmitic acid has 16
carbons including carboxyl carbon. Arachidonic acid has 20 carbon
atoms including the carboxyl carbon. Fatty acids could be saturated
(without double bond) or unsaturated (with one or more C=C double
bonds). Another simple lipid is glycerol which is trihydroxy propane. Many
lipids have both glycerol and fatty acids. Here the fatty acids are found
esterified with glycerol. They can be then monoglycerides, diglycerides
and triglycerides. These are also called fats and oils based on melting
point. Oils have lower melting point (e.g., gingelly oil) and hence remain
as oil in winters. Can you identify a fat from the market? Some lipids
have phosphorous and a phosphorylated organic compound in them.
These are phospholipids. They are found in cell membrane. Lecithin is
one example. Some tissues especially the neural tissues have lipids with
more complex structures.
Living organisms have a number of carbon compounds in which
heterocyclic rings can be found. Some of these are nitrogen bases –
adenine, guanine, cytosine, uracil, and thymine. When found attached to
a sugar, they are called nucleosides. If a phosphate group is also found
esterified to the sugar they are called nucleotides. Adenosine, guanosine,
thymidine, uridine and cytidine are nucleosides. Adenylic acid, thymidylic
acid, guanylic acid, uridylic acid and cytidylic acid are nucleotides. Nucleic
acids like DNA and RNA consist of nucleotides only. DNA and RNA function
as genetic material.
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Cholesterol
Phospholipid (Lecithin)
Fats and oils (lipids)
(CH )
2 14
CH
3
COOH
Fatty acid
(Palmitic acid)
Glycerol
Triglyceride (R
1
, R
2
and R
3
are fatty acids)
Nitrogen bases
OH
OH
Adenine
O
OCH
2
P
HO
OH
O
Adenylic acid
Nucleotide
OH
OH
HOCH
2
Adenine
O
OH
OH
HOCH
2
Uracil
O
Adenosine
Uridine
Nucleosides
OH
OH
OH
HOCH
2
O
OH
OH
HO
OH
CH OH
2
O
C
6
H
12
O
6
(Glucose)
C
5
H
10
O
5
(Ribose)
Sugars (Carbohydrates)
SerineGlycine
Amino acids
Alanine
Figure 9.1 Diagrammatic representation of small molecular weight organic
compounds in living tissues
O
O
H
N
N
H
Adenine (Purine)
Uracil (Pyrimidine)
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9.2 PRIMARY AND SECONDARY METABOLITES
The most exciting aspect of chemistry deals with isolating thousands of
compounds, small and big, from living organisms, determining their
structure and if possible synthesising them.
If one were to make a list of biomolecules, such a list would have
thousands of organic compounds including amino acids, sugars, etc.
For reasons that are given in section 9.10, we can call these biomolecules
as ‘metabolites’. In animal tissues, one notices the presence of all such
categories of compounds shown in Figure 9.1. These are called primary
metabolites. However, when one analyses plant, fungal and microbial cells,
one would see thousands of compounds other than these called primary
metabolites, e.g. alkaloids, flavonoids, rubber, essential oils, antibiotics,
coloured pigments, scents, gums, spices. These
are called secondary metabolites (Table 9.3).
While primary metabolites have identifiable
functions and play known roles in normal
physiologial processes, we do not at the moment,
understand the role or functions of all the
‘secondary metabolites’ in host organisms.
However, many of them are useful to ‘human
welfare’ (e.g., rubber, drugs, spices, scents and
pigments). Some secondary metabolites have
ecological importance. In the later chapters and
years you will learn more about this.
9.3 BIOMACROMOLECULES
There is one feature common to all those compounds found in the acid
soluble pool. They have molecular weights ranging from 18 to around
800 daltons (Da) approximately.
The acid insoluble fraction, has only four types of organic compounds
i.e., proteins, nucleic acids, polysaccharides and lipids. These classes of
compounds with the exception of lipids, have molecular weights in the
range of ten thousand daltons and above. For this very reason,
biomolecules, i.e., chemical compounds found in living organisms are of
two types. One, those which have molecular weights less than one
thousand dalton and are usually referred to as micromolecules or simply
biomolecules while those which are found in the acid insoluble fraction
are called macromolecules or biomacromolecules.
The molecules in the insoluble fraction with the exception of lipids
are polymeric substances. Then why do lipids, whose molecular weights
do not exceed 800 Da, come under acid insoluble fraction, i.e.,
macromolecular fraction? Lipids are indeed small molecular weight
Pigments Carotenoids, Anthocyanins,
etc.
Alkaloids Morphine, Codeine, etc.
Terpenoides Monoterpenes, Diterpenes etc.
Essential oils Lemon grass oil, etc.
Toxins Abrin, Ricin
Lectins Concanavalin A
Drugs Vinblastin, curcumin, etc.
Polymeric Rubber, gums, cellulose
substances
TABLE 9.3 Some Secondary Metabolites
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Component % of the total
cellular mass
Water 70-90
Proteins 10-15
Carbohydrates 3
Lipids 2
Nucleic acids 5-7
Ions 1
TABLE 9.4 Average Composition of Cells
compounds and are present not only as such but also
arranged into structures like cell membrane and other
membranes. When we grind a tissue, we are disrupting
the cell structure. Cell membrane and other
membranes are broken into pieces, and form vesicles
which are not water soluble. Therefore, these
membrane fragments in the form of vesicles get
separated along with the acid insoluble pool and hence
in the macromolecular fraction. Lipids are not strictly
macromolecules.
The acid soluble pool represents roughly the
cytoplasmic composition. The macromolecules from
cytoplasm and organelles become the acid insoluble
fraction. Together they represent the entire chemical
composition of living tissues or organisms.
In summary if we represent the chemical
composition of living tissue from abundance point of
view and arrange them class-wise, we observe that
water is the most abundant chemical in living
organisms (Table 9.4).
9.4 PROTEINS
Proteins are polypeptides. They are linear chains of
amino acids linked by peptide bonds as shown in
Figure 9.3.
Each protein is a polymer of amino acids. As there
are 20 types of amino acids (e.g., alanine, cysteine,
proline, tryptophan, lysine, etc.), a protein is a
heteropolymer and not a homopolymer. A
homopolymer has only one type of monomer repeating
‘n’ number of times. This information about the amino
acid content is important as later in your nutrition
lessons, you will learn that certain amino acids are
essential for our health and they have to be supplied
through our diet. Hence, dietary proteins are the
source of essential amino acids. Therefore, amino acids
can be essential or non-essential. The latter are those
which our body can make, while we get essential amino
acids through our diet/food. Proteins carry out many
functions in living organisms, some transport
nutrients across cell membrane, some fight infectious
organisms, some are hormones, some are enzymes,
TABLE 9.5 Some Proteins and their
Functions
Protein Functions
Collagen Intercellular ground
substance
Trypsin Enzyme
Insulin Hormone
Antibody Fights infectious agents
Receptor Sensory reception
(smell, taste, hormone,
etc.)
GLUT-4 Enables glucose
transport
into cells
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CH OH
2
CH OH
2
CH
2
OH
OH
OH
OH
OH
O
O
O
O
O
O
O
O
O
O
O
O
O
Figure 9.2 Diagrammatic representation of a portion of glycogen
etc. (Table 9.5). Collagen is the most abundant protein in animal world
and Ribulose bisphosphate Carboxylase-Oxygenase (RuBisCO) is the
most abundant protein in the whole of the biosphere.
9.5 POLYSACCHARIDES
The acid insoluble pellet also has polysaccharides (carbohydrates) as
another class of macromolecules. Polysaccharides are long chains of
sugars. They are threads (literally a cotton thread) containing different
monosaccharides as building blocks. For example, cellulose is a
polymeric polysaccharide consisting of only one type of monosaccharide
i.e., glucose. Cellulose is a homopolymer. Starch is a variant of this but
present as a store house of energy in plant tissues. Animals have another
variant called glycogen. Inulin is a polymer of fructose. In a
polysaccharide chain (say glycogen), the right end is called the reducing
end and the left end is called the non-reducing end. It has branches as
shown in the form of a cartoon (Figure 9.2). Starch forms helical
secondary structures. In fact, starch can hold I
2
molecules in the helical
portion. The starch-I
2
is blue in colour. Cellulose does not contain
complex helices and hence cannot hold I
2
.
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Plant cell walls are made of cellulose. Paper made from plant pulp
and cotton fibre is cellulosic. There are more complex polysaccharides
in nature. They have as building blocks, amino-sugars and chemically
modified sugars (e.g., glucosamine, N-acetyl galactosamine, etc.).
Exoskeletons of arthropods, for example, have a complex
polysaccharide called chitin. These complex polysaccharides are mostly
homopolymers.
9.6 NUCLEIC ACIDS
The other type of macromolecule that one would find in the acid
insoluble fraction of any living tissue is the nucleic acid. These are
polynucleotides. Together with polysaccharides and polypeptides these
comprise the true macromolecular fraction of any living tissue or cell.
For nucleic acids, the building block is a nucleotide. A nucleotide has
three chemically distinct components. One is a heterocyclic compound,
the second is a monosaccharide and the third a phosphoric acid or
phosphate.
As you notice in Figure 9.1, the heterocyclic compounds in nucleic
acids are the nitrogenous bases named adenine, guanine, uracil,
cytosine, and thymine. Adenine and Guanine are substituted purines
while the rest are substituted pyrimidines. The skeletal heterocyclic ring
is called as purine and pyrimidine respectively. The sugar found in
polynucleotides is either ribose (a monosaccharide pentose) or 2’
deoxyribose. A nucleic acid containing deoxyribose is called
deoxyribonucleic acid (DNA) while that which contains ribose is called
ribonucleic acid (RNA).
9.7 STRUCTURE OF PROTEINS
Proteins, as mentioned earlier, are heteropolymers containing strings
of amino acids. Structure of molecules means different things in
different contexts. In inorganic chemistry, the structure invariably
refers to the molecular formulae (e.g., NaCl, MgCl
2
, etc.). Organic
chemists always write a two dimensional view of the molecules while
representing the structure of the molecules (e.g., benzene,
naphthalene, etc.). Physicists conjure up the three dimensional views
of molecular structures while biologists describe the protein structure
at four levels. The sequence of amino acids i.e., the positional
information in a protein – which is the first amino acid, which is
second, and so on – is called the primary structure (Figure 9.3a) of a
protein. A protein is imagined as a line, the left end represented by
the first amino acid and the right end represented by the last amino
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150 BIOLOGY
acid. The first amino acid is also
called as N-terminal amino acid. The
last amino acid is called the C-
terminal amino acid. A protein
thread does not exist throughout as
an extended rigid rod. The thread is
folded in the form of a helix (similar
to a revolving staircase). Of course,
only some portions of the protein
thread are arranged in the form of a
helix. In proteins, only right handed
helices are observed. Other regions
of the protein thread are folded into
other forms in what is called the
secondary structure (Fig. 9.4 b). In
addition, the long protein chain is
also folded upon itself like a hollow
woolen ball, giving rise to the
tertiary structure (Figure 9.4 c).
This gives us a 3-dimensional view
of a protein. Tertiary structure is
absolutely necessary for the many
biological activities of proteins.
Some proteins are an assembly
of more than one polypeptide or
subunits. The manner in which
these individual folded polypeptides
or subunits are arranged with
respect to each other (e.g. linear
string of spheres, spheres arranged
one upon each other in the form of
a cube or plate etc.) is the
architecture of a protein otherwise
called the quaternary structure of
a protein (Fig. 9.4 d). Adult human
haemoglobin consists of 4 subunits.
Two of these are identical to each
other. Hence, two subunits of α type
and two subunits of β type together
constitute the human haemoglobin
(Hb).
(a)
(b)
N
C
Figure 9.4 Cartoon showing : (a) A secondary structure
and (b) A tertiary structure of proteins
Figure 9.3 Various levels of Protein Structure
(a) Primary
(b) Secondary
(d) Quaternary
Hydrogen
Disulphide bond
Beta–plated sheet
Polypeptide
Tertiary
Alpha–Helix
(c)
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9.8 NATURE OF BOND
LINKING MONOMERS IN A POLYMER
In a polypeptide or a protein, amino acids are linked by a peptide
bond which is formed when the carboxyl (-COOH) group of one amino
acid reacts with the amino (-NH
2
) group of the next amino acid with
the elimination of a water moiety (the process is called dehydration).
In a polysaccharide the individual monosaccharides are linked by a
glycosidic bond. This bond is also formed by dehydration. This bond
is formed between two carbon atoms of two adjacent monosaccharides.
In a nucleic acid a phosphate moiety links the 3’-carbon of one sugar
of one nucleotide to the 5’-carbon of the sugar of the succeeding
nucleotide. The bond between the phosphate and hydroxyl group of
sugar is an ester bond. As there is one such ester bond on either side,
it is called phosphodiester bond (Figure 9.5).
Nucleic acids exhibit a wide variety of secondary structures. For
example, one of the secondary structures exhibited by DNA is the
famous Watson-Crick model. This model says that DNA exists as a
double helix. The two strands of polynucleotides are antiparallel i.e.,
run in the opposite direction. The backbone is formed by the sugar-
phosphate-sugar chain. The nitrogen bases are projected more or less
perpendicular to this backbone but face inside. A and G of one strand
compulsorily base pairs with T and C, respectively, on the other strand.
Figure 9.5 Diagram indicating secondary structure of DNA
5'
3'
3'
5'
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152 BIOLOGY
There are two hydrogen bonds between A and T and three hydrogen
bonds between G and C. Each strand appears like a helical staircase.
Each step of ascent is represented by a pair of bases. At each step of
ascent, the strand turns 36
°
. One full turn of the helical strand would
involve ten steps or ten base pairs. Attempt drawing a line diagram.
The pitch would be 34Å. The rise per base pair would be 3.4Å. This
form of DNA with the above mentioned salient features is called B-
DNA. In higher classes, you will be told that there are more than a
dozen forms of DNA named after English alphabets with unique
structural features.
9.9 DYNAMIC STATE OF BODY CONSTITUENTS – CONCEPT OF
METABOLISM
What we have learnt till now is that living organisms, be it a simple bacterial
cell, a protozoan, a plant or an animal, contain thousands of organic
compounds. These compounds or biomolecules are present in certain
concentrations (expressed as mols/cell or mols/litre etc.). One of the greatest
discoveries ever made was the observation that all these biomolecules have
a turn over. This means that they are constantly being changed into some
other biomolecules and also made from some other biomolecules. This
breaking and making is through chemical reactions constantly occuring
in living organisms. Together all these chemical reactions are called
metabolism. Each of the metabolic reactions results in the transformation
of biomolecules. A few examples for such metabolic transformations are:
removal of CO
2
from amino acids making an amino acid into an amine,
removal of amino group in a nucleotide base; hydrolysis of a glycosidic
bond in a disaccharide, etc. We can list tens and thousands of such
examples. Majority of these metabolic reactions do not occur in isolation
but are always linked to some other reactions. In other words, metabolites
are converted into each other in a series of linked reactions called metabolic
pathways. These metabolic pathways are similar to the automobile traffic
in a city. These pathways are either linear or circular. These pathways criss-
cross each other, i.e., there are traffic junctions. Flow of metabolites through
metabolic pathway has a definite rate and direction like automobile traffic.
This metabolite flow is called the dynamic state of body constituents. What
is most important is that this interlinked metabolic traffic is very smooth
and without a single reported mishap for healthy conditions. Another feature
of these metabolic reactions is that every chemical reaction is a catalysed
reaction. There is no uncatalysed metabolic conversion in living systems.
Even CO
2
dissolving in water, a physical process, is a catalysed reaction in
living systems. The catalysts which hasten the rate of a given metabolic
conversation are also proteins. These proteins with catalytic power are
named enzymes.
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9.10 METABOLIC BASIS FOR LIVING
Metabolic pathways can lead to a more complex structure from a simpler
structure (for example, acetic acid becomes cholesterol) or lead to a simpler
structure from a complex structure (for example, glucose becomes lactic
acid in our skeletal muscle). The former cases are called biosynthetic
pathways or anabolic pathways. The latter constitute degradation and
hence are called catabolic pathways. Anabolic pathways,
as expected,
consume energy. Assembly of a protein from amino acids requires energy
input. On the other hand, catabolic pathways lead to the release of energy.
For example, when glucose is degraded to lactic acid in our skeletal muscle,
energy is liberated. This metabolic pathway from glucose to lactic acid which
occurs in 10 metabolic steps is called glycolysis. Living organisms have
learnt to trap this energy liberated during degradation and store it in the
form of chemical bonds. As and when needed, this bond energy is utilised
for biosynthetic, osmotic and mechanical work that we perform. The most
important form of energy currency in living systems is the bond energy in
a chemical called adenosine triphosphate (ATP).
How do living organisms derive their energy? What strategies have they
evolved? How do they store this energy and in what form? How do they
convert this energy into work? You will study and understand all this under
a sub-discipline called ‘Bioenergetics’ later in your higher classes.
9.11 THE LIVING STATE
At this level, you must understand that the tens and thousands of
chemical compounds in a living organism, otherwise called metabolites,
or biomolecules, are present at concentrations characteristic of each of
them. For example, the blood concentration of glucose in a normal healthy
individual is 4.2 mmol/
L–6.1 mmol/L, while that of hormones would
be nanograms/mL. The most important fact of biological systems is that
all living organisms exist in a steady-state characterised by
concentrations of each of these biomolecules. These biomolecules are in
a metabolic flux. Any chemical or physical process moves spontaneously
to equilibrium. The steady state is a non-equilibrium state. One should
remember from physics that systems at equilibrium cannot perform
work. As living organisms work continuously, they cannot afford to reach
equilibrium. Hence the living state is a non-equilibrium steady-
state to be able to perform work; living process is a constant effort to
prevent falling into equilibrium. This is achieved by energy input.
Metabolism provides a mechanism for the production of energy. Hence
the living state and metabolism are synonymous. Without metabolism
there cannot be a living state.
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9.12 ENZYMES
Almost all enzymes are proteins. There are some nucleic acids that behave
like enzymes. These are called ribozymes. One can depict an enzyme by a
line diagram. An enzyme like any protein has a primary structure, i.e.,
amino acid sequence of the protein. An enzyme like any protein has the
secondary and the tertiary structure. When you look at a tertiary structure
(Figure 9.4 b) you will notice that the backbone of the protein chain folds
upon itself, the chain criss-crosses itself and hence, many crevices or
pockets are made. One such pocket is the ‘active site’. An active site of an
enzyme is a crevice or pocket into which the substrate fits. Thus enzymes,
through their active site, catalyse reactions at a high rate. Enzyme catalysts
differ from inorganic catalysts in many ways, but one major difference
needs mention. Inorganic catalysts work efficiently at high temperatures
and high pressures, while enzymes get damaged at high temperatures
(say above 40°C). However, enzymes isolated from organisms who normally
live under extremely high temperatures (e.g., hot vents and sulphur
springs), are stable and retain their catalytic power even at high
temperatures (upto 80°-90°C). Thermal stability is thus an important
quality of such enzymes isolated from thermophilic organisms.
9.12.1 Chemical Reactions
How do we understand these enzymes? Let us first understand a chemical
reaction. Chemical compounds undergo two types of changes. A physical
change simply refers to a change in shape without breaking of bonds.
This is a physical process. Another physical process is a change in state
of matter: when ice melts into water, or when water becomes a vapour.
These are also physical processes. However, when bonds are broken and
new bonds are formed during transformation, this will be called a chemical
reaction. For example:
Ba(OH)
2
+ H
2
SO
4
→
BaSO
4
+ 2H
2
O
is an inorganic chemical reaction. Similarly, hydrolysis of starch into
glucose is an organic chemical reaction. Rate of a physical or chemical
process refers to the amount of product formed per unit time. It can be
expressed as:
rate =
δ
δ
P
t
Rate can also be called velocity if the direction is specified. Rates of physical
and chemical processes are influenced by temperature among other
factors. A general rule of thumb is that rate doubles or decreases by half
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155
for every 10°C change in either direction. Catalysed reactions proceed at
rates vastly higher than that of uncatalysed ones. When enzyme catalysed
reactions are observed, the rate would be vastly higher than the same but
uncatalysed reaction. For example
CO
2
+ H
2
O
→
Carbonic anhydrase
H
2
CO
3
carbon dioxide water carbonic acid
In the absence of any enzyme this reaction is very slow, with about
200 molecules of H
2
CO
3
being formed in an hour. However, by using the
enzyme present within the cytoplasm called carbonic anhydrase, the
reaction speeds dramatically with about 600,000 molecules being formed
every second. The enzyme has accelerated the reaction rate by about 10
million times. The power of enzymes is incredible indeed!
There are thousands of types of enzymes each catalysing a unique
chemical or metabolic reaction. A multistep chemical reaction, when each
of the steps is catalysed by the same enzyme complex or different enzymes,
is called a metabolic pathway. For example,
Glucose
→
2 Pyruvic acid
C
6
H
12
O
6
+ O
2
→
2C
3
H
4
O
3
+ 2H
2
O
is actually a metabolic pathway in which glucose becomes pyruvic acid
through ten different enzyme catalysed metabolic reactions. When you
study respiration in Chapter 14 you will study these reactions. At this
stage you should know that this very metabolic pathway with one or two
additional reactions gives rise to a variety of metabolic end products. In
our skeletal muscle, under anaerobic conditions, lactic acid is formed.
Under normal aerobic conditions, pyruvic acid is formed. In yeast, during
fermentation, the same pathway leads to the production of ethanol
(alcohol). Hence, in different conditions different products are possible.
9.12.2 How do Enzymes bring about such High Rates of
Chemical Conversions?
To understand this we should study enzymes a little more. We have already
understood the idea of an ‘active site’. The chemical or metabolic conversion
refers to a reaction. The chemical which is converted into a product is
called a ‘substrate’. Hence enzymes, i.e. proteins with three dimensional
structures including an ‘active site’, convert a substrate (S) into a product
(P). Symbolically, this can be depicted as:
S
→
P
It is now understood that the substrate ‘S’ has to bind the enzyme at
its ‘active site’ within a given cleft or pocket. The substrate has to diffuse
←
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towards the ‘active site’. There is thus, an
obligatory formation of an ‘ES’ complex. E
stands for enzyme. This complex formation is
a transient phenomenon. During the state
where substrate is bound to the enzyme active
site, a new structure of the substrate called
transition state structure is formed. Very soon,
after the expected bond breaking/making is
completed, the product is released from the
active site. In other words, the structure of
substrate gets transformed into the structure
of product(s). The pathway of this
transformation must go through the so-called
transition state structure. There could be
many more ‘altered structural states’ between
the stable substrate and the product. Implicit
in this statement is the fact that all other
intermediate structural states are unstable. Stability is something related
to energy status of the molecule or the structure. Hence, when we look at
this pictorially through a graph it looks like something as in Figure 9.6.
The y-axis represents the potential energy content. The x-axis
represents the progression of the structural transformation or states
through the ‘transition state’. You would notice two things. The energy
level difference between S and P. If ‘P’ is at a lower level than ‘S’, the reaction
is an exothermic reaction. One need not supply energy (by heating) in
order to form the product. However, whether it is an exothermic or
spontaneous reaction or an endothermic or energy requiring reaction,
the ‘S’ has to go through a much higher energy state or transition state.
The difference in average energy content of ‘S’ from that of this transition
state is called ‘activation energy’.
Enzymes eventually bring down this energy barrier making the
transition of ‘S’ to ‘P’ more easy.
9.12.3 Nature of Enzyme Action
Each enzyme (E) has a substrate (S) binding site in its molecule so that a
highly reactive enzyme-substrate complex (ES) is produced. This
complex is short-lived and dissociates into its product(s) P and the
unchanged enzyme with an intermediate formation of the enzyme-product
complex (EP).
The formation of the ES complex is essential for catalysis.
E + S
ES
→
EP
→
E + P
Activationenergy
withoutenzyme
PotentialEn
ergy
Activation
energywithenzyme
Substrate(s)
Product(P)
Progressofreaction
Transitionstate
Figure 9.6 Concept of activation energy
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The catalytic cycle of an enzyme action can be described in the following
steps:
1. First, the substrate binds to the active site of the enzyme, fitting
into the active site.
2. The binding of the substrate induces the enzyme to alter its shape,
fitting more tightly around the substrate.
3. The active site of the enzyme, now in close proximity of the
substrate breaks the chemical bonds of the substrate and the
new enzyme- product complex is formed.
4. The enzyme releases the products of the reaction and the free
enzyme is ready to bind to another molecule of the substrate and
run through the catalytic cycle once again.
9.12.4 Factors Affecting Enzyme Activity
The activity of an enzyme can be affected by a change in the conditions
which can alter the tertiary structure of the protein. These include
temperature, pH, change in substrate concentration or binding of specific
chemicals that regulate its activity.
Temperature and pH
Enzymes generally function in a narrow range of temperature and pH
(Figure 9.7). Each enzyme shows its highest activity at a particular
temperature and pH called the optimum temperature and optimum pH.
Activity declines both below and above the optimum value. Low
temperature preserves the enzyme in a temporarily inactive state whereas
high temperature destroys enzymatic activity because proteins are
denatured by heat.
Figure 9.7 Effect of change in : (a) pH (b) Temperature and (c) Concentration of
substrate on enzyme activity
V
max
Velocity of r
eaction
(V)
[S]
V
2
max
K
m
(a)
(b)
(c)
pH
Temperature
E
nzyme activity
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Concentration of Substrate
With the increase in substrate concentration, the velocity of the enzymatic
reaction rises at first. The reaction ultimately reaches a maximum velocity
(V
max
) which is not exceeded by any further rise in concentration of the
substrate. This is because the enzyme molecules are fewer than the
substrate molecules and after saturation of these molecules, there are no
free enzyme molecules to bind with the additional substrate molecules
(Figure 9.7).
The activity of an enzyme is also sensitive to the presence of specific
chemicals that bind to the enzyme. When the binding of the chemical
shuts off enzyme activity, the process is called inhibition and the chemical
is called an inhibitor.
When the inhibitor closely resembles the substrate in its molecular
structure and inhibits the activity of the enzyme, it is known as
competitive inhibitor. Due to its close structural similarity with the
substrate, the inhibitor competes with the substrate for the substrate-
binding site of the enzyme. Consequently, the substrate cannot bind and
as a result, the enzyme action declines, e.g., inhibition of succinic
dehydrogenase by malonate which closely resembles the substrate
succinate in structure. Such competitive inhibitors are often used in the
control of bacterial pathogens.
9.12.5 Classification and Nomenclature of Enzymes
Thousands of enzymes have been discovered, isolated and studied. Most
of these enzymes have been classified into different groups based on the
type of reactions they catalyse. Enzymes are divided into 6 classes each
with 4-13 subclasses and named accordingly by a four-digit number.
Oxidoreductases/dehydrogenases: Enzymes which catalyse
oxidoreduction between two substrates S and S’ e.g.,
S reduced + S’ oxidised
→
S oxidised + S’ reduced.
Transferases: Enzymes catalysing a transfer of a group, G (other than
hydrogen) between a pair of substrate S and S’ e.g.,
S - G + S’
→
S + S’ - G
Hydrolases: Enzymes catalysing hydrolysis of ester, ether, peptide,
glycosidic, C-C, C-halide or P-N bonds.
Lyases: Enzymes that catalyse removal of groups from substrates by
mechanisms other than hydrolysis leaving double bonds.
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Isomerases: Includes all enzymes catalysing inter-conversion of optical,
geometric or positional isomers.
Ligases: Enzymes catalysing the linking together of 2 compounds, e.g.,
enzymes which catalyse joining of C-O, C-S, C-N, P-O etc. bonds.
9.12.6 Co-factors
Enzymes are composed of one or several polypeptide chains. However,
there are a number of cases in which non-protein constituents called co-
factors are bound to the the enzyme to make the enzyme catalytically
active. In these instances, the protein portion of the enzymes is called the
apoenzyme. Three kinds of cofactors may be identified: prosthetic groups,
co-enzymes and metal ions.
Prosthetic groups are organic compounds and are distinguished from
other cofactors in that they are tightly bound to the apoenzyme. For
example, in peroxidase and catalase, which catalyze the breakdown of
hydrogen peroxide to water and oxygen, haem is the prosthetic group
and it is a part of the active site of the enzyme.
Co-enzymes are also organic compounds but their association with
the apoenzyme is only transient, usually occurring during the course of
catalysis. Furthermore, co-enzymes serve as co-factors in a number of
different enzyme catalyzed reactions. The essential chemical components
of many coenzymes are vitamins, e.g., coenzyme nicotinamide adenine
dinucleotide (NAD) and NADP contain the vitamin niacin.
A number of enzymes require metal ions for their activity which form
coordination bonds with side chains at the active site and at the same
time form one or more cordination bonds with the substrate, e.g., zinc is
a cofactor for the proteolytic enzyme carboxypeptidase.
Catalytic activity is lost when the co-factor is removed from the enzyme
which testifies that they play a crucial role in the catalytic activity of the
enzyme.
SUMMARY
Although there is a bewildering diversity of living organisms, their chemical
composition and metabolic reactions appear to be remarkably similar. The
elemental composition of living tissues and non-living matter appear also to be
similar when analysed qualitatively. However, a closer examination reveals that
the relative abundance of carbon, hydrogen and oxygen is higher in living systems
when compared to inanimate matter. The most abundant chemical in living
organisms is water. There are thousands of small molecular weight (<1000 Da)
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biomolecules. Amino acids, monosaccharide and disaccharide sugars, fatty acids,
glycerol, nucleotides, nucleosides and nitrogen bases are some of the organic
compounds seen in living organisms. There are 20 types of amino acids and 5
types of nucleotides. Fats and oils are glycerides in which fatty acids are esterified
to glycerol. Phospholipids contain, in addition, a phosphorylated nitrogenous
compound.
Only three types of macromolecules, i.e., proteins, nucleic acids and
polysaccharides are found in living systems. Lipids, because of their association
with membranes separate in the macromolecular fraction. Biomacromolecules
are polymers. They are made of building blocks which are different. Proteins
are heteropolymers made of amino acids. Nucleic acids (RNA and DNA) are
composed of nucleotides. Biomacromolecules have a hierarchy of structures –
primary, secondary, tertiary and quaternary. Nucleic acids serve as genetic
material. Polysaccharides are components of cell wall in plants, fungi and also
of the exoskeleton of arthropods. They also are storage forms of energy (e.g.,
starch and glycogen). Proteins serve a variety of cellular functions. Many of
them are enzymes, some are antibodies, some are receptors, some are hormones
and some others are structural proteins. Collagen is the most abundant protein
in animal world and Ribulose bisphosphate Carboxylase-Oxygenase (RuBisCO)
is the most abundant protein in the whole of the biosphere.
Enzymes are proteins which catalyse biochemical reactions in the cells.
Ribozymes are nucleic acids with catalytic power. Proteinaceous enzymes
exhibit substrate specificity, require optimum temperature and pH for maximal
activity. They are denatured at high temperatures. Enzymes lower activation
energy of reactions and enhance greatly the rate of the reactions. Nucleic acids
carry hereditary information and are passed on from parental generation to
progeny.
EXERCISES
1. What are macromolecules? Give examples.
2. Illustrate a glycosidic, peptide and a phospho-diester bond.
3. What is meant by tertiary structure of proteins?
4. Find and write down structures of 10 interesting small molecular weight
biomolecules. Find if there is any industry which manufactures the compounds
by isolation. Find out who are the buyers.
5. Proteins have primary structure. If you are given a method to know which amino
acid is at either of the two termini (ends) of a protein, can you connect this
information to purity or homogeneity of a protein?
6. Find out and make a list of proteins used as therapeutic agents. Find other
applications of proteins (e.g., Cosmetics etc.)
7. Explain the composition of triglyceride.
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8. Can you describe what happens when milk is converted into curd or yoghurt,
from your understanding of proteins.
9. Can you attempt building models of biomolecules using commercially available
atomic models (Ball and Stick models).
10. Attempt titrating an amino acid against a weak base and discover the number
of dissociating ( ionizable ) functional groups in the amino acid.
11. Draw the structure of the amino acid, alanine.
12. What are gums made of? Is Fevicol different?
13. Find out a qualitative test for proteins, fats and oils, amino acids and test any
fruit juice, saliva, sweat and urine for them.
14. Find out how much cellulose is made by all the plants in the biosphere and
compare it with how much of paper is manufactured by man and hence what is
the consumption of plant material by man annually. What a loss of vegetation!
15. Describe the important properties of enzymes.
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