Enzymes,Their Classification, Mechanism of Action,and Factors Affecting Enzymatic Activity
Enzymes:
Enzymes are catalysts of biological systems (hence are called as biocatalysts),
colloidal, thermolabile and protein in nature.
Certain enzymes with only one polypeptide chain in their structure are called as monomeric enzymes, e.g. ribonuclease.
Several enzymes possess more than one polypeptide chain and are called as oligomeric enzymes, e.g. lactate dehydrogenase, hexokinase, etc. Each single polypeptide chain of oligomeric enzymes is called as subunit.
When many different enzyme catalyzing reaction sites are located at different sites of the same macromolecule, it is called as multienzyme complex.
The complex becomes inactive when it is
fractionated into smaller units each bearing individual enzyme activity, e.g.
fatty acid synthetase, carbamoyl phosphate synthetase II, pyruvate
dehydrogenase, prostaglandin synthase,
Substrate: on
which enzymes act to convert them into products are called substrates.
Factors Affecting Catalytic Activity of Enzymes:
Enzymes have immense catalytic
power and accelerate reactions at least a million times, by reducing the
energy of activation.
Activation
Energy: Before a chemical reaction can occur, the reacting molecules are
required to gain a minimum amount of energy, this is called the energy
of activation.
Effect
of Temperature on Activation Energy: It can be decreased by increasing
the temperature of the reaction medium. But in human body which maintains a
normal body temperature fairly constant, it is achieved by enzymes.
Chemical Composition of Enzymes:
In general, with the exception of
ribozymes which are few RNA molecules with enzymatic activity, all the enzymes
are protein in nature with large mol. wt. Few enzymes
are simple proteins while some are conjugated proteins. In such enzymes the
Nomenclature And Classification of Enzymes
Enzymes
are generally named after adding the suffix ‘ase’ to the name of the
substrate, e.g.
Enzymes
acting on nucleic acids are known as nucleases,
Enzymes
hydrolyzing dipeptides are called dipeptidases.
Names Other Than Nomenclature
Even
though few exceptions such as trypsin, pepsin, and chymotrypsin are
still in use.
In
Active Enzymes
Further,
few enzymes exist in their inactive forms and are called as proenzymes or
zymogens, e.g. pepsin has pepsinogen as its zymogen. The zymogens become
active after undergoing some prior modification in its structure by certain
agents.
Autocatalysis:
Many
times the active form of enzyme acts on zymogen and catalysis its conversion
into active form and this process is called as autocatalysis.
In
order to have a uniformity and unambiguity in identification of enzymes,
International Union of Biochemistry (IUB) adopted a nomenclature system based
on chemical reaction type and reaction mechanism. According to this system,
enzymes are grouped in six main classes.
•
Each enzyme is characterized by a code number (enzyme code No. or E C No)
comprising four figures (digits) separated by points, the first being that of
the main class (one of the six).
•
The second figure indicates the type of group involved in the reaction.
•
Third figure denotes the reaction more precisely indicating substrate on which
the group acts.
•
The fourth figure is the serial number of the enzyme. Briefly, the four digits characterize
class, sub-class, sub-sub-cl.
Six Classes Are:
1.
Oxidoreductase: Enzymes involved in oxidations and reductions of their substrates,
e.g. alcohol dehydrogenase, lactate dehydrogenase, xanthine oxidase,
glutathione reductase, glucose-6-phosphate dehydrogenase.
2.
Transferases: Enzymes that catalyze transfer of a particular group from one
substrate to another, e.g. aspartate and alanine transaminase (AST/ALT),
hexokinase, phosphoglucomutase, hexose-1-phosphate uridyltransferase,
ornithine carbamoyl transferase, etc.
3.
Hydrolases: Enzymes that bring about hydrolysis, e.g. glucose-6-phosphatase,
pepsin, trypsin, esterase, glycoside hydrolases, etc.
4.
Lyases: Enzymes that facilitate removal of small molecule from a large
substrate, e.g. fumarase, arginosuccinase, histidine decarboxylase.
5.
Isomerases: Enzymes involved in isomerization of substrate, e.g. UDP-glucose,
epimerase, retinal isomerase, racemases, triosephosphate isomerase.
6.
Ligases: Enzymes involved in joining together two substrates, e.g.
alanyl-t. RNA synthetase, glutamine synthetase, DNA ligases.
Many
times the word ‘OTHLIL’ is used to remember the six classes.
What is Specificity of Enzymes
The ability of an enzyme to select a specific
substrate from a range of chemically similar compounds is known as specificity. It is
important property of enzymes The specificity is of three different types
namely:
1.
Stereochemical specificity
2.
Reaction specificity
3.
Substrate specificity.
1. Stereochemical Specificity
Stereospecificity is the property of a
reaction mechanism that leads to different stereoisomeric reaction
products from different stereoisomeric reactants, or which operates on only one
(or a subset) of the stereoisomers.
Specific
Enzyme for Specific Isomer
There
can be many optical isomers of a substrate. However, it is only one of the
isomers which acts as a substrate for an enzyme action, e.g. for the oxidation
of D- and L-amino acids, there are two types of enzyme which will act on D- and
L-isomers of amino acids.
Product
of an Enzyme is a specific Isomer
There can be a product of enzyme action which can have isomers. However, it is only one kind of isomer which will be produced as a product, e.g., Succinic dehydrogenase while acting on succinic acid will give only fumaric acid and not malic acid which is its isomer.
2.Reaction
Specificity
A
substrate can undergo many reactions but in a reaction specificity one
enzyme can catalyze only one of the various reactions. For example,
oxaloacetic acid can undergo several reactions but each reaction is catalyzed
by its own separate enzyme which catalysis only that reaction and none of the
others.
3.Substrate
Specificity
The
extent of substrate specificity varies from enzyme to enzyme. There are two
types of substrate specificity viz, absolute specificity and relative
specificity
•
Absolute specificity is comparatively rare such as urease which catalysis hydrolysis of
urea.
•
Relative substrate specificity is further divided as:
Group dependent
Bond dependent.
Examples
of group specificity are trypsin, chymotrypsin. Trypsin hydrolyses the residues
of only lysine and arginine, while chymotrypsin hydrolyses residues of only
aromatic amino acids.
4.Bond
Specificity
Bond
specificity is observed in case of proteolytic enzymes, glycosidases and
lipases which act on peptide bonds, glycosidic bonds and ester bonds
respectively.
Mechanism Of Enzyme Action
Michaelis and Menten have proposed a hypothesis for enzyme action, which is most acceptable. According to their hypothesis, the enzyme molecule (E) first combines with a substrate molecule (S) to form an enzyme substrate (ES) complex which further dissociates to form product (P) and enzyme (E) back.
Enzyme
once dissociated from the complex is free to combine with another molecule
of substrate and form product in a similar way.
“The
ES complex is an intermediate or transient complex”
Sometimes
two substrates can bind to an enzyme molecule and such reactions are called as
substrate reactions.
Active Site
The
site to which a substrate can bind to the enzyme molecule is extremely specific
and is called as active site or catalytic site
The
active site is made up of several amino acid residues that come together as a
result of folding of secondary and tertiary structures of the enzyme.
Few groups of active site amino acids are bound to substrate while few groups bring about change in the substrate molecule.
Reference:
Notes Made By The Help of "The Text Book of Medical Biochemistry By MN. Chatterjea 8th Edition"
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