Download Consortium for Educational Communication

Consortium for
Educational
Communication
Module on
Discovery, Nomenclature and
Characteristics of Enzymes
By
Ms Hadiya Shafi
Research scholar
Department of Botany
University of Kashmir
Srinagar
Consortium for Educational Communication
TEXT
Enzymes
Enzymes are the biological molecules that catalyze specific
biochemical reactions. They are active under mild conditions of
temperature and pressure found in the cell. They are usually
single or multiple chain proteins that act as biological catalysts
that catalyze intracellular and extracellular biochemical reactions.
On the basis of their composition, enzymes are of two types,
i.e., simple enzymes (Pepsin, trypsin, Urease and amylase) and
conjugated enzymes. The simple enzymes are entirely made up
of proteins while as the conjugated enzymes consist of a protein
part and a non protein part. The protein part is called apoenzyme
and the non protein part is called prosthetic group, coenzyme or
cofactor. The complete enzyme including prosthetic group and or
coenzyme or cofactor is called holoenzyme. Apopenzyme consists
of one or more polypeptide chains of 10’s to 100’s of amino acids.
The type of amino acid and the number of subunits determine the
size and composition of the protein part. The Prosthetic group is
tightly bound to the enzyme and is usually a much smaller inorganic
portion of an enzyme. It is attached to the protein part of an enzyme
by covalent bonds and is essential for its catalytic activity. The
simple prosthetic groups are metal ions, such as, Cu, Zn, Mn and
Mo. Some prosthetic groups may also be organic compounds which
include cytochromes, Flavonoids, pyridoxe or biotinal phosphate,
heme or biotin, etc. These groups are bound to active site of many
enzymes that participate in catalysis. Coenzyme or cofactor is not
tightly bound to the enzyme. The former is an organic molecule
and the later is inorganic in nature. Coenzymes serve as carriers of
various types of chemical groups. NAD+ (Nicotineamide adenine
Consortium for Educational Communication
dinucleotide) functions as carrier of electrons in oxidation reduction
reactions. Several other coenzymes are involved in the transfer of
a variety of additional chemical groups (e.g. carboxyl groups and
acyl goups). Cofactors are essential for the catalytic activity of
an enzyme. Some examples of cofactors are Clˉ, Cu2+, Fe2+, Zn2+,
Co2+, K+, Ca2+, etc. There are also some special types of enzymes.
These are isozymes and extremozymes. Isozymes are the multiple
molecular forms of a single enzyme which are normally formed
under normal conditions. They can differ in various physical and
chemical properties and their intracellular localization may also
differ in nature of cofactor and kinetic or regulatory properties.
Extremoxymes are the enzymes which are active under extreme
conditions of pH, temperature and pressure. These enzymes are
well adapted to tolerate such harsh conditions. Enzymes have
the ability to increase the reaction rate or biocatalysis without
being destroyed or changed in the process. Because of this, one
enzyme molecule can theoretically change an infinite amount
of substrate, if given an infinite amount of time. Increasing the
amount of enzyme, decreases the time required for completing
the reaction. The enzymes enhance reaction rates by a factor of
105 to 10 17 times than the uncatalysed reaction and facilitate
the attainment of chemical equilibrium fast. Enzymes function by
lowering the free energy of the reactions transition state. They are
effective in very small amounts. They are extremely unstable and
are inactivated or denatured by high temperature or very alkaline
or acidic conditions. For an enzyme to work, it must have access
to its substrate, the material upon which it exerts its action. If
no substrate is available to the enzyme, the enzyme performs no
function. The substances on which an enzyme exerts its action are
called reactants and the substances produced at the end of the
reaction are called products.
Consortium for Educational Communication
Enzymes control all biochemical reactions in every living
thing, from viruses to man. Breathing, digestion, heart action,
formation of body tissues, movement of muscles and many more
processes depend on enzymes. So without enzymes, life is not
possible. More than 100,000 different types of enzymes have
been identified, most of which are colorless solid, soluble in water
or dilute solutions, but some are blue, green or greenish brown.
Discovery of Enzymes
The term ‘enzyme’ was introduced by Kuhne in 1878. It is taken
from the Greek word Zume (Zyme) meaning ‘Leaven’. Enzymes
are the principles found in Leaven. The first observation was carried
out on vegetable and animal cell free extracts. Berzilius in 1836
coined the term catalysis which in Greek means ‘to discover’. After
this, a certain number of enzymes called ‘diastases’ or ‘ferments’
were identified. Claude Bernard in 1848 discovered the role
of Pancreatic juices and described the action of Pancreatin, an
albuminous substance. In 1864, Bechamp observed that the
enzyme Zymase is involved in the transformation of Saccharose
into glucose. Raphael Dubois in 1886 discovered luciferase, the
enzyme responsible for bioluminescence. Later in 1896, several
oxidative enzymes were discovered by Gabriel Bertrand. He
introduced the concept of Cofactor. In 1897, Hill discovered that
maltase catalyzes a reversible reaction. Edward Buchner in 1897
showed that a yeast extract completely lacking cells could just
as well stimulate alcoholic fermentation. He hypothesized that
yeast cells ferment sugars not inside the cell but by secreting
proteins into their environment, which is the actual mechanism.
This posed challenge to vitalism by showing that the presence
of living yeast cells was not necessary for fermentation. He was
Consortium for Educational Communication
awarded Nobel Prize in Chemistry in 1907 for this valuable work.
Biocatalysis has been the basis of some of the oldest chemical
transformations (e.g fermentation and brewing) and it involves
enzyme catalysis in the living cells. The discovery of enzymes
can be traced back to nineteenth century.
In 1926, James Batcheller Sumner demonstrated that urease
could be isolated and crystallized and showed by chemical tests
that it was a protein. This was the first experimental proof that
an enzyme is a protein. In 1937, he succeeded in isolating and
crystallizing a second enzyme, catalase and devised a general
crystallization method for enzymes, and also showed that all
enzymes are proteins. By similar methods, John
Howard
Northrop also obtained other crystalline enzymes by this time,
starting with pepsin in 1929. W. M. Stanley is also known for
the preparation of enzymes and virus proteins in a pure form.
In 1946, James Batcheller Sumner shared the Nobel Prize in
Chemistry with John Howard Northrop and Wendell Meredith
Stanley for the brilliant work on enzymes.
Table 1: Enzymes and their discoverers.
Enzyme
Ptyalin
Maltase
Singrase
Pepsin
Discoverer
Leuchs
Payen and Persoz
Faure
Schwann
Year of discovery
1831
1833
1835
1836
Nomenclature of enzymes
Many enzymes have usually been named by adding the suffix
“ase” to the name of their substrate or the catalytic reaction.
Some enzymes have been named in an arbitrary way. Some
of the examples are trypsin, pepsin, chymotrypsin, rennin and
emulsion. The earlier system of nomenclature has been modified
Consortium for Educational Communication
in 1992 by International Union of Biochemistry and Molecular
Biology (IUBMB) nomenclature system of enzymes, commonly
known as Enzyme Commision (EC) numbers. It developed certain
rules for nomenclature and classification of enzymes. This system
divided enzymes into six classes, each with subclasses and subsubclasses on the basis of the reaction catalysed. According to
standard nomenclature, each enzyme is assigned a four digit code
number following the abbreviation EC (Enzyme Commission) and
a systematic name, which identifies the reaction catalysed by it.
Examples:
ATP + D-glucose → ADP +D-glucose 6-phosphate
Common name: Hexokinase
Systematic name:
ATP: glucose phosphotransferase
Enzyme Commision number [EC no.] is EC 2.7.1.1
The first number 2 denotes that it belongs to class 2 of enzymes, i.e.,
transferases which catalyze group transfer reactions.The second
number 7 denotes the subclass 7, i.e., phosphotransferases,
which catalyse transfer of phosphoryl group.The third number
1 denotes the sub-subclass i.e., a phosphotransferase with a
hydroxyl group as acceptor and the fourth number 1 indicates
that the phosphoryl group acceptor is D- glucose.
The six classes of enzymes are mentioned below:1- Oxidoreductases
2- Transferases
3- Hydrolases
4- Lyases
5- Isomerases
Consortium for Educational Communication
6- Ligases
1-Oxidoreductases
Oxidoreductases bring about redox reactions or oxidation-reduction
reactions involving transfer of hydrogen or oxygen atoms, hydride
ions or electrons between the molecules i.e., from one molecule
(the reductant or the hydrogen or electron donar) to another
molecule (the oxidant or the hydrogen or electron acceptor). Their
systematic name is in the form of donar: acceptor oxidoreductase.
If a hydrogen donating substrate is being oxidized, the enzyme is
commonly called donar dehydrogenase. If oxygen is the acceptor,
the term donar oxidase is used. The second number in the EC code
of oxidoreductases denotes the sub-subclass of the enzyme and
indicates the hydrogen acceptor. Some of the common examples of
oxidoreductases are Alcohol dehydrogenase, Cytochrome oxidase
and Heme oxygenase.
The general reaction catalysed by oxidoreductases is mentioned
below:
Areduced + Boxidized
→ A oxidized + Breduced
Table 2 : Classification of oxidoreductases upto the subclass level
SubClass
1.1.
1.2
1.3
Oxidoreductases
Example
Acts on the CH–OH group of donors.
Alcohol dehydrogenase
Acts on the aldehyde or oxo group of F o r m a l d e h y d e
donors.
dehydrogenase
Acts on the CH–CH group of donors.
Fumarate
reductase
(NADH)
Consortium for Educational Communication
1.4
1.5
1.6
1.7
1.8
1.9
1.10
Acts on the CH–NH2 group of donors.
Glutamate
synthase
(NADPH)
Acts on the CH–NH group of donors.
Flavin reductase
Acts on NADH2 or NADPH2.
NAD(P)+ transhydrogenase
Acts on other nitrogenous compounds as Nitrite reductase (NAD(P)
donors.
H)
Acts on a sulfur group of donors.
Cystine reductase
Acts on a haem group of donors.
Cytochrome oxidase
1.15.
1.16.
1.17.
1.18.
Acts on diphenols and related substances
as donors.
Acts on a peroxide as acceptor
(peroxidases).
Acts on hydrogen as donor.
Acts on single donors with incorporation
of molecular oxygen.
Acts on paired donors with incorporation
of molecular oxygen.
Acts on superoxide radicals as acceptor.
Oxidizing metal ions.
Acts on –CH2– groups.
Acts on reduced ferredoxin as donor.
1.19.
Acts on reduced flavodoxin as donor.
Superoxide dismutase
Mercury(II) reductase
Ribonucleotide reductase
Ferredoxin—NADP(+)
reductase
nitrogenase
1.97.
Other oxidoreductases.
Chlorate reductase
1.11
1.12
1.13
1.14
Coenzyme Q – cytochrome
c reductase
Catalase
Hydrogen dehydrogenase
Indole 2,3-dioxygenase
Heme oxygenase
2. Transferases
Transferases catalyze the transfer of functional groups from one
substrate (donar) to another (acceptor). Thus, they are involved
in group transfer reactions. The systematic name is donar:
acceptor group transferase. The recommended name is acceptor
grouptransferase or donar grouptransferase. Transferases
catalyze the transfer of an atom (other than oxygen or hydrogen)
Consortium for Educational Communication
or a group of atoms like acyl-, alkyl-, glycosyl, etc. Some of the
common examples of transferases are Hexokinase, Glutathione
S-transferase and Methionine synthase.
The general reaction of transferases is given below:
A-C + B → A + B-C
Table 3. Classification of transferases upto the subclass level.
SubClass
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
Transferases
Transfers one-carbon groups.
Transfers aldehyde or ketone residues.
Acyltransferases.
Glycosyltransferases.
Transfers alkyl or aryl groups, other than
methyl groups.
Transfers nitrogenous groups.
Transfers phosphorus-containing groups.
Transfers sulfur-containing groups.
Transfers selenium-containing groups.
Example
Methionine synthase
Transketolase
Fatty acid synthase
Glycogen synthase
Glutathione s transferase
Transaminase
Hexokinase
Biotin synthase
L- seryl-tRNASec
transferase
selenium
3. Hydrolases
Hydrolase catalyzes the hydrolytic cleavage of a chemical
bond such as C–O, C–N, C–C and some other bonds, including
phosphoric anhydride bonds. The systematic name is substrateXhydrolase, where X is the group removed by hydrolysis. In many
cases, the recommended name is formed by the name of the
substrate with the suffix “ase”. This suffix indicates that it is a
hydrolytic enzyme. The main physiological function of the enzyme
is the reaction with water as the acceptor. The first number in
Consortium for Educational Communication
the EC code indicates that it belongs to the class hydrolases. The
second number indicates the nature of the hydrolysed bond, and
the third number denotes the nature of the substrate. Some of
the common examples of hydrolases are Alkaline phosphatase,
Uracil DNA glycosylase and Exonuclease.
For example,the hydrolase catalyzes the following reaction:A–B + H2O → A–OH + B–H
Table 4: Classification of hydrolases upto the sub class level.
Subclass
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
3.9
3.10
3.11
3.12
4. Lyases
Hydrolases
Acts on ester bonds.
Glycosidases.
Acts on ether bonds.
Acts on peptide bonds (peptidase).
Acts on carbon–nitrogen bonds,
other than peptide bonds.
Acts on acid anhydrides.
Acts on carbon–carbon bonds.
Acts on halide bonds.
Acts on phosphorus–nitrogen
bonds.
Example
Alkaline phosphatase
Urasil DNA glycosylase
Adenosylmethionine hydrolase
carboxypeptidase A
Aminohydrolase
Acylphosphatase
Acetylpyruvate hydrolase
Alkylhalidase
phosphoamidase
Acts on sulfur–nitrogen bonds.
Cyclamate sulfohydrolase
Acts on carbon–phosphorus Phosphonopyruvate hydrolase
bonds.
Acts on sulfur–sulfur bonds.
Trithionate hydrolase
Consortium for Educational Communication
Lyases catalyze the cleavage of various chemical bonds
such as C–N, C–O, C–C, C-S and others by reactions other
than hydrolysis and oxidation. Thus, they remove a group of
atoms from the substrate and lead to the formation of a new
double bond or a new ring structure. Lyases require only one
substrate for the unidirectional reaction, but two substrates for
the reverse reaction. The systematic name is substrate grouplyase. If the reverse reaction is much more important, or is the
only one demonstrated, the name synthase may be used rather
than synthetase. Some of the common examples of lyases are
Fumarase, Isocitrate lyase and pyruvate decarboxylase.
One of the reactions catalyzed by lyases is given below:
ATP → cAMP + PPi
Table 5. Classification of Lyases.
SubClass
4. 1.
4. 2.
4. 3.
4. 4.
4. 5.
4. 6.
4.99
Lyases
Carbon–carbon lyases
Carbon–oxygen lyases
Carbon–nitrogen lyases
Carbon–sulfur lyases
Carbon–halide lyases
Phosphorus-oxygen lyases
Other lyases
Example
Isocitrate lyase
Fumarase
Aspartate ammonia-lyase
Cysteine lyase
DDT-dehydrochlorinase
Adenylate cyclase
Aliphatic aldoxime dehydratase
5.Isomerases
Isomerase catalyzes the interconversion or structural
rearrangement of optical, positional or geometric isomers. These
enzymes catalyze structural or geometric changes within one
molecule. The first number indicates that they are isomerases.
Consortium for Educational Communication
The second number denotes the type of isomerism involved, and
the third number shows the type of substrate. In certain cases,
the intermolecular oxidoreduction reaction is involved. They are
classified as isomerases and not oxidoreductases because the
donor and the acceptor groups are present in the same molecule.
They may also contain firmly bound NAD or NADP. Some of
the common examples of isomerases are Inositol-3-phosphate
synthase, Phosphoglucomutase and Type II topoisomerase.
Thus, isomerases catalyze reactions of the form:
A→C
where C is an isomer of A.
Table 6. Classification of isomerases.
Subclass
5.1.
5.2.
5.3.
5.4.
5.5.
5.99.
Isomerases
Racemases and epimerases
cis–trans-Isomerases
Intramolecular oxidoreductases
Intramolecular transferases
Intramolecular lyases
Other isomerases
Example
Alanine racemase
Retinal isomerase
Ribose-5-phosphate isomerase
Phosphoglucomutase
Inositol-3-phosphate synthase
DNA gyrase
6. Ligases
The word ligase has been taken from the Latin verb ligare which
means “to bind” or “to glue together”. It catalyzes the joining
of two large molecules to form a new covalent bond coupled
with the hydrolysis of a small dependent chemical group.
For example, a pyrophosphate bond in ATP, GTP or a similar
compound is hydrolyzed by ligase. The enzyme links together
two compounds and catalyzes the joining of C-N, C-S, C-O, C-C
bonds etc. The systematic name of enzyme is A: B ligase. The
recommended name is A–B ligase. To emphasize the synthetic
nature of the reaction, sometimes, the name synthase is used
Consortium for Educational Communication
for the recommended name. The name synthetase can also be
sometimes used instead of synthase in the names of enzymes
in this class. Some of the common examples of ligases are DNA
Ligase, Pyruvate carboxylase and Succinyl co A synthetase.
The general reaction catalysed by Ligase is shown below:Ab + cD → A–D + b + c
Here, the lower case letters indicate the small dependent groups.
Table 7: Classification of ligases upto the subclass level
SubClass
6. 1.
6. 2.
6. 3.
6. 4.
6. 5.
Ligases
Forming carbon–oxygen bonds
Forming carbon–sulfur bonds
Forming carbon–nitrogen bonds
Forming carbon–carbon bonds
Forming phosphoric ester bonds
Example
Aminoacyl tRNA synthetase
Succinyl co A synthetase
Glutamine synthetase
Pyruvate caboxylase
DNA ligase
Characteristics of enzymes
Enzymes can easily be identified by characteristics possessed by
them. The various characteristic features of enzymes are specificity,
reversibility of action, temperature sensitivity, pH sensitivity, high
catalytic rate, colloidal properties, molecular weight, chemical
nature, inhibition by poisons. All these properties are explained
below:
1. Specificity
Enzymes are highly specific in the type of reaction they catalyze
and the choice of reactants they act upon. A particular enzyme can
catalyze only a particular type of reaction. Each enzyme acts on
Consortium for Educational Communication
a single substrate or small group of closely related substrates.
The presence of specific functional groups adjacent to the bond
to be cleaved determines the specificity of the enzyme. The
active sites of enzymes are highly specific centre’s composed of
varying number and sequence of amino acids and a particular
binding site which complexes only with a specific substrate (in a
lock and key process).
2. Reversibility of action
The enzyme catalyzed reactions are reversible as well as
irreversible. In the presence of an appropriate enzyme, the
conversion of substrate to products is accelerated without
altering the equilibrium between the reactants and the products.
Although, the rate at which the chemical equilibrium is established
is enhanced. The enzymes affect only the rate and not the
direction. They can accelerate the reaction in either direction.
Many enzymes which are required for the synthesis of starch and
sucrose are responsible for their breakdown also. However, by
removing the products of enzymatic reaction as quickly as they
are formed, the reversion can be checked. The enzyme must
accelerate both the forward and backward reactions equally.
The reversible reaction can be written as:
S⇌P
Example:
Alcohol hydrogenase act in both directions.
CH3CH2OH + NAD+
(Alcohol)
⇌
CH3CHO +NADH + H+
(Aldehyde)
On the other hand, some enzymes carry out irreversible reactions
Consortium for Educational Communication
only in one direction, e.g., hydrolytic enzymes catalyzing cleavage
of bonds between two atoms. In fact large molecules such as fats,
proteins, starch and nucleic acids are synthesized by one enzyme
and degraded by the other. Synthetic and degradative enzymes
are often kept separate from each other by membranes or are
formed at different stages, so that competition between them is
minimized.
3. Temperature sensitivity
The enzymes are sensitive to heat and hence are thermo labile.
They function best at optimum temperature range of 250C to
400C. Both low as well as high temperatures are unfavorable for
enzymatic activity. Their activity decreases with decrease as well
as increase in temperature. Enzymatic reactions occur slowly at
00C because of the lower level of the molecular kinetic energy
which limits substrate enzyme collision and attainment of the
transition state. With increase in temperature, both these events
are favoured. With every 100 rise in temperature, the reaction
rate shows enhancement. Most enzymes are inactivated or
denatured by temperature about 600C. It is because majority of
the enzymes are proteinaceous in nature. Early denaturation by
heat is observed in enzymes present in moist conditions. However
the enzymes in drier or dehydrated conditions are quite stable.
Denaturation disrupts the secondary structure of an enzyme and
it looses its active site and hence the catalytic power.
However, some enzymes are resistant to high temperature and
continue to work at high temperature conditions.
4.pH sensitivity
Consortium for Educational Communication
In the catalytic site of an enzyme, the state of ionization of
amino acid residues is pH dependent. Since, the specific states
of ionization of these residues determine the enzyme’s catalytic
activity, enzymes activity is also pH dependent. Different enzymes
function at different pH ranges. There is a restrictive pH range
which remains constant for each enzyme and is its characteristic
property. The activity of an enzyme decreases with increase
or decrease in optimum pH. pH changes also modify the ionic
substrates. For example, if a COO- group of an aspartate is used
to bind a positively charged substrate, protonation would reduce
the force of attraction and decreases the affinity of the enzyme
for the substrate. Most enzymes show maximal activity around
neutral pH, but there are many exceptions.
Table 8.Optimum pH of different enzymes
Enzyme
Pepsin
Egg white lysozyme
Acid phosphatase
Pancreatic alpha amylase
yeast hexokinase
Alpha-chymotrypsin
Alkaline phosphatase
Fumarase
Ribonuclease
Optimum pH
1.5-2.0
4.5-5.0
5.0
6.9
7.4
8-8.5
9.5-10
7.0
7.0
3. Catalytic properties
Enzymes increase reaction rate by a factor of 105-1017 than that
of an unanalyzed reaction. Enzymes are active in extremely small
amounts, e.g., one molecule of catalase can effectively catalyze
conversion of 50, 00,000 molecules of H202 into H20 and H2.The
Consortium for Educational Communication
enzyme remains unaffected at the end of the reaction. It also
doesn’t alter the position of chemical equilibrium of a reversible
reaction but helps in its quick establishment. Weak interactions
(hydrogen bonds and hydrophobic and ionic interactions)
between substrate and enzyme provide a significant part of the
energy used for reaction rate enhancement. Carbonic anhydrase
is one of the fastest enzymes known. It catalyzes the hydration
of CO2 and transfers it from tissues into the blood and then to the
alveolar air. Each enzyme molecule can hydrate 105 molecules of
CO2 per second.
5. Colloidal properties of enzymes
Enzymes are large proteinaceous substances of very high
molecular weight. They have the size in the range of colloidal
particles (0.001µm-0.1µm in diameter). Their colloidal nature
provides large surface area for reactions to take place. Enzymes
display all the colloidal properties. They don’t pass through a
collodion (or parchment membrane) but can pass through filter
paper. This helps in their separation from true solution. The
enzymes, being colloidal particles, have positive or negative
charges.
7. Molecular weight
Catalase has a molecular weight of 250,000 Daltons and is one
of the largest enzymes. On the other side, peroxidase, one of
the smaller enzymes has a molecular weight of 40,000 Daltons.
Thus enzymes are larger than usual simple organic molecules.
Consortium for Educational Communication
8. Chemical nature of enzymes
Enzymes are mostly proteinaceous in nature except a small group
of catalytic RNA and DNA molecules. There catalytic activity
depends on the integrity of their native protein conformation.
If an enzyme is dissociated into subunits or is denatured, it
looses its catalytic activity. Breaking down an enzyme into its
component amino acids always destroys its activity completely.
Thus, the primary, secondary, tertiary and quaternary structures
of proteinaceous enzymes are absolutely important for their
catalytic activity.
9. Inhibition by poison
The activity of an enzyme is altered by many foreign substances
that combine with it in a way that influences its chemical substrate
binding property and turnover number. Substances or agents that
reduce the activity of an enzyme or inactivate it in this way are
called as inhibitors or enzyme poisons. Inhibitor binds to enzyme
either reversibly or irreversibly. Thus, the inhibitors are of two
types:-
A. Reversible inhibitors
Reversible inhibitors can easily be removed from the enzyme
because they bind reversibly to the enzyme by noncovalent
interactions. There are three main types of reversible inhibition
of enzymes. These are mentioned below.
a. competitive reversible inhibition
A competitive inhibitor binds at and blocks the binding site of
Consortium for Educational Communication
substrate on the enzyme. Most of them resemble the substrate
and combine with the enzyme to form an EI complex but don’t
lead to the catalysis. By adding more substrate, its combination
with enzyme is favoured.
b. Uncompetitive reversible inhibition
An uncompetitive inhibitor binds only to site distinct from
substrate active site and binds only to ES complex. Ultimately, it
leads to no reaction.
c. Mixed reversible inhibition
A mixed reversible inhibitor binds either to ES complex or directly
to the enzyme. It also binds to a site which is distinct from
substrate binding site.
B. Irreversible inhibitors
These are the inhibitor which bind covalently with an enzyme and
destroys a functional group on it that is essential for its catalytic
activity. Some inhibitors form a particular stable non covalent
association with the enzyme but the formation of a covalent bond
between a irreversible inhibitor and an enzyme is common.