Enzymes are protein catalysts that accelerate biochemical reactions without being consumed. They achieve high specificity and reaction rates by lowering the activation energy of reactions. Enzymes are classified based on the type of reaction they catalyze and identified by EC numbers. Many factors influence enzyme activity, including temperature, pH, and substrate concentration. Enzymes precisely bind substrates in their active sites to form enzyme-substrate complexes that stabilize transition states and yield products.

1. ENZYMES
DEPARTMENT OF ZOOLOGY

2. Enzymes
• Enzymes are proteins that act as biological catalysts
(biocatalysts).
• Catalysts accelerate chemical reactions.
• The molecules upon which enzymes may act are called
substrates, and the enzyme converts the substrates into
different molecules known as products.
• Almost all metabolic processes in the cell need enzyme catalysis
in order to occur at rates fast enough to sustain life.

3. Classification of enzymes and their nomenclature
• Enzymes are classified on the basis of reaction they catalyze.
• Usually enzymes are named by adding the suffix “ase” to the name of
their substrate or to a word describing their activity.
• Urease – Catalyzes the hydrolysis of urea.
• DNA Polymerase- Catalyzes the reaction of DNA
• Many enzymes have common names that provide little information
about the reactions that they catalyze.
• Trypsin: A proteolytic enzyme secreted by pancreas.

4. Rules for the Nomenclature and Classification of enzymes
• Rules for the Nomenclature and Classification of enzymes was prepared by The International
Union of Biochemistry (IUB) in 1964
• IUB is now IUBMB (International Union of Biochemistry and Molecular Biology)
• In the IUB system each enzyme has a name and a unique identification number.
• The systematic name of each enzyme has two parts
• The name of substrate(s) followed by a word ending in ase- specifying the type of reaction.
• Example: Alcohol Dehydrogenase
• The enzymes have a common name and The Unique Identification Number of the enzyme
called Enzyme Number (E. C Number).

5. IUB system of Nomenclature- Enzyme classification
• All enzymes were categorized in to seven major classes(Previously six) based on the type of the reaction they
catalyze.
• These groups were divided and further subdivided into so many categories.
• Based upon this classification a 4 digit Unique Identification Number(EC number is assigned to each enzyme as an
identification code.
• The Concept of EC Number: (IUB system)
1. Oxidoreductases
2. Transferases
3. Hydrolases
4. Lyases
5. Isomerases
6. Ligases
7. Translocases

6. 1.Oxidoreductases
Oxidoreductases Catalyzes oxidation and reduction
reactions(oxidoreductases).
Reaction involves transfer of protons or electrons between substrate.
• Eg. Alcohol Dehydrogenase, oxidase
•
• Oxidoreductase
• A red +B ox - ----------------------- A ox +B red

7. 2. Transferases
Transferases: Catalyze the reactions of a functional group from one substance to
another.
• groups usually transferred by these enzymes are - methyl, ethyl, amino or
phosphate etc.
• Ex. 1.Transaminase,
• 2. Nucleoside Monophosphate Kinase(NMP Kinase)
•
• Transferase
• A-B + C ----------------------- A + CB

8. 3.Hydrolases
Hydrolases
• They catalyze (breakdown) the hydrolysis reaction with water
(transfer of functional groups to water).
• Ex. Lipase,
• Amylase,
• Peptidase
•
• Hydrolases
• A-B + H2 O ----------------------- AH + BOH

9. 4. Lyases:
Lyases: Catalyze the addition groups to double bonds and formation of double
bonds by the removal of groups.
Ex. Fumarase,
Decarboxylase
Lyases
-----------------------
A = B A - B
<--------------------------

10. 5. Isomerases
Isomerases: They catalyze the transfer of groups within molecules to yield
isomeric forms (isomeric reactions)
• Catalyzes the intramolecular group transfer (within the molecule)
• Ex. Triose phosphate isomerase,
• Phosphoglucomutase
•
• Isomerase
• A - B – C ----------------------- A – C - B
• <--------------------------

11. 6. Ligases:
Ligases:
• Catalyze the condensation of two molecules(joining) with the expense
of energy from ATP hydrolysis.
• Catalyzes the formation of C-C, C-S, C-O and C-N bonds by the
condensation reaction coupled with ATP cleavage.
• Ex. Amino Acyl -tRNA Synthetase, DNA Ligase
•
•
• Ligases
• A + B + ATP ----------------------- A – B + ADP + Pi

12. 7. Translocases:
Translocases:
• This class is a newly added major class of IUBMB Enzyme
classification.
• Translocases catalyze the movement of ions or molecules
across membranes or their separation within membranes.
• Ex. ATP Synthase

13. Features of IUB Classification
• All enzymes have been classified into 7 major classes.
• Each major class is divided into sub classes.
• Each sub class is further divided into sub-sub classes
• Each enzyme has been assigned with a specific code number.
• This code number is called Enzyme Commission Number (EC Number)
• E. C number consists of 4 digits, separated by periods.
• Each digit in the E.C Number indicates a specific category in the
classification.

14. Example: Alcohol dehydrogenase(E C No. 1. 1. 1. 1. dehydrogenase )
• First digit indicates the Major Class
• The second digit indicates the sub class
• Third digit indicates the sub sub class.
• Fourth digit indicates the systematic specific name of the enzyme.
• The systematic specific name of the enzyme consists of two parts.
• The first part indicates the name of the substrate
• The second part indicates the nature of the reaction
• Ex. Alcohol Dehydrogenase(ADH)

15. E.C. Number indicates:
The Following - reaction in glycolysis (example)
• ATP + D Glucose---------- ADP + D- Glucose 6 Phosphate
• Enzyme responsible for this reaction is Hexokinase
• The name of Hexokinase according to IUMB is ATP Glucose Phosphotransferase.
• The name indicates the transfer of Phosphoryl group from ATP to Glucose.
• E. C. Number of Hexokinase is 2. 7. 1. 1.
• The first number (2) indicates the Class name Transferases
• Second number (7) the sub class Phosphotransferases
• Third number (1) a phosphotransferase with a hydroxyl group acceptor
• For any enzyme a trivial name is commonly in use.
• Here The enzyme name is Hexokinase.
Factors Affecting Enzyme Activity
The conditions of the reaction have a great impact on the
activity of the enzymes. Enzymes are particular about the
optimum conditions provided for the reactions such as
temperature, pH, alteration in substrate concentration, etc.

16. FACTORS AFFECTING ENZYME ACTIVITY
• Factors Affecting Enzyme Activity
• The conditions of the reaction have a great impact on the
activity of the enzymes. Enzymes are particular about the
optimum conditions provided for the reactions such as
temperature, pH, alteration in substrate concentration, etc.

17. Functions of Enzymes
• The enzymes perform a number of functions in our bodies. These include:
1.Enzymes help in signal transduction. The most common enzyme used in the
process includes protein kinase that catalyzes the phosphorylation of proteins.
2.They break down large molecules into smaller substances that can be easily
absorbed by the body.
3.They help in generating energy in the body. ATP synthase is the enzymes
involved in the synthesis of energy.
4.Enzymes are responsible for the movement of ions across the plasma
membrane.
5.Enzymes perform a number of biochemical reactions, including oxidation,
reduction, hydrolysis, etc. to eliminate the non-nutritive substances from the body.
6.They function to reorganize the internal structure of the cell to regulate cellular
activities.

18. TEMPERATURE
• enzyme activities are accelerated with increasing temperatures.
As enzymes are functional in cells, the feasible conditions for
nearly all enzymes are temperatures that are moderate.
• At higher temperatures, given a specific point, there is a drastic
decrease in the activity with the denaturation of enzymes.
• The International Union of Biochemistry suggests the standard
assay temperature to be 30 °C.
• human enzymes = 35°- 40°C
• body temp = 37°C

20. pH
• Enzymes require an optimum temperature and pH for their
action. The temperature or pH at which a compound shows its
maximum activity is called optimum temperature or optimum
pH, respectively.
• As mentioned earlier, enzymes are protein compounds.
• A temperature or pH more than optimum may alter the
molecular structure of the enzymes. Generally, an optimum pH
for enzymes is considered to be ranging between 5 and 7.
•

24. SUBSTRATE CONCENTRATION
• Enzymes have a saturation point, i.e., once all the enzymes
added are occupied by the substrate molecules, its activity will
be ceased.
• When the reaction begins, the velocity of enzyme action keeps
on increasing on further addition of substrate.
• However, at a saturation point where substrate molecules are
more in number than the free enzyme, the velocity remains the
same.

26. Enzyme-Substrate Interactions
• Enzymes are the biocatalysts with high molecular weight
proteinous compound
• The substrate which has an opposite charge of the enzyme fits
into these spaces, just like a key fits into a lock. This substrate
binding site is called the active site of an enzyme (E).
• The favourable model of enzyme-substrate interaction is called
the induced-fit model. This model states that the interaction
between substrate and enzyme is weak, and these weak
interactions induce conformational changes rapidly and
strengthen binding and bring catalytic sites close enough to
substrate bonds.

28. Action of enzymes
• Once substrate (S) binds to this active site, they form a complex
(intermediate-ES)
• which then produces the product (P) and the enzyme (E).
• The substrate which gets attached to the enzyme has a specific
structure and that can only fit in a particular enzyme. Hence, by
providing a surface for the substrate, an enzyme slows down the
activation energy of the reaction. The intermediate state where the
substrate binds to the enzyme is called the transition state. By
breaking and making the bonds, the substrate binds to the enzyme
(remains unchanged), which converts into the product and later splits
into product and enzyme. The free enzymes then bind to other
substrates and the catalytic cycle continues until the reaction
completes.

29. MECHANISM OF ENZYME REACTION
• The enzyme action basically happens in two steps:
• Step1: Combining of enzyme and the reactant/substrate.
• E+S → [ES]
• Step 2: Disintegration of the complex molecule to give the
product.
• [ES]→E+P
• Thus, the whole catalyst action of enzymes is summarized as:
• E + S → [ES] → [EP] → E + P

31. Enzymes: Properties and Mechanism of Enzyme Action
• Proteinous nature:
• Nearly all enzymes are proteins although some catalytically active RNA
molecules have been identified.
• In the protoplasm, enzymes exist as hydrophilic colloids. Due to colloidal
nature, they are isolated by dialysis.
• Substrate specificity:
• A given enzyme only catalyzes one reaction or a similar type of reaction.
• For example, maltase acts only on maltose while pancreatic lipase acts in a
variety of fats.
• Sometimes, different enzymes may act on the same substrate to produce
different end products.
• The substrate specificity of enzyme is based on amino acids sequence in the
catalytic site as well as the optical isomeric form of the substrate.

32. Catalytic properties:
• (1)Enzyme require in small concentration for any chemical change,
• (ii) They don’t initiate the catalysis but accelerate the rate of catalysis
by lowering the activation energy,
• (iii) They remain unchanged at the end of reaction,
• (iv) Their presence don’t alter the properties of end products,
• (v) Enzymes accelerate the forward or reverse reactions to attain the
equilibrium but don’t shift the equilibrium,
• (vi) Usually enzyme catalyzed reactions are reversible, but not
always,
• (vii) They require hydration for activity.

33. Turn over Number (Enzyme
efficiency):
• It is the number of substrate molecules changed per unit of time
per enzyme. Typical turn over number varies form 102 to
103 sec-1. For example the turn over number for sucrase is 104,
that means, one sucrase molecule convert 10,000 sucrose into
products. Similarly, it is 36 million for carbonic anhydrase
(fastest enzyme) and 5 million for catalase (2nd fastest enzymes).
Enzyme efficiency is very low in Lysozyme.

34. Sensitivity:
• Enzymes are highly sensitive to change in pH, temperature and
inhibitors. Enzymes work best at a narrow range of condition
called optimum.

35. Temperature
•
• The optimum temp of enzymes is 20-35°C. They become
inactivated at very low temperature and denatured (destroyed)
at very high temp i.e. greater than 45°C.
• Low molecular weight enzymes are comparatively more heat
stable.
• In archebacterium Pyrococcus furious, the optimum
temperature of hydrogenise is greater than 95°C. This heat-
stable enzyme enables Pyrococcus to grow at 100°C.

36. • pH:
• The optimum pH of most endoenzyme is pH 7.0 (neutral pH).
However, digestive enzymes can function at different pH. For
example, salivary amylase act best at pH 6.8, pepsin act best at
pH2 etc. Any fluctuation in pH from the optimum causes
ionization of R-groups of amino acids which decrease the
enzyme activity. Sometime a change in pH causes the reverse
reaction, e.g. at pH 7.0 phosphorylase break down starch into
glucose 1- phosphate while at pH5 the reverse reaction occurs.

37. Inhibitors:
• Enzymes are also sensitive to inhibitors. Inhibitors are any
molecules like cellular metabolites, drugs or toxins which
reduce or stop enzyme activity. Enzyme inhibitors are of 2 types
i.e. reversible and irreversible.

39. Irreversible Inhibitors
• Irreversible Inhibitors (=Inactivators) Covalently bind to amino
acid residue of catalytic site, and permanently inactivate the
enzyme,
• e.g., penicillin covalently attach to a serine residue in the active
site of the glycopeptide transpeptidase enzyme that forms cross-
linking in bacterial cell wall.
• Phenylmethylsulfonyl fluoride forms a covalent bond with the
catalytic site serine residue of proteases like trypsin,
chymotrypsin etc. and thus used during enzyme isolation.

40. inhibitors
• A competitive inhibitor competes with a substrate to bind
reversibly to catalytic site. The action of such type of inhibitor is
overcome by increasing substrate concentration, dilution or
dialysis.
• An uncompetitive inhibitor binds reversibly to the enzyme
substrate complex, but not the free enzyme
• A non-competitive inhibitor or mixed inhibitor binds to both
free enzyme and the enzyme- substrate complex.
• An allosteric or feedback inhibitor is the end product of a
metabolic pathway that inhibits the activity of the first enzyme
of that pathway