Enzymes are protein catalysts that accelerate chemical reactions in living organisms. They are highly specific and efficient, allowing reactions to proceed thousands of times faster than without enzymes. Enzymes have an active site that binds substrates and stabilizes transition states to lower the activation energy of reactions. They are classified based on the type of reactions they catalyze and many require non-protein cofactors to function. Enzyme activity can be regulated by cells to control metabolic pathways.

1. Enzymes
Lect-1
Introduction and classification

2. ENZYMES
• The enzymes speak : “We are
the catalysts of the living world
! Protein in nature, and in
action specific, rapid and
accurate; Huge in size but with
small active centres; Highly
exploited for disease diagnosis
in lab centres.”

3. Definition
• Enzymes are biocatalysts – the catalysts of life.
• A catalyst is defined as a substance that
increases the velocity or rate of a chemical
reaction without itself undergoing any change
in the overall process.

4. • Enzymes may be defined as biocatalysts synthesized by
living cells.
• They are protein in nature (exception – RNA acting as
ribozyme), colloidal and thermolabile in character, and
specific in their action.
• In the laboratory, hydrolysis of proteins by a strong
acid at 100°C takes at least a couple of days.
• The same protein is fully digested by the enzymes in
gastrointestinal tract at body temperature (37°C)
within a couple of hours.
• This remarkable difference in the chemical reactions
taking place in the living system is exclusively due to
enzymes. The very existence of life is unimaginable
without the presence of enzymes.

5. Category
• Enzymes are sometimes considered under two
broad categories :
• (a) Intracellular enzymes – They are functional
within cells where they are synthesized.
• (b) Extracellular enzymes – These enzymes are
active outside the cell; all the digestive
enzymes belong to this group.

6. Classification
• IUB system of enzyme classification divided into six
major classes.
1. Oxidoreductases : Enzymes involved in oxidation-
reduction reactions.
2. Transferases : Enzymes that catalyse the transfer of
functional groups.
3. Hydrolases : Enzymes that bring about hydrolysis of
various compounds.
4. Lyases : Enzymes specialised in the addition or removal
of water, ammonia, CO2 etc.
5. Isomerases : Enzymes involved in all the isomerization
reactions.
6. Ligases : Enzymes catalysing the synthetic reactions
(Greek : ligate—to bind) where two molecules are
joined together and ATP is used.

8. Nomenclature
Each enzyme is assigned two names.
• The first is its short, recommended name, convenient for everyday use.
• The second is the more complete systematic name.
A. Recommended name
• Most commonly used enzyme names have the suffix “-ase” attached to
the substrate of the reaction , for example, glucosidase and urease), or
to a description of the action performed for example, lactate
dehydrogenase and adenylyl cyclase).
B. Systematic name
• In the systematic naming system, enzymes are divided into six
major classes , each with numerous subgroups.
• For a given enzyme, the suffix -ase is attached to a fairly complete
• description of the chemical reaction catalyzed, including
• the names of all the substrates; for example, lactate:NAD+
oxidoreductase.

9. Properties of enzymes
• Enzymes are protein catalysts that increase the
velocity of a chemical reaction, and are not
consumed during the reaction.
• A. Active sites : Enzyme molecules contain a
special pocket or cleft called the active site.
• The active site contains amino acid side chains
that participate in substrate binding and
catalysis.
• The substrate binds the enzyme, forming an
enzyme–substrate (ES) complex.
• Binding is thought to cause a conformational
change in the enzyme (induced fit) that allows
catalysis. ES is converted to an enzyme–
product (EP) complex that subsequently
dissociates to enzyme and product.

10. B. Catalytic efficiency
• Enzyme-catalyzed reactions are highly efficient, proceeding
from
103–108 times faster than uncatalyzed reactions.
The number of molecules of substrate converted to
product per enzyme molecule per second is called the
turnover number, or kcat and typically is 102–104 s-1.
• C. Specificity
• Enzymes are highly specific, interacting with one or a few
substrates and catalyzing only one type of chemical
reaction.

11. D. Holoenzymes
• Some enzymes require molecules
other than proteins for enzymic
activity.
• The term holoenzyme refers to
the active enzyme with its
nonprotein component, whereas
the enzyme without its
nonprotein moiety is termed an
apoenzyme and is inactive.
• If the nonproteinmoiety is a metal
ion such as Zn2+ or Fe2+, it is
called a cofactor.
• If it is a small organic molecule, it
is termed a coenzyme.

12. • Coenzymes that only
transiently associate with the
enzyme are called cosubstrates.
• Cosubstrates dissociate from
the enzyme in an altered state
(NAD+ is an example).
• If the coenzyme is permanently
associated with the enzyme
and returned to its original
form, it is called a prosthetic
group (FAD is an example).
• Coenzymes frequently are
derived from vitamins. For
example, NAD+contains niacin
and FAD contains riboflavin

13. E. Regulation
• Enzyme activity can be regulated, that is, increased or
decreased,
• so that the rate of product formation responds to
cellular need.
F. Location within the cell
• Many enzymes are localized in specific organelles
within the cell
• Such compartmentalization serves to isolate the
reaction substrate or product from other competing
reactions.
• This provides a favorable environment for the reaction,
and organizes the thousands of enzymes present in the
cell into purposeful pathways.

14. HOW ENZYMES WORK ?
The mechanism of enzyme action can be viewed from two different
perspectives.
A. Energy changes occurring during the reaction
1. Virtually all chemical reactions have an energy ,called the free energy of
activation.
This free energy of activation is the energy difference between that of the
reactants and a high-energy intermediate that occurs during the formation
of product.
• Because of the high free energy of activation, the rates of uncatalyzed
chemical reactions are often slow.

15. 2. Rate of reaction: For molecules to react, they must contain
sufficient energy to overcome the energy barrier of the
transition state.
• In the absence of an enzyme, only a small proportion of a
population of molecules may possess enough energy to
achieve the transition state between reactant and product.
• In general, the lower the free energy of activation, the more
molecules have sufficient energy to pass through the transition
state, and, thus, the faster the rate of the reaction.

16. 3. An enzyme allows a reaction to proceed rapidly
under conditions prevailing in the cell by
providing an alternate reaction pathway with a
lower free energy of activation.
• The enzyme does not change the free energies
of the reactants or products and, therefore,
does not change the equilibrium of the
reaction.
• It does, however, accelerate the rate with
which equilibrium is reached.

17. • B. Chemistry of the active site-
• A number of factors are responsible for the catalytic efficiency of
enzymes, including the following:
• The active site is a complex molecular that facilitate conversion
of substrate to product.
• It acts as a flexible molecular template that binds the substrate
and initiates its conversion to the transition state .
• By stabilizing the transition state, the enzyme greatly increases
the concentration of the reactive intermediate that can be
converted to product and, thus, accelerates the reaction.