Enzymes are Biological Catalysts
Introduction
Catalysts are substances that increase the rates of chemical reactions. Life requires that many chemical
reactions occur within organisms. The human body employs over a thousand chemical reactions. Many
of these reactions would occur too slowly to be useful in the absence of a catalyst. Nature provides
humans and other biological organisms with proteins that are capable of catalyzing reactions.
Enzymes are protein, organic or biological catalysts that increase the velocity of a chemical reaction and
are not consumed during the reaction they catalyze. This process is called catalysis and enzymes thus
catalyze biochemical reactions. Scientists who specialize in studying enzymes are called enzymologists.
In enzymatic reactions, the molecules present at the beginning of the reaction (reactants) are called
substrates. Enzymes convert substrates into different molecules, called products. All processes in nature
require enzymes in order to occur at significant rates.
Enzyme kinetics is a fundamental way of describing, predicting and calculating how
enzymes bind substrates, turn these into products and also how fast and efficiently this is
happening.
Properties of Enzymes
1. Enzymes are Proteins: Most of the enzyme are simple or conjugated proteins. Some enzymes are
simple proteins. On hydrolysis they yield amino acids only. Eg. Amylase, Urease etc. Many enzymes
possess chemical group that are non-amino acid in nature, ie, the protein part of the enzyme is
conjugated with a non protein part. These are called conjugated proteins or holoenzymes.
Several types of groups are associated with enzymes (Figure ). These groups are related to the
structure and function of an enzyme.
Fig. 7.1: Schematic representation of an enzyme.
Apoenzyme: The protein part of an enzyme molecule is known as apoenzyme and is inactive.
Cofactor: The nonprotein part of an enzyme is called a cofactor. These cofactors are basically the
additional chemical groups, which appear in those enzymes that are conjugated protein molecules. Some
cofactors may be metal ions such as Mg2+, Zn2+ or complex organic molecules, such as nicotinamide
adenine dinucleotide. Whatever be the nature of a cofactor, both are required for enzyme activity
(participate in substrate binding or in catalysis).
The term holoenzyme refers to the active enzyme with its non-protein component, whereas the enzyme
without its non-protein moiety is termed an apoenzyme and is inactive.
, COURSE CODE/ TITLE: BCH 371/ INTRODUCTION TO ENZYMOLOGY
PROPERTIES AND CHARACTERISTICS OF ENZYMES BY C. B. LUKONG (PhD) @2019/2020
Holoenzyme: The combination of an apoenzyme and the cofactor is known as the holoenzyme and is
active:
Apoenzyme + Cofactor Holoenzyme
Coenzyme: When the cofactor of an enzyme is a nonprotein organic molecule, the cofactor is known as
a coenzyme. They are often derivatives of vitamins (for example, NAD, FAD, coenzyme A.
Prosthetic group: A cofactor, which is tightly bound to the apoenzyme, is generally called a prosthetic
group. They are either small organic molecules or inorganic metal ions. Examples include pyridoxal
phosphate, flavin mononucleotide (FMN), flavin adenine dinucleotide (FAD), thiamin pyrophosphate,
and biotin. Metal ions constitute the most common type of prosthetic group. The roughly one-third of all
enzymes that contain tightly bound Fe, Co, Cu, Mg, Mn, and Zn are termed metalloenzymes.
Activator: In case the cofactor of an enzyme molecule is a metal ion such as, Mg2+, Ca2+, K+, Na+ etc
and associate reversibly, the cofactor is known as an activator. Enzymes that require a reversibly metal
ion cofactor are termed metal-activated enzymes to distinguish them from the metalloenzymes for which
bound metal ions serve as prosthetic groups.
Active site: Enzyme molecules contain a special pocket or cleft called the active site where all the events
of the catalytic process occur. The active site contains amino acid side chains that create a three-
dimensional surface complementary to the substrate. The active site binds the substrate, forming an
enzyme-substrate (ES) complex. ES is converted to enzyme-product (EP), which subsequently
dissociates to enzyme and product
The active site consists of a few amino acid side chains, some known as binding groups and the other as
catalytic groups. The binding side chains can bind to different parts of the substrate by hydrogen bonds
and electrostatic or hydrophobic interactions. Thus, binding amino acids can either be polar or nonpolar.
However, the amino acid side chains of the catalytic groups are of polar type only, because they bring
about changes in the electronic structure of the substrate, which is a pre-stage to chemical
transformations.
2. They are Catalysts: In the presence of enzymes, certain chemical reactions proceed faster than in
their absence. An enzyme changes only the rate of the reaction, not the direction or equilibirium. Like all
catalysts, enzymes work by lowering only the activation energy for a reaction. This is
illustrated in Fig. 1.
Figure 1: The Lowering of the activation energy (ΔG‡) and the standard free-energy (ΔGo') in the
enzyme catalyzed and uncatalyzed reaction.
Catalysts, like enzymes, act by lowering the energy difference between the substrates (reactants) and the
transition state. This lowers the activation barrier for the reaction, allowing it to
proceed more rapidly.
Since lowering of the kinetic barrier also accelerates the reverse reaction, the equilibrium
of the reaction remains unchanged. As with all catalysts, enzymes are not consumed by the
2
Introduction
Catalysts are substances that increase the rates of chemical reactions. Life requires that many chemical
reactions occur within organisms. The human body employs over a thousand chemical reactions. Many
of these reactions would occur too slowly to be useful in the absence of a catalyst. Nature provides
humans and other biological organisms with proteins that are capable of catalyzing reactions.
Enzymes are protein, organic or biological catalysts that increase the velocity of a chemical reaction and
are not consumed during the reaction they catalyze. This process is called catalysis and enzymes thus
catalyze biochemical reactions. Scientists who specialize in studying enzymes are called enzymologists.
In enzymatic reactions, the molecules present at the beginning of the reaction (reactants) are called
substrates. Enzymes convert substrates into different molecules, called products. All processes in nature
require enzymes in order to occur at significant rates.
Enzyme kinetics is a fundamental way of describing, predicting and calculating how
enzymes bind substrates, turn these into products and also how fast and efficiently this is
happening.
Properties of Enzymes
1. Enzymes are Proteins: Most of the enzyme are simple or conjugated proteins. Some enzymes are
simple proteins. On hydrolysis they yield amino acids only. Eg. Amylase, Urease etc. Many enzymes
possess chemical group that are non-amino acid in nature, ie, the protein part of the enzyme is
conjugated with a non protein part. These are called conjugated proteins or holoenzymes.
Several types of groups are associated with enzymes (Figure ). These groups are related to the
structure and function of an enzyme.
Fig. 7.1: Schematic representation of an enzyme.
Apoenzyme: The protein part of an enzyme molecule is known as apoenzyme and is inactive.
Cofactor: The nonprotein part of an enzyme is called a cofactor. These cofactors are basically the
additional chemical groups, which appear in those enzymes that are conjugated protein molecules. Some
cofactors may be metal ions such as Mg2+, Zn2+ or complex organic molecules, such as nicotinamide
adenine dinucleotide. Whatever be the nature of a cofactor, both are required for enzyme activity
(participate in substrate binding or in catalysis).
The term holoenzyme refers to the active enzyme with its non-protein component, whereas the enzyme
without its non-protein moiety is termed an apoenzyme and is inactive.
, COURSE CODE/ TITLE: BCH 371/ INTRODUCTION TO ENZYMOLOGY
PROPERTIES AND CHARACTERISTICS OF ENZYMES BY C. B. LUKONG (PhD) @2019/2020
Holoenzyme: The combination of an apoenzyme and the cofactor is known as the holoenzyme and is
active:
Apoenzyme + Cofactor Holoenzyme
Coenzyme: When the cofactor of an enzyme is a nonprotein organic molecule, the cofactor is known as
a coenzyme. They are often derivatives of vitamins (for example, NAD, FAD, coenzyme A.
Prosthetic group: A cofactor, which is tightly bound to the apoenzyme, is generally called a prosthetic
group. They are either small organic molecules or inorganic metal ions. Examples include pyridoxal
phosphate, flavin mononucleotide (FMN), flavin adenine dinucleotide (FAD), thiamin pyrophosphate,
and biotin. Metal ions constitute the most common type of prosthetic group. The roughly one-third of all
enzymes that contain tightly bound Fe, Co, Cu, Mg, Mn, and Zn are termed metalloenzymes.
Activator: In case the cofactor of an enzyme molecule is a metal ion such as, Mg2+, Ca2+, K+, Na+ etc
and associate reversibly, the cofactor is known as an activator. Enzymes that require a reversibly metal
ion cofactor are termed metal-activated enzymes to distinguish them from the metalloenzymes for which
bound metal ions serve as prosthetic groups.
Active site: Enzyme molecules contain a special pocket or cleft called the active site where all the events
of the catalytic process occur. The active site contains amino acid side chains that create a three-
dimensional surface complementary to the substrate. The active site binds the substrate, forming an
enzyme-substrate (ES) complex. ES is converted to enzyme-product (EP), which subsequently
dissociates to enzyme and product
The active site consists of a few amino acid side chains, some known as binding groups and the other as
catalytic groups. The binding side chains can bind to different parts of the substrate by hydrogen bonds
and electrostatic or hydrophobic interactions. Thus, binding amino acids can either be polar or nonpolar.
However, the amino acid side chains of the catalytic groups are of polar type only, because they bring
about changes in the electronic structure of the substrate, which is a pre-stage to chemical
transformations.
2. They are Catalysts: In the presence of enzymes, certain chemical reactions proceed faster than in
their absence. An enzyme changes only the rate of the reaction, not the direction or equilibirium. Like all
catalysts, enzymes work by lowering only the activation energy for a reaction. This is
illustrated in Fig. 1.
Figure 1: The Lowering of the activation energy (ΔG‡) and the standard free-energy (ΔGo') in the
enzyme catalyzed and uncatalyzed reaction.
Catalysts, like enzymes, act by lowering the energy difference between the substrates (reactants) and the
transition state. This lowers the activation barrier for the reaction, allowing it to
proceed more rapidly.
Since lowering of the kinetic barrier also accelerates the reverse reaction, the equilibrium
of the reaction remains unchanged. As with all catalysts, enzymes are not consumed by the
2
