ENZYMES
by Mahwish Kazmi

An Introduction to enzymes....
• Biocatalysts - Substances that speed up a chemical
reaction without itself being chemically changed at the
end of the reaction.
• They mainly proteins, produced by living cells.
• Louis Pasteur called them as ferments and inseparable
from living cells.
• Eduard Buchner discovered that these ferments
continued to function after removal from cells.
• Frederick W. Kühne called these molecules enzymes.

An Introduction to enzymes....
• James Sumner provided a breakthrough in early enzyme
studies with isolation and crystallization of urease.
• Haldane made the remarkable suggestion that weak
bonding interactions between an enzyme and its
substrate might be used to catalyze a reaction.

Most Enzymes are Proteins
• All enzymes are protein except a small group of catalytic
RNA
• All metabolic and digestive enzymes are protein in
nature.
• They require some chemical groups for their proper
function called Prosthetic group which may be a
• Cofactor -- either one or more tightly bound inorganic
ions, such as Fe2+
, Mg2+
, Mn2+
, or Zn2+
• Coenzyme -- an easily removed complex organic or
metalloorganic molecule, e.g. many vitamins

• A complete, catalytically active enzyme together with its
bound coenzyme and/or metal ions is called a
holoenzyme.
• The protein part of an enzyme is called the apoenzyme
or apoprotein.
Human pancreatic amylase

General characteristics of enzymes
1. Biological catalysts (activity can be regulated),
2. Efficient, specific, stereospecific,
3. Speeds up the rate of a reaction (activation energy) but
does not change the equilibrium,
4. Speeds up both forward and reverse reactions,
5. Product purity is 100% (due to reaction specificity - lack
of formation of wasteful by-products),
6. Activity is affected by temperature, pH,
7. Inhibited by inhibitors.

Enzyme Classification
• International agreement of biochemists has adopted a
system for naming and classifying enzymes.
• This system divides enzymes into six classes, each with
subclasses, based on the type of reaction catalyzed.

Enzyme Nomenclature
• Each enzyme is assigned a four-part classification
number and a systematic name, which identifies the
reaction it catalyzes.
e.g., ATP:Glucose phosphotransferase catalyzes the
transfer of a phosphoryl group from ATP to glucose. Its
Enzyme Commission number (E.C. number) is 2.7.1.1.

Enzyme Nomenclature
E.C 2.7.1.1
2 denotes class name (transferase)
7 subclass (phosphotransferase)
1 a phosphotransferase with a hydroxyl group as
acceptor;
1 D-glucose as the phosphoryl group acceptor.

How Enzymes work...
• Substrate -- the compound on which an enzyme acts.
• Active Site -- Enzyme's specific region where substrate
binds and catalysis occurs. Amino acid's R groups play
effective role here.
• When a substrate binds to an enzyme's active site, an
Enzyme-Substrate (ES) complex is formed.
• Activation Energy -- an initial input of energy to start
reaction.
• During this part of the reaction the molecules are said to
be in a transition state.

Reaction Coordinate
free
energy
R
P

What is the source of the energy for lowering
activation energy?
1. Much of the energy required to lower activation energies is
derived from weak, noncovalent interactions between
substrate and enzyme's active site.
• The energy derived from enzyme-substrate interaction is
called binding energy.
• Binding energy is a major source of free energy used by
enzymes to lower the activation energies of reactions.
• The binding energy can be used to cause a conformational
change in the enzyme (induced fit). Binding energy also
accounts for the exquisite specificity of enzymes for their
substrates.

2. Rearrangement of covalent interactions between enzymes
and substrates lower the activation energy (and thereby
accelerate the reaction) by providing an alternative, lower-
energy reaction path.
• Chemical reactions of many types take place between
substrates and enzymes's functional groups (specific amino
acid side chains, metal ions, and coenzymes).

Mechanism of Enzyme Action
1. Lock and Key Model
• Propounded by Emil Fisher,
• Enzymes are structurally complementary to their
substrates, so that they fit together
• Like a key fits into a lock very precisely.
• Temporary structure called the enzyme-substrate
complex formed.
• Products have a different shape from the substrate.
• Once formed, they are released from the active site.
• Leaving it free to become attached to another substrate.

• Lock and Key Model
Enzyme may
be used again
Enzyme-
substrate
complex
E
S
P
E
E
P
Reaction coordinate

• Some proteins can change their shape (conformation) so
the active site is flexible, not rigid.
• When a substrate combines with an enzyme, it induces a
change in the enzyme’s conformation.
• The active site is then moulded into a precise
conformation.
• The shapes of enzyme, active site and substrate adjust
to maximum fit which improves catalysis.
• Making the chemical environment suitable for the
reaction.
• The bonds of the substrate are stretched to make the
reaction easier (lowers activation energy).
Induced-fit Model

Induced-fit Model

Modern notion of enzyme catalysis
• Michael Polanyi (1921), Haldane (1930), Linus Pauling
(1946) elaborated this concept.
• An enzyme must be complementary to the reaction
transition state. This means that optimal interactions
between substrate and enzyme occur only in the
transition state.

• An enzyme must provide functional groups for ionic,
hydrogen-bond, and other interactions, and also must
precisely position these groups so that binding energy is
optimized in the transition state.
• Adequate binding is accomplished most readily by positioning
a substrate in the active site where it is effectively removed
from water.
• The size of proteins reflects the need for superstructure to
keep interacting groups properly positioned and to keep the
cavity from collapsing.