make 100 choose questions on this text with answers Lecture No. 1

Enzymology

Dr. Obeid Shanab.
Assistant professor of Biochemistry, Clinical Biochemistry and
Cancer Biology

ïƒ˜

Definition: Enzymes are proteins that act as catalysts

for chemical reactions in the body.

ïƒ˜

Enzyme-catalyzed reactions have three basic steps:

1) Binding of substrate (reactant) : E + S â†’ ES
2) Conversion of bound substrate to bound product:ESâ†”EP
3) Release of product: EPâ†”E + P.

ïƒ˜ Enzymes increase the speed of reactions and regulate the
rate of metabolic pathways in the body (by a factor of 1011
or higher).

ïƒ˜ If the enzymes are modified during the reaction, they
return to their original form at the end.

ïƒ˜ Enzyme Components:

â€¢ Enzymes are chemically classified into:
1) Simple protein enzymes (consist only of protein as pepsin)

2) Conjugated

protein

enzymes

(Holozymes

or

Holoenzymes): which consist of :

a) Protein part (called apoenzyme).

b) Non-protein part which is either:
i. Loosely attached to apoenzyme (non-covalently) and
called Coenzyme or Cofactor.

ii. Firmly attached to apoenzyme (covalently) and called
Prosthetic group.

ïƒ˜ Basic Reactions and Classes of Enzymes

â€¢ The Enzyme Commission has divided the basic reaction
types and the enzymes catalyzing them into six broad

numbered classes:

1) Oxidoreductases.
2) Transferases.
3) Hydrolases.
4) Lyases.

5) Isomerases
6) Ligases.

1) Oxidoreductases:

â€¢ It catalyzes oxidation-reduction reactions which are very
common in biochemical pathways.

â€¢ In this type of reaction, at least one substrate gains
electrons and becomes reduced, and another substrate
loses electrons and becomes oxidized.

â€¢ If the reactions include acceptance and donation of
hydride ions (H) or hydrogen atoms, the enzyme is called

dehydrogenases

which

usually

need

an

transferring coenzyme, such as NAD+/NADH.
Hydrogen carrier

electron-

â€¢ If the reactions include acceptance and donation of oxygen
atoms, the enzymes are called:

a) Hydroxylases and oxidases: when one oxygen atom is
incorporated into a substrate and the other oxygen atom
into water, or both atoms are incorporated into water.

b) Oxygenases: when both atoms of oxygen are incorporated
into the acceptor.

2) Transferases.
ïƒ˜ They catalyze the transfer of a functional group from one
molecule to another, e.g.:

â€¢ If the transferred group is a high-energy phosphate, the
enzyme is a kinase.

â€¢ If it is a fatty acyl group, the enzyme is an acyltransferase.
â€¢ If it is amino group, the enzymes are called transaminases
or aminotransferases (PLP is a coenzyme).

3) Hydrolases:
â€¢ It catalyzes hydrolyses of C-O, C-N or C-S bonds by the
addition of H2O in the form of OH- and H+ to the atoms

forming the bond.

4) Lyases.
â€¢ It catalyzes hydrolyses of C-C, C-O and C-N bonds by
means

other

than

hydrolysis

or

oxidation,

e.g.,

decarboxylases (when carbon dioxide is released from a

substrate).

5) Isomerases:
â€¢ They rearrange the bond structure of a compound.
â€¢ If they catalyze movement of a phosphate from one atom
to another, the Enzymes are called mutases.

6) Ligases:

â€¢ They synthesize C-C, C-S, C-O, and C-N bonds in
reactions, e.g., Carboxylases which add CO2 to another
compound in a reaction requiring ATP cleavage to provide
energy and biotin as a coenzyme.
vit b2

ï¶ Properties of enzymes

ïƒ˜ Functional sites of enzyme system
I. Catalytic (active site):

â€¢

It is the site on the enzyme surface that catalyzes the
chemical reaction i.e. carries out the chemical reaction.

II. Substrate binding site:

â€¢

It is the site on the enzyme surface at which the substrate
specifically binds.

o

Both sites:
â€“

Either separated by large or small distance or
combined into one site.

â€¢

They are also either:

a) Rigid model "Lock & key model": No changes in their
shape after binding of the substrate.

b) Flexible model: substrate binding induce conformational
changes in their shapes to induce proper fitting of the
enzyme-substrate complex.

III.Allosteric site:

â€¢ It is site on the enzyme surface other than the catalytic and
substrate binding site to which allosteric effector binds.

â€¢ Allosteric effector is a low Mwt
substance with no or little
structural similarities to the
substrate that bind at sites other
than the active catalytic site and
regulate the enzyme through
conformational

changes

affecting the catalytic site

S

â€¢

Allosteric effector could be:

1) Allosteric activator: lead to increase on the rate of
enzyme

reaction,

e.g.,

glucose-6-phosphate

is

an

allosteric activator to glycogen synthase.

2) Allosteric inhibitor: lead to decrease in rate of enzyme,
e.g., glucose-6-phosphate is an allosteric inhibitor of

hexokinase.

ïƒ˜

Enzyme Specificity (5 types):

â€¢

Enzymes show different degrees of specificity:

I.

Relative, low or bond specificity:

â€¢

In this type the enzyme acts on substrates that are
similar in structure and contain the same type of bonds,

e.g.

a)

Lipase: that hydrolyzes ester bonds in different TGs.

b) Protease: that catalyzes the hydrolysis of peptide bonds
of different proteins.

II. Moderate, structural or group specificity:
â€¢

In this type, the enzyme is specific not only to the type
of bond but also to the surrounding structure (chemical

groups or atoms) e.g.

a) Pepsin: is an endopeptidase that hydrolyzes central
peptide bonds in which the amino group belongs to
aromatic aa.

b) Trypsin: is an endopeptidase that hydrolyzes central
peptide bonds in which the amino group belongs to
basic aa.

III. Absolute, high or substrate specificity:

â€¢

In this type of specificity the enzyme acts only on one
substrate e.g.

a)

Uricase: acts only on uric acid.

b) Arginase: acts only on arginine.
c)

Carbonic anhydrase: acts only on carbonic acid.

d) Lactase: acts only on lactose.
e)

Sucrase: acts only on sucrose.

f)

Maltase: acts only on maltose.

IV. Optical or Streo-specificity:
â€¢

The enzyme is specific not only to the substrate but also
to it's optical configuration (act only on specific isomer):

a)

Î±-glycosidase acts only on Î±-glycosidic bonds, which are
present in starch, dextrin and glycogen.

b) Î²-glycosidase acts only on Î²- glycosidic bonds, which
are present in cellulose.

o

N.B: We can digest glycogen and starch due to presence
of Î±-glycosidase but we can't digest cellulose due to
absence of Î²-glycosidase.

V. Dual specificity:
â€¢

There are two types of dual specificity:

a)

The enzyme may acts on two substrate by one reaction

type e.g. xanthine oxidase enzyme acts on xanthine
and hypoxanthine (two substrates) by oxidation (one
reaction type):

b) The enzyme may acts on one substrate by two
different reaction types e.g. isocitrate dehydrogenase
enzyme acts on isocitrate (one substrate) by

oxidation followed by decarboxylation (two different
reaction types):