ENZYMES - INTRODUCTION - PART I
(Introduction
- Historical aspects - nomenclature)
Q-1
What is the importance of enzymes in living
systems?
A-1 Enzymes
are the reaction catalysts of the biological systems.
They
have extraordinary catalytic power, often greater than that of synthetic
catalysts. They have high degree of substrate specificity & accelerate
specific chemical reactions. They function in an aqueous medium under very mild
conditions of temperature& pH. Very few non-biological catalysts show all
these properties.
Q-2 If
enzymes are not present in body what will happen?
A-2 The
biochemical reactions taking place in living cells at body temperature at
sufficiently rapid in a regular order. Such reactions would have been extremely
slow, had they not been catalyzed by enzymes, which are present in every living
cell and can also act independently of the cell. (e.g. hydrolysis of starch by
salivary amylase).
Q-3
Were enzyme actions were known in earlier
time?
A-3
Yes, Biological catalysis was first recognized
& described in the early 1800 in studies of digestion of meat by
secretions of stomach & conversion of starch into sugar by saliva &
various plant extracts. In 1850 Louise Pasteur concluded that fermentation
of sugar into alcohol by yeast is catalyzed by” FERMENTS “& later named
than enzymes, are inseparable from living yeast cells, a view that prevailed for
many years.
Q-4
What was Buchner’s important discovery
regarding enzyme? (Or fermentation?)
A-4
In 1897 Buchner showed yeast extract (cell
free) could ferment sugar to alcohol. This encouraged biochemists to attempt the
isolation of many different enzymes & to examine their catalytic properties
Q-5
What was the important discovery made by
James Sumner?
A-5 In 1926 James Sumner crystallized
UREASE& found it is as a protein and postulated that all enzymes are
proteins. This idea remained controversial for some time.
Q-6
What was contribution of J.B.S.HALDANE
regarding enzyme?
A-6
During this period (1930) J.B.S.HALDANE wrote a
treatise entitled enzymes, even though molecular nature of enzymes was not fully
yet appreciated. It contained remarkable suggestions of weak bonding
interactions and substrate catalyzed reaction.
Q-7
What is an enzyme?
A-7
Enzymes are the reaction catalysts of the
biological systems. They are protein in nature (With exception of small group of
catalytic RNA molecules). Molecular weight ranges from 12000 to over million. They are
specific in action e.g. Urease Their catalytic activity depends upon the
integrity of their native protein conformation. (If enzyme is denatured or
dissociated into subunits catalytically activity is usually lost)
The
enzymes carry out transformation of molecules and also mediate transformation of
energy e.g. PHOTOSYNTHESIS
Q-8
What are simple and complex enzymes?
A-8
Some enzymes require no chemical groups other
than amino acids for activity, Enzymes composed of only protein are known as
simple enzymes. Complex enzyme composed of protein plus a relatively small
organic molecule, (which is required for enzyme activity)
Q-9
What is apo-enzyme?
A-9 Apo-enzyme
is the protein part of an enzyme without any cofactors or prosthetic group that
may be required for the enzyme to be functional. The apo-enzyme is catalytically
inactive.
Q-10
What is
coenzyme or prosthetic group?
A-10
A non-protein component of an enzyme, which is
required for catalytic activity, is known as co-enzyme or prosthetic group.
Q-11
What is co-factor?
A-11
Is small organic or inorganic molecules that an
apo-enzyme requires for its activity. E.g. Lysine oxidase copper is loosely
bound
Q-12
What is the difference between prosthetic
group and co-enzyme?
A-12.
A coenzyme or metal ion that is COVALENTLY bound to
the enzyme protein is called prosthetic group. For e.g. in the cytochromes, the
heme prosthetic group is very tightly bound and requires strong acids to
dissociate from its apo enzyme.
Q-13
Do some Enzymes contain vitamin derivatives?
A-12
YES, Many
prosthetic groups and coenzymes are water-soluble derivatives of vitamins. It
should be noted that the main clinical symptoms of dietary vitamin insufficiency
generally arise from the malfunction of enzymes, which lack sufficient cofactors
derived from vitamins to maintain homeostasis.
Q-13
What is a holo enzyme?
A-13 A complete catalytically active
enzyme together with its coenzyme and/or metal ions called a holoenzyme.
Apo-enzyme
+ co enzyme ==holoenzyme
Q-14
What is metalloenzyme and metal activated
enzymes?
A-14
Enzymes require
a metal in their composition (such as Fe+2,Mg2+.
Mn2+,Zn2+ )
are known as metalloenzymes if they bind and retain their metal atom(s) under
all conditions, that is with very high affinity. Those, which have a lower
affinity for metal ion, but still require the metal ion for activity, are known
as metal-activated enzymes.
Nomenclature
Q-15
How enzymes are named?
A-15
Many enzymes have been named by adding the
suffix “-ase” to the name of their substrate or to a word or phrase
describing their activity. e.g. UREASE catalyze hydrolysis of urea, MALTASE act on
maltose, and DNA polymerase catalyze the synthesis of DNA.
Other enzymes such as PEPSIN and TRYPSIN have names they do not denote
their substrates.
Q-16
What is IUB system?
A-16
Sometimes the enzyme may have two or more
names, or two different enzymes have the same name. Because of such ambiguities,
and ever increasing number of
newly discovered enzymes a system for naming and classifying enzymes has been
adopted by inter national agreement, International Union of Biochemistry (IUB-system)
Q-17
How enzymes are classified according to IUB
system?
A-17
According to IUB system enzymes are grouped in
six major classes.
Each
with sub classes based on the type of reaction catalyzed.
Q-18
According to IUB system which enzymes are
included in group 1?
A-18
OXIDO-REDUCTASE
(EC-1)
Enzymes
of this group add or remove hydrogen atoms during the catalysis. They include
dehyrogenases & oxidases, and are mostly concerned with biological
oxidation.
Dehydrogenases
removes H+ from substrate in the presence of H+ acceptor group. e.g. Lactate
Dehydrogenase
AH2
+B à
A + BH2
OXIDASES
– transfer two electrons from the donor to oxygen resulting usually in
hydrogen peroxide formation (H2O2) e.g. Glucose oxidase.
CYTOCHROME oxidase produces H2O rather than H2O2.
Q-19
According
to IUB system which enzymes are included in group 2?
A-19
TRANSFERASES
(EC-2)
These
enzymes transfer functional groups between donors and acceptors.
The
AMINO, ACYL, PHOSPHATE, ONE CARBON and GLYCOSYL are the major groups that are
transferred. E.g.
A— X+ B
à
A+ B—X
A.
METHYL group---àe.g. Transmethylase
B. ALDEHYDE or KETONIC group e.g. Transaldolase or transketolase.
C.
ACYL GROUP e.g.Aceyltransferase
D.
SUGAR GROUP e.g.
Glucosyltransferase
E. AMINO-KETO GROUP- Aminotransferase or transaminases
F. KINASES are specialized trnsferases that regulate metabolism by transferring phosphate from ATP to other moleculs e.g. Hexokinase:
ATP
+Glucose -----àG-6-P+
ADP
Q-20
According
to IUB system which enzymes are included in group 3?
A-20
HYDROLASES
(EC-3)
A
special class of transferases in which the donor group is transferred to water.
The generalized reaction involves the hydrolytic cleavage of C-O, C-N, O-P
and C-S bonds. In other words enzymes, which add, water to the substrate and
hydrolyze or decompose it to give products.
A—B + H2O à
AH+
BOH
1.
LIPASES-------e.g. Glycerol ester hydrolase
2.
PHOSPHATASES------e.g.Glucose-6-Phosphatase
3.
CHOLINE ESTERASE hydrolysases acetylcholine
4.
PEPTIDASES-----hydrolyses peptides
5.
NUCLEASES e.g.nucleotidase,
nucleosidase
6.
CARBOHYDRASES e.g. Amylase act on amylose Lactase, Maltase
7.
Enzymes acting on C—N linkage Urease converts urea into ammonia, Asparginase.
Glutaminase, Arginase
Q-21
According to IUB system which enzymes are
included in group 4?
A-21
LYASES
(EC4)
Lyases
add or remove water, ammonia, or carbon dioxide from the substrates.
A-B
+ X-Y
---- > A-X + B-Y
1.
DECARBOXYLASE removes CO2 from a
orb
keto acids or aminoacids.
2.
Carbonic anhydrase
3.
Cysteine desulfurase
Q-22
According
to IUB system which enzymes are included in group5?
A-22
ISOMERASES
(EC5)
A
hetrogeneous group of enzymes catalyze transfer of groups within molecules to
yield isomeric forms e.g. isomerazition of!
1) Optical isomers 2) geometrical isomers
A-----à
A’
1.
Epimerases or Racemases catalyze inversion at asymmetric carbon atoms
2.
MUTASES involve intramolecular transfer of a group such as a phosporyl
3.
CIS-TRANS ISOMERASE e.g. all trans retinene isomerase
Q-23
According to IUB system which enzymes are included in group 6?
A-23
LIGASES (EC6)
TO
liagate means to bind, Formation of C-C, C-S, C-O& C-N bonds by condensation
reactions. These enzymes carry out synthetic reactions where two molecules
joined at the utilization of a “high energy phosphate bond of ATP. ”
REACTION: A+B---à
A-B
1.
Pyruvate carboxylase Pyruvate+CO2+ATP-----àOxaloacetate+
ADP+Pi
2.
GLUTAMINE SYNTHETASE
3.
Acetyl Co A CARBOXYLASE
Q-24
The
following enzyme is classified as EC –2.7.1.1.
What does this mean?
A-24
The
name of the enzyme catalyzing following reaction is ATP:glucose phospo
transferase. Its enzyme classification number (E.C.number) is
2.7.1.1
ATP+ D-Glucose---àD-Glucose-6-phosphate+
ADP
The
first digit (2) denotes the class number (transferase)
The
second digit (7) denotes sub-class - phospo transferase;
The
third digit (1)-Hydroxyl group as
an acceptor;
The
fourth digit (1) D-glucose as the phosphate group acceptor.
When
the systematic name of an enzyme is long or cumbersome, a trivial name may be
used in this case Hexokinase.