The enzymes
Enzymes are another important group of macromolecules synthesized by living cells.
They are catalysts of biological systems(hence are called as biocatalysts), colloidal,
thermo labile, and protein in nature.
They also mediate the transformation of different forms of energy.
The striking characteristics of enzymes are their catalytic power and specificity.
Actions of most enzymes are under strict regulation in variety of ways.
Substances on which enzymes act to convert them into products are called
substrates.
Because of its catalytic power(activity), enzymes accelerate reactions at least a
million times, by reducing the energy of activation.
Before a chemical reaction can occur, the reacting molecules are required to gain
minimum amount of energy, this is called the energy of activation. It can be
decreased by increasing the temperature of the reaction medium. But in human
body which maintains a normal body temperature fairly constant, it is achieved by
enzymes.
All the enzymes are protein in nature with large mol.wt. Few enzymes are simple
proteins while some are conjugated proteins.
Certain enzymes with only one polypeptide chain in their structure are called as
monomeric enzymes, e.g.ribonuclease.
Several enzymes possess more than one polypeptide chain and are called as
oligomeric enzymes, e.g. lactate dehydrogenase, hexokinase, etc. Each single
polypeptide chain of oligomeric enzymes is called as subunit.
When many different enzyme catalyzing reaction sites are located at different sites
of the same macromolecule, it is called multienzyme complex, e.g. fatty acid
synthetase, carbamoyl phosphate synthetase ||, pyruvate dehydrogenase,
prostaglandin synthes, etc.
Co-enzyme:
Since the involvement of co-enzyme in a given reaction on substrate is so intimate
that co-enzyme is often called a s co-substrate or second substrate.
Many co-enzymes are derived as the physiologically active forms from the
constituents of vitamin B-complex viz, Pantothenic acid: CoASH, vitamin B12:
Cobamide, Folic acid: Tetrahydrofolate (FH4), Niacin: NAD
+,
NADP, Riboflavin: FMN,
FAD, Pyridoxin: Pyridoxal phosphate, Thiamine: TPP.
It is generally observed that reactions involving oxidoreduction, group transfers,
isomerization and covalent bond formation require coenzyme.
Classification of Co-enzymes:
Coenzymes can be classified according to the group whose transfer they
facilitate. Based on this concept we may classify coenzymes as follows:
A. For transfer of groups other than hydrogen:
Sugar phosphates, CoASH, Thiamine pyrophosphate (TPP), Pyridoxal
phosphate, Folate coenzymes, Biotin, Cobamide coenzyme, Lipoic acid.
B. For transfer of hydrogen:
- NAD
+
, NADP
+
- FMN, FAD
- Lipoic acid
- Coenzym Q
In addition:
- Heme act as co-enzyme in cytochromes, peroxidases and PG synthase
complex.
- Many co-enzymes contain adenine, ribose and phosphate and are
derivatives of adenosine monophosphate (AMP) such as NAD, FAD.
Role of Metal Ions in Enzymes:
The activity of many enzymes dependes on the presence of certain metal ions such
as K
+
, Mg
++
, Ca
++
, Zn
++
, Cu
++
.
A- Metal activated enzyme:
In certain enzymes the metals form a loose and easily dissociable complex.
Such enzymes are called metal-activated enzymes.
The metal ions can be removed by dialysis or any other such method from
the enzyme without causing any denaturation of apo-enzyme.
B- Metallo-enzymes:
In this case metal ion is bound tightly to the enzyme and is not dissociated
even after several extensive steps of purification.
Metals play variety of roles such as:
1. They help in either maintaining or producing (or both), active structural
conformation of the enzyme.
2. Formation of enzyme-substrate complex.
3. Making structural changes in substrate molecule
4. Accept or donate electrons.
5. Activating or functioning as nuclophiles, and
6. Formation of ternary complexes with enzyme or substrate.
