Enzyme | types of Enzymes

Enzyme

Enzymes are Complex macromolecules with high molecular weight.

All enzymes are proteins but all proteins are not enzymes.(except ribozyme, which is nucleic acid (RNA) that behave like enzyme.

Enzymes do not start a reaction however they help in accelerating it.

The catalyse biochemical reaction in a Cell and affect the rate of biochemical reaction not the direction.

enzymes help in The breakdown of large molecules into smaller molecules or bring together to smaller molecules to form a larger molecule hence are also called biocatalyst.

Enzymes are specific in action.

Enzymatic activity decreases with increase in temperature.

They show maximum activity at optimum pH of 6 to 8.

inorganic catalyst work at high temperature and pressure but enzymes get damaged at high temperature. (>40°c)

*Thermophilic organisms have enzymes which are stable at high temperature. (Up to 80-90°c)

carbonic anhydrase is the fastest enzyme it accelerates the following reaction 10 million times.

in the absence of enzymes only 200 molecules of H2CO3 are formed in an hour.

In the presence of carbonic anhydrase about 600,000 molecules are formed per second.

Most of the enzymes have high turnover number.

Turnover number-

turnover number of an enzyme is the number of molecules of a substance that is acted upon by an enzyme per minute.

High turnover number of enzymes increase the efficiency of reaction.

the velocity of enzyme increases with increase in substrate concentration and then ultimately reaches maximum velocity.

Active site-

like all proteins, enzymes have Primary , secondary and tertiary structure.

the tertiary structure of an enzyme has some pockets that called active  site , into which the substrate fits.

Process of enzyme action-

substrate(S) is the chemical that is converted into product by the action of an enzyme(E).

First of all the Substrate "S" bindes to the enzyme "E"at its active site. This leads to formation of Enzyme--Substrate "E-S" complex.

transition state structure is the new structure of the substrate being formed during the state  when the substrate is bound to the enzyme. It is the transition structure between the substrate and the product.

finally the structure of substrate gets converted into the structure of the product, and the product is released from the active site.

transition state is the state of higher energy and lesser stability as compared to the product.

the difference in average energy content of 'S' from its transition state is called activation energy.

Catalytic cycle of enzyme

First of all, the substrate binds to the active site of enzyme(E+S).

this induces some change in enzyme so that the substrate tightly bound with active site of enzyme(ES).

The active site breaks chemical bonds of the substrate(EP).

The enzyme releases the products and the free ennzyme is ready to bind to other molecules of the substrate (E+P).
  
     {   E + S <-------> ES ------> EP -----> E + P   }

The pathway of this transformation must go through the so called transition state structure.

Classification and nomenclature of enzymes

Enzymes are classified into 6 classes based on the reaction they catalyse.

(1) Oxido-reductase / dehydrogenases:
these enzymes catalyse the oxidation or reduction between two substrates, S and S' .

S reduced + s' oxydized -----> S oxydized + s' reduced

E.g. cytochrome oxidase, succinate dehydrogenase, nitrate reductase.

(2) Transferases: 
these enzymes catalyse the transfer of a group (other than hydrogen) from one substrate to another.

                S-G + S' -----> S'-G + S

E.g. glutamate-pyruvate transaminase transfers amino group from glutamate to pyruvate during synthesis of alanine.

(3) Hydrolases:
these enzymes catalyse The breakdown of large molecule into smaller molecules with the addition of Water. They catalyse hydrolysis of bonds like ester, ether, peptide, glycosidic, C-C, C-halide, P-N etc.
E.g. Amylase hydeohydro starch, sucrase, lactase etc.

(4) Lyases: 
these enzymes catalyst removal of groups by mechanism other than hydrolysis living double bonds.
            X-C-C-Y ------> X-Y + C=C
E.g. histidine decarboxylase breaks histidine to histamine and CO2.

(5) Isomerases: 
these enzymes catalyse the arrangement of atoms in a molecule to form its isomer.
There are three types of isomerases:
a. Isomerases:
Aldose to ketose group or vice -versa.
E.g.  Glucose-6-Phosphate to Fructose-6-Phosphate.

b. Epimerases:
 change in position of one constituent or Carbon group.
E.g. xylulose phosphate to ribulose phosphate.

c. Mutase:
shifting the position of side group.
E.g. glucose 6 phosphate to glucose 1 phosphate

(6) Ligases:
these enzymes catalyse the covalent bonding between two substance to form larger molecules.
E.g. enzyme catalysing joining of C-O, C-S, C-N, P-O etc.
E.g. pyruvate carboxylase. It combines pyruvic acid with CO2 to form oxaloacetic acid.