3.1.3. Coenzymes, Prosthetic group, Effectors

Sometimes the surface cavity does not act as a catalytic site until it is modified by a second incoming molecule. These participants known as the coenzymes are non-peptide molecules capable of completing the binding site for the transition state. Other molecules that do the similar function are prothetic group, and effectors.

Coenzyme. Coenzyme is a non-peptide molecule capable of completing the binding site for the transition state. Examples include many vitamin derivative such as coenzyme A, thiamine, pyrophosphate, vitamin B12

Prosthetic Group. Prosthetic group is the same as the coenzyme but are tightly bound to the enzyme. When they are split off, the enzyme is mostly denatured. Examples include flavin nucleotides and heme.

Effectors. Effectors accelerate (activators) or block (inhibitors) enzyme reaction. Examples of activators include Mg++, Ca++, Zn++, K+, and Na+, while the examples for the inhibitors include Hg, and substrate analogs.Table 3.1. lists functions of some of the important coenzymes and prostshetic groups.

Table 3.1. Function of some important coenzymes and prosthetic groups.

Compound

Function

(a) Oxidoreduction

Nicotinamide adenine dinucleotide (NAD)

hydrogen transfer

Nicotinamide adenine dinucleotide phosphate (NADP)

hydrogen transfer

Flavin monocucleotide

hydrogen transfer

Flavin adenine dinucleotide (FAD)

electron transfer

Heme (cytochromes)

Ferredoxins

electron transfer

(b) Group Transfer

Pyridoxalphosphate

transamination, decarboxylation

Adenosine triphosphate

phosphate group donor

Tetrahydrofolic acid

C1 group transfer

Biotin

carboxylation, decarboxylation

Coenzyme A

transacylation

Thiamine pyrophosphate

(Vitamin B1)

C2-group transfer

Riboflavin

hydrogen transfer

5'-deoxyadenosyl-cobalamine

transfer of methyl group, isomerization