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The following points highlight the two main groups of coenzymes. The groups are: 1. Specific Coenzymes 2. General Activators.
Group # 1. Specific Coenzymes:
These are usually organic compounds of complicated structure often acting as carriers of some chemical group. Many coenzymes are either vitamins or derivatives of vitamins.
Depending upon the nature of chemical group which they carry, the coenzymes are classified as follows:
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(a) Hydrogen Carriers (Oxidation-Reduction Reactions):
(i) Coenzyme I (Co I) or NAD+ (Nicotinamide Adenine Dinucleotide):
Its reduced form is NADH + H+
Previously known as DPN (Diphospho Pyridine Dinucleotide) contains vitamin Nicotinic acid (Niacin) in amide form.
(ii) Coenzyme II (Co II) or NADP+ (Nicotinamide Adenine Dinucleotide Phosphate):
Previously known as TPN (Triphospho Pyridine Dinucleotide).
(iii) Lipoic Acid:
Helps in oxidative decarboxylation of α- keto acids.
(iv) Ascrobic Acid (Vitamin C):
(v) Glutathione (GSH reduced form)/GSSG (Oxidised form):
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It is a tripeptide (Ƴ-glutamyl cyseinly glycine) that acts as coenzyme in some oxidation-reduction reaction in cells.
(vi) Flavin Adenine Dinucleotid (FAD):
It is derivation of Vitamin B2 i.e., Riboflavin. Often it acts as prosthetic group.
(vii) Flavin Mono Nucleotide (FMN):
It is also derivation of Vitamin B2 and often acts as prosthetic group.
(viii) Coenzyme Q (CO-Q) or Ubiquinone:
(b) Actyl Group Carrier:
Coenzyme-A (Co-A or CoA-SH):
It consists phosphopantetheine which is a derivation of the Vitamin pantothenic acid. It was discovered by Fritz Albert Lipman. Nobel Laureate of 1953 in Physiology or Medicine category.
(c) Amino Group Carrier:
Pyridoxal Phosphate:
It is derivation of Vitamin B6 i.e., pyridoxine.
(d) CO2 Carrier:
Biotin (Coenzyme R):
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This is a Vitamin and often acts as prosthetic group.
(e) Thiamine Pyrophosphate (TPP):
This is a derivation of Vitamin B1 i.e., thiamine and plays an important role as coenzyme in the oxidative decarboxylase of α- keto aciods (e.g., pyruvic acid α- ketoglutaric acid).
(f) Carrier of One-Carbon Componds (e.g., formic acid, formaldehyde):
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Coenzyme F (Tetrahydrofolic acid):
It is derivation of Vitamin folic acid.
(g) Adenosine Tri Phosphate (ATP):
It acts as coenzyme and help in catalysing many enzyme reactions by activating either the substrate or the enzyme and providing energy, It also acts as carrier of phosphate group.
(h) UDP (Uridine Diphosphate):
This is carrier of sugar molecules such as glucose, galactose or their derivatives. For instance, when UDP picks up a glucose molecule, it becomes converted into UDPG (Uridine Diphosphate Glucose).
(i) PAPS (3′ Phosphoadenosine -5′-Phosphosuipliate):
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This is carrier of sulphate group.
Group # 2. General Activators:
These are usually some metallic ions, the presence of which in some way activates the enzyme reaction.
Holoenzymes and Apoenzymes:
An enzyme and a coenzyme together form an enzymatically active conjugated protein called the holoenzyme. The enzyme (protein) component of the holoenzyme is called an apoenzyme. The non-protein coenzyme is called the prosthetic group.
Thus:
Holoenzyme = Apoenzyme + Prosthetic group.
The major coenzymes of dehydrogenases are the two dinucleotides, NAD and NADP. The coenzyme (prosthetic group) may be tightly bound to the apoenzyme or may easily dissociate from it.
The prosthetic group remains attached to the apoenzyme while undergoing oxidation and reduction. For example, the porphyrin groups of cytochromes are more or less permanently attached to their apoenzymes.
The coenzyme on the other hand may undergo reduction while attached to one apoenzyme, and then migrate to another apoenzyme where it can be oxidised. Thus NAD, NADP and CoA are considered to be coenzymes while hemes, flavins and biotin are considered to be prosthetic groups.
The coenzyme usually acts as an accepter or donor of an atom or a group, e.g., NAD, NADP, FMN and CoQ are hydrogen carrires, ATP transfers the phosphate group and CoA the acyl group.
Many coenzymes are closely related to vitamins and are the derivation of vitamins. The B group vitamins (except biotin and lipoic acid) function as parts of some coenzyme, e.g., CoA is a derivative of pantothenic acid. When there is vitamin deficiency the coenzyme concentration decreases. Consequently enzyme function is depressed. Nucleotides may also function as coenzymes in certain metabolic reactions.
Activators:
In same cases enzymes may be activated by simple substance like metal ions which are then called activators. The substrate forms a complex with the metal ions and then reacts with the enzyme.
Examples of metal ions which function as cofactors are:
Na+, K+, Ca++, Co++, Mg++, Mn++, Cd++, Fe++, Cr+++ and Al+++
