Enzyme cofactors

Complex enzymes are made up of protein and nonprotein components.

The protein part of the enzyme is called Apoenzymes or Apozyme.

The non-protein part of the enzyme is called Cofactor.

Cofactors are two types based on their chemical property such as metal cofactors and Organic cofactors.

Unlike complex enzymes, a simple enzymes can be made up of only protein component and they can complete the enzymatic reaction alone without cofactor.

The combination of apoenzyme and cofactor together is called the Holoenzyme.

The intact of holoenzyme is biologically active and contain the catalytic activity.

Alone apoenzyme and cofactor will not be biologically active, hence they cannot fulfill the enzymatic reaction.


Metal cofactors

Metalloenzymes or metallozymes

Enzymes having the transition metal ions as cofactor are called metalloenzymes or metallozymes.

The transition metal ion can participate in the formation of active which is place for substrate binding.

The shape of the active site in the enzyme is crucial and once the transition metal bound to enzyme then it will be always bound to the active site.

Zn2+ is the most common metal ion found in enzyme Alcohol dehydrogenase.

The dinitrogenase complex is one of the metalloenzymes involves in the biological nitrogen fixation (BNF) containing more than one transitional metal ion, dinitrogenase contains both Mo and Fe.


Metal-activated enzymes

Some enzymes require alkali or alkaline earth metal ions are called metal-activated enzymes.

In such enzymes, the metal ion requires only at the time of reaction to activate the enzymes or to complex with the substrate.

Once the enzyme-substrate complex formed or reaction completed then the metal will be separated from enzyme, where as these ions will not take part in the enzyme active site formation.


Enzyme inhibitors

Some of the metals can inhibit the overall enzymatic reaction

Pb2+, Hg2+, Hg+ and Co2+ metals are best examples for enzyme inhibitors.

The strong metal inhibitors Pb2+ and Hg2+ are binding with cysteinyl SH group of the enzyme and removes the catalytic activity of enzymes.


Organic cofactors

The organic cofactors are derivatives of B-complex group of vitamins.

These are further divided into coenzymes and prosthetic groups.


A prosthetic group

A prosthetic group is an organic cofactor that is tightly bound to the enzyme and involves in the active site formation. Like metal ion cofactors, once the organic cofactors bind to enzyme then they always remain with the enzyme complex.

The prosthetic group cannot be removed from the enzyme until structure of enzyme gets denatured.

ATP  dependent carboxylation enzymes such as pyruvate carboxylase, acetyl Co.A carboxylase, propionyl Co.A contain Biotin [vitamin H] as the prosthetic group.

Avidin is a tetrameric biotin-binding protein synthesized in the oviducts of birds, reptiles and amphibians and finally deposited in the whites of their eggs. The extremely high affinity of avidin for biotin makes it a potent inhibitor of biotin-dependent enzymes and it leads to defects in carbohydrate metabolism [gluconeogenesis] and lipid biosynthesis.

The flavin nucleotides (FMN and FAD) are prosthetic groups that take part in oxidoreductase which is an enzyme that catalyzes the transfer of electrons from one molecule, the reductant (electron donor) to another molecule, the oxidant (electron acceptor) in electron transfer system.



Coenzymes are organic cofactors that are loosely bound to the enzymes and they will participate in the formation of the active site.

Coenzymes can bind the enzyme only at the time of reaction.

In some cases, coenzymes act as co-substrate and can be removed from the enzyme without damaging the structure of enzyme.

Pyridoxal phosphate is the cofactor that is capable of catalysis even in the absence of apoenzyme (protein part) even though the catalytic activity is relatively low compared to the Holoenzyme.

The pyridine nucleotides (NAD+ and NADP+) and both NAD and NADP play crucial roles in pro-oxidant and antioxidant metabolism.

Pyridine nucleotides are involved in many other defense and signaling reactions, for example, production of nitric oxide and metabolism of reactive lipid derivatives.

NAD and NADP are also used as respective substrates for the production of the calcium agonists cyclic ADP-ribose and nicotinate adenine dinucleotide phosphate.

Co-enzyme A as cofactor involves in synthesis and oxidation of fatty acids, and the oxidation of pyruvate in the citric acid cycle.

ATP is one of the coenzymes involved in different types of enzymatic reactions as a cofactor.




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