Structure of Coenzyme A

Coenzyme A (CoA) is an important chemical required in the metabolic regulation of fatty acids and also for the energy generating cycle of the body, that is the citric acid cycle. The name itself suggests that it acts along with an enzyme while carrying out its function.

Before we start off, it is important that we understand what a coenzyme is and how does it differ from the commonly confused terms such as 'cofactors' and 'prosthetic groups'.

A coenzyme is a non proteinaceous group that binds or interacts with an enzyme, or rather apoenzyme, to activate it. When a cofactor binds tightly to an enzyme, it is termed as a prosthetic group. When loosely bound to an enzyme, the cofactor is called a coenzyme.

Coenzymes are groups that are a part of an enzyme catalyzed reaction. They play an important role in enzyme functions. During this process, it undergoes certain changes which can be reversed once the reaction is complete.

The structure of these coenzymes is quite unique and therefore, it is necessary to know the structure of these while studying complex enzyme mediated biochemical reactions. CoA is an important one, which plays a vital role in many biochemical reactions of metabolism.

Its structure is synthesized in a step by step process.

• Vitamin B₅ gets phosphorylated to form 4'- phosphopantothenate, by the enzyme pantothenate kinase
• A cysteine molecule is added to the 4'-phosphopantothenate and the new molecule formed is 4'- phospho- N-pantothenoylcysteine. This reaction is catalyzed by phosphopantothenoylcysteine synthetase.

• The molecule is then decarboylated to form 4'- phosphopantetheine, catalyzed by the enzyme phosphopantothenoylcysteine decarboylase.
• The molecule is then subjected to adenylation that forms dephospho-CoA.
• Dephospho-CoA is then phosphorylated with ATP to form CoA. This reaction is catalyzed by dephosphocoenzyme A kinase.

Its structure is such that it reacts with carboxylic acids and forms thio-esters. When it forms these bonds, CoA is referred to as an acyl group carrier. This ability of the CoA helps in transfer of fatty acids from the cytoplasm to the mitochondria for its oxidation.

Pyruvate is converted to acetyl CoA in the last step of glycolysis, after which it enters the citric acid cycle. This acetyl CoA is necessary for the conversion of oxaloacetate to citrate, which is the first step of the cycle in aerobic cellular respiration. Thus it assists citrate synthase and transfers the acetyl group to the oxaloacetate to form citrate.

This coenzyme activates a lot of acyl groups which are transferred to other molecules in various biochemical reactions. They are

• Acetyl-CoA
• Propionyl CoA
• Acetoacyl CoA
• Comaryl CoA

Apart from this, there are various other groups with which it interacts.

Thus the structure of CoA is such that it helps in transferring Carbon atoms to different molecules in enzyme catalyzed reactions.