Coenzyme A (CoA) is a small organic coenzyme that carries acyl groups between biochemical reactions.

An acyl group has the general form

$$\mathrm{R{-}C(=O){-}},$$

and CoA carries it through a sulfur-containing thioester linkage:

$$\mathrm{R{-}C(=O){-}S{-}CoA}.$$

The most important introductory example is acetyl-CoA:

$$\mathrm{CH_3{-}C(=O){-}S{-}CoA}.$$

It carries a two-carbon acetyl group.

CoA is a carrier, not part of the transferred carbon skeleton

When an acetyl group is transferred from acetyl-CoA to another molecule, CoA is released and can be reused:

$$\mathrm{acetyl{-}CoA+acceptor\rightarrow acetyl{-}acceptor+CoA}.$$

The large CoA portion therefore acts as a molecular handle that enzymes recognize. The transferred material is the attached acyl group.

Why thioesters are useful transfer intermediates

Acyl transfer from an ordinary carboxylic acid can be thermodynamically or kinetically difficult. Converting the acyl group into a thioester changes its chemical reactivity and can make subsequent group transfer favorable as part of a coupled pathway.

This does not mean that a particular bond contains a packet of usable energy. The relevant quantity is the Gibbs free-energy change of the complete reaction that converts reactants into products.

Acetyl-CoA connects pathways

Acetyl-CoA can be produced from several carbon sources and can donate its acetyl group to several pathways. In cellular respiration, pyruvate is converted to acetyl-CoA before its two-carbon unit enters the citric-acid cycle.

The same carrier also appears in lipid metabolism and many biosynthetic reactions.

The reusable pattern is therefore

free CoA
  ↓ receives an acyl group
acyl-CoA
  ↓ transfers the acyl group
free CoA regenerated

Coenzyme A is a recyclable acyl-group carrier whose thioester derivatives connect carbon transformations across metabolic pathways.