A thioester is a type of chemical compound formed when a carboxylic acid reacts with a thiol. In this reaction, the hydroxyl group of the acid is replaced by a sulfur atom, resulting in a structure that contains a carbonyl group (C=O) adjacent to a sulfur atom (S). Thioesters are important in biochemistry, particularly in the metabolism of fatty acids and the synthesis of certain biomolecules. One of the most well-known thioesters is acetyl-CoA, which plays a crucial role in the Krebs cycle and fatty acid synthesis. Thioesters are generally more reactive than regular esters, making them key intermediates in various biochemical reactions. Their unique properties allow them to participate in important cellular processes.