The $ CoASH $ can be compared to a courier truck. For example, it can load acetyl groups and transmit them.
Its suppliers of acetyl are either pyruvate (from carbohydrates) or amino acids (from proteins) ( 2 ) or fatty acids (from lipids) ( 3 ). Its receiver is either the Krebs cycle, an energy provider ( 4 ), or systems that remake new amino acids ( 5 ) or fatty acids ( 6 ):
1 : adenine 2 : ribosephosphate 3 : diphosphate group 4 : amide group of pantothenic acid 5 : group β-mercaptoethylamine
The $ CoASC=O R $ is therefore a thioester The most important case is the formation of acetylCoA ($ R = CH_3 $), but other acyl groups can also be involved!