Long chain fatty acyl CoA esters have the ability to interact with certain proteins and thereby serve as effectors in cell metabolism. In particular, they can displace nucleotides from specific nucleotide dependent or binding proteins and interfere with their action. The ADP/ATP carrier and uncoupling protein are two examples where the interplay of nucleotide and acyl CoA binding to the proteins regulate their function. Other proteins such as glucokinase can be considered in this group. In certain tissues like liver they are affected during fasting and insulin deficiency, and when serum fatty acids and liver acyl CoA levels are elevated. More recently, an acyl CoA binding protein in E. coli has been found to be a transcription factor for gene regulation of fatty acid metabolism enzymes. There appears to be some consensus in the amino acid sequence for acyl CoA binding sites on these proteins which serve a variety of important roles in cellular metabolism.