ATP formation during enzymatic decomposition of coenzyme B 12

Abstract

This chapter discusses about adenosyltransferases. In the chapter, existing knowledge about the two enzymes known to catalyze adenosyl transfer reactions have been discussed. Both the enzyme responsible for the formation of S-adenosylmethionine and the one responsible for the formation of adenosyl-B12 are widespread in biological systems. Studies of the purified enzymes have demonstrated that there are many features common to the two reactions. In each case, adenosine triphosphate (ATP) is the adenosyl donor. The adenosyl acceptor, in the first instance, is the sulfur atom of methionine and in the second instance, the reduced cobalt atom of cobalamin. The sulfur of methionine possesses an unshared pair of electrons that presumably spearhead a nucleophilic attack upon the 5'-carbon of ATP, resulting in transfer of the adenosyl moiety and displacement of PPPi. The cobalt atom of B12, after reduction, similarly possesses an unshared pair of electrons; thus, in this instance, too, the reaction may be visualized as a nucleophilic attack upon the 5'-carbon of ATP with PPPi, serving as the leaving group.