Theoretical study of the methyl transfer in guanidinoacetate methyltransferase.

The reaction mechanism of the guanidinoacetate methyltransferase (GAMT) enzyme has been investigated by means of density functional theory using the B3LYP hybrid functional. GAMT catalyzes the S-adenosyl-L-methionine (SAM)-dependent methylation of guanidinoacetate (GAA) to form creatine. A quantum chemical model was built on the basis of the recent crystal structure of GAMT complexed with S-adenosylhomocysteine (SAH) and GAA. The methyl group transfer from SAM to N(E) of GAA is shown to occur concertedly with a proton transfer from NE to the neighboring OD1 of Asp134. Good agreement is found between the calculated barrier and the experimental rate.