During exercise, a substance called creatine phosphate is used to

replenish ATP in the muscle. An enzyme called arginine:glycine amidinotransferase

(AGAT) is responsible for the first step in the biosynthesis of creatine. This

enzyme catalyses the transfer of an amidino group from arginine to glycine to

produce ornithine and guanidinoacetic acid.

Analysis of the structure of this enzyme bound to one of its

substrates, arginine, revealed the location of the active site and interactions

that stabilise the arginine molecule bound in the active site. The figure below

shows schematic representations of these interactions, and the impact of

different amino acid substitutions on the enzymatic activity.