Catalyzes the biosynthesis of guanidinoacetate, the immediate precursor of creatine. Creatine plays a vital role in energy metabolism in muscle tissues. May play a role in embryonic and central nervous system development. May be involved in the response to heart failure by elevating local creatine synthesis. glycine + L-arginine = guanidinoacetate + L-ornithine Amine and polyamine biosynthesis; creatine biosynthesis; creatine from L-arginine and glycine: step 1/2. Homodimer. There is an equilibrium between the monomeric and dimeric forms, shifted towards the side of the monomer. Probably attached to the outer side of the inner membrane. Expressed in brain, heart, kidney, liver, lung, salivary gland and skeletal muscle tissue, with the highest expression in kidney. Biallelically expressed in placenta and fetal tissues. Expression is elevated in the myocardium during heart failure, and decreased in inter-uterine growth restriction (IUGR)-associated placenta. One chain folds into a compact single domain composed of repeating units, five beta-beta-alpha-beta modules, which surround the central active site. The disease is caused by variants affecting the gene represented in this entry. The disease is caused by variants affecting the gene represented in this entry. Belongs to the amidinotransferase family. Extended N-terminus.