MMAA is a GTPase protein essential for intracellular vitamin B12 (cobalamin) metabolism 1. It mediates cobalamin transport into mitochondria and catalyzes the final steps of adenosylcobalamin (AdoCbl) synthesis, functioning as a G-protein chaperone that facilitates AdoCbl cofactor delivery to methylmalonyl-CoA mutase (MMUT) 12. MMAA exhibits dual protective and reactivating roles for MMUT. It acts as a protectase by decreasing the formation of oxidized, inactive hydroxocobalamin (OH2Cbl) during catalysis, and as a reactivase by promoting replacement of damaged cofactor with active AdoCbl through GTP hydrolysis 34. MMAA forms nucleotide-selective complexes with MMUT apoenzyme, with complex formation and cofactor exchange regulated by GTP binding and hydrolysis 1. Mutations in MMAA cause cblA-type methylmalonic aciduria, a rare inborn error of metabolism characterized by accumulation of methylmalonic acid in blood and urine 56. Clinical presentation varies from mild to life-threatening, typically manifesting in early infancy with vomiting, hypotonia, and developmental delay 5. Understanding MMAA's structural and functional properties is crucial for developing therapeutic interventions for affected patients 7.