GTPase, binds and hydrolyzes GTP (PubMed:20876572, PubMed:21138732, PubMed:28497574, PubMed:28943303). Involved in intracellular vitamin B12 metabolism, mediates the transport of cobalamin (Cbl) into mitochondria for the final steps of adenosylcobalamin (AdoCbl) synthesis (PubMed:20876572, PubMed:28497574). Functions as a G protein chaperone that assists AdoCbl cofactor delivery from MMAB to the methylmalonyl-CoA mutase (MMUT) (PubMed:20876572, PubMed:28497574). Plays a dual role as both a protectase and a reactivase for MMUT (PubMed:21138732, PubMed:28943303). Protects MMUT from progressive inactivation by oxidation by decreasing the rate of the formation of the oxidized inactive cofactor hydroxocobalamin (OH2Cbl) (PubMed:21138732, PubMed:28943303). Additionally acts a reactivase by promoting the replacement of OH2Cbl by the active cofactor AdoCbl, restoring the activity of MMUT in the presence and hydrolysis of GTP (PubMed:21138732, PubMed:28943303). {ECO:0000269|PubMed:20876572, ECO:0000269|PubMed:21138732, ECO:0000269|PubMed:28497574, ECO:0000269|PubMed:28943303}. This is the function of MMAA (metabolism of cobalamin associated A, Ensembl gene identifier ENSG00000151611).