FUNCTION 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).CATALYTIC ACTIVITY GTP + H2O = GDP + phosphate + H(+)ACTIVITY REGULATION GTPase activity is stimulated by MMUT.BIOPHYSICOCHEMICAL PROPERTIES kcat is 0.201 min(-1) for GTP hydrolysis (PubMed:28497574). kcat is 0.03 min(-1) for GTP hydrolysis (PubMed:20876572).SUBUNIT Homodimer (PubMed:20876572). Interacts with MMUT (the apoenzyme form); the interaction is GTP dependent (PubMed:20876572, PubMed:21138732, PubMed:28497574, PubMed:28943303).INTERACTION Widely expressed. Highest expression is observed in liver and skeletal muscle.DISEASE The disease is caused by variants affecting the gene represented in this entry.SIMILARITY Belongs to the SIMIBI class G3E GTPase family. ArgK/MeaB subfamily.