Structurally, ArgK (also known as YgfD) belongs to the G3E family of P-loop GTPases, a family defined by the glutamate residue in the Walker B motif and an intact NKxD, members of which include: UreG, HypB, CobW, and MeaB []. ArgK is involved in the transport of positively charged amino acids (lysine, arginine, and ornithine) and has arginine kinase activity. It is found in a small, but phylogenetically diverse array of bacteria and archaea, and in Caenorhabditis and Leishmania among the eukaryotes []. Although ArgK was characterised as an ATPase originally [], it has been shown to binds and cleaves GTP. Therefore, its actual substrate in vivo is GTP rather than ATP []. This entry also includes methylmalonic aciduria type A (MMAA), which is associated with the fatal disease methylmalonyl aciduria []. Caenorhabditis elegan MMAA may have a role in propionyl-CoA metabolism and adenosylcobalamin synthesis []. The structure of MMAA has been revealed [].
