Three-dimensional structure of ATP : corrinoid adenosyltransferase from Salmonella typhimurium in its free state, complexed with MgATP, or complexed with hydroxycobalamin and MgATP

In Salmonella typhimurium, formation of the cobalt-carbon bond in the biosy nthetic pathway for adenosylcobalamin is catalyzed by the product of the co bA gene which encodes a protein of 196 amino acid residues. This enzyme is an ATP:co(I)rrinoid adenosyltransferase which transfers an adenosyl moiety from MgATP to a broad range of co(I)rrinoid substrates that are believed to include cobinamide, its precursor cobyric acid and probably others as yet unidentified, and hydroxocobalamin. Three X-ray structures of CobA are repo rted here: its substrate-free form, a complex of CobA with MgATP, and a ter nary complex of CobA with MgATP and hydroxycobalamin to 2.1, 1.8, and 2.1 A ngstrom resolution, respectively. These structures show that the enzyme is a homodimer. In the apo structure, the polypeptide chain extends from Arg(2 8) tow Lys(181) and consists of an alpha/beta structure built from a six-st randed parallel beta -sheet with strand order 324516. The topology of this fold is very similar to that seen in RecA protein, helicase domain, F(1)ATP ase, and adenosylcobinamide kinase/adenosylcobinamide guanylyltransferase w here a P-loop is located at the end of the first strand. Strikingly, the nu cleotide in the MgATP . CobA complex binds to the P-loop of CobA in the opp osite orientation compared to all the other nucleotide hydrolases. That is, the gamma -phosphate binds at the location normally occupied by the alpha -phosphate. The unusual orientation of the nucleotide arises because this e nzyme transfers an adenosyl group rather than the gamma -phosphate. In the ternary complex, the binding site for hydroxycobalamin is located in a shal low bowl-shaped depression at the C-terminal end of the beta -sheet of one subunit; however, the active site is capped by the N-terminal helix from th e symmetry-related subunit that now extends from Gln(7) to Ala(24). The low er ligand of cobalamin is well-ordered and interacts mostly with the N-term inal helix of the symmetry-related subunit. Interestingly, there are few in teractions between the protein and the polar side chains of the corrin ring which accounts for the broad specificity of this enzyme. The corrin ring i s oriented such that the cobalt atom is located similar to6.1 Angstrom from C5' of the ribose and is beyond the range of nucleophilic attack. This sug gests that a conformational change occurs in the ternary complex when Co(II I) is reduced to Co(I).