Crystal structure of human ornithine transcarbamylase complexed with carbamoyl phosphate and L-norvaline at 1.9 angstrom resolution

The crystal structure of human ornithine transcarbamylase (OTCase) complexe d with carbamoyl phosphate (CP) and L-norvaline (NOR) has been determined t o 1.9-Angstrom resolution. There are significant differences in the interac tions of CP with the protein, compared with the interactions of the CP moie ty of the bisubstrate analogue N-(phosphonoacetyl)-L-ornithine (PALO), The carbonyl plane of CP rotates about 60 degrees compared with the equivalent plane in PALO complexed with OTCase, This positions the side chain of NOR o ptimally to interact with the carbonyl carbon of CP, The mixed-anhydride ox ygen of CP, which is analogous to the methylene group in PALO, interacts wi th the guanidinium group of Arg-92; the primary carbamoyl nitrogen interact s with the main-chain carbonyl oxygens of Cys-303 and Leu-304, the side cha in carbonyl oxygen of Gln-171, and the side chain of Arg-330, The residues that interact with NOR are similar to the residues that interact with the o rnithine (ORN) moiety of PALO, The side chain of NOR is well defined and cl ose to the side chain of Cys-303 with the side chains of Leu-163, Leu-200, Met-268, and Pro-305 forming a hydrophobic wall. C-delta of NOR is close to the carbonyl oxygen of Leu-304 (3.56 Angstrom), S-gamma atom of Cys-303 (4 .19 Angstrom), and carbonyl carbon of CP (3.28 Angstrom). Even though the N -epsilon atom of ornithine is absent in this structure, the side chain of N OR is positioned to enable the N-epsilon Of ornithine to donate a hydrogen to the S-gamma atom of Cys-303 along the reaction pathway. Binding of CP an d NOR promotes domain closure to the same degree as PALO, and the active si te structure of CP-NOR-enzyme complex is similar to that of the PALO-enzyme complex. The structures of the active sites in the complexes of aspartate transcarbamylase (ATCase) with various substrates or inhibitors are similar to this OTCase structure, consistent with their common evolutionary origin . (C) 2000 Wiley-Liss, Inc.