THE CRYSTAL-STRUCTURE OF 8-AMINO-7-OXONONANOATE SYNTHASE - A BACTERIAL PLP-DEPENDENT, ACYL-COA-CONDENSING ENZYME

8-Amino-7-oxononanoate synthase (or 8-amino-7-ketopelargonate synthase ; EC 2.3.1.47; AONS) catalyses the decarboxylative condensation of L-a lanine and pimeloyl-CoA in the first committed step of biotin biosynth esis. We have cloned, over-expressed and purified AONS from Escherichi a coli and determined the crystal structures of the apo and PLP-bound forms of the enzyme. The protein is a symmetrical homodimer with a ter tiary structure and active site organisation similar to, but distinct from, those of other PLP-dependent enzymes whose three-dimensional str uctures are known. The critical PLP-binding lysine of AONS is located at the end of a deep cleft that allows access of the pantothenate arm of pimeloyl-CoA. A cluster of positively charged residues at the entra nce to this cleft forms a putative diphosphate binding site for CoA. T he structure of E. coli AONS enables identification of the key residue s of the PLP-binding site and thus provides a framework with which to understand the biochemical mechanism, which is similar to that catalys ed by 5-aminolevulinate synthase and two other alpha-oxoamine synthase s. Although AONS has a low overall sequence similarity with the cataly tic domains of other alpha-oxoamine synthases, the structure reveals t he regions of significant identity to be functionally important. This suggests that the organisation of the conserved catalytic residues in the active site is similar for all enzymes of this sub-class of PLP-de pendent enzymes and they share a common mechanism. Knowledge of the th ree-dimensional structure of AONS will enable characterisation of the structural features of this enzyme sub-family that are responsible for this important type of reaction. (C) 1998 Academic Press.