title: Mechanism of 8-amino-7-oxononanoate synthase: spectroscopic, kinetic, and crystallographic studies.
abstract: 8-Amino-7-oxononanoate synthase (also known as 7-keto-8-aminopelargonate synthase, EC 2.3.1.47) is a pyridoxal 5'-phosphate-dependent enzyme which catalyzes the decarboxylative condensation of L-alanine with pimeloyl-CoA in a stereospecific manner to form 8(S)-amino-7-oxononanoate. This is the first committed step in biotin biosynthesis. The mechanism of Escherichia coli AONS has been investigated by spectroscopic, kinetic, and crystallographic techniques. The X-ray structure of the holoenzyme has been refined at a resolution of 1.7 A (R = 18.6%, R(free) = 21. 2%) and shows that the plane of the imine bond of the internal aldimine deviates from the pyridine plane. The structure of the enzyme-product external aldimine complex has been refined at a resolution of 2.0 A (R = 21.2%, R(free) = 27.8%) and shows a rotation of the pyridine ring with respect to that in the internal aldimine, together with a significant conformational change of the C-terminal domain and subtle rearrangement of the active site hydrogen bonding. The first step in the reaction, L-alanine external aldimine formation, is rapid (k(1) = 2 x 10(4) M(-)(1) s(-)(1)). Formation of an external aldimine with D-alanine, which is not a substrate, is significantly slower (k(1) = 125 M(-)(1) s(-)(1)). Binding of D-alanine to AONS is enhanced approximately 2-fold in the presence of pimeloyl-CoA. Significant substrate quinonoid formation only occurs upon addition of pimeloyl-CoA to the preformed L-alanine external aldimine complex and is preceded by a distinct lag phase ( approximately 30 ms) which suggests that binding of the pimeloyl-CoA causes a conformational transition of the enzyme external aldimine complex. This transition, which is inferred by modeling to require a rotation around the Calpha-N bond of the external aldimine complex, promotes abstraction of the Calpha proton by Lys236. These results have been combined to form a detailed mechanistic pathway for AONS catalysis which may be applied to the other members of the alpha-oxoamine synthase subfamily.
authors.affiliation: Edinburgh Centre for Protein Technology, Department of Chemistry, University of Edinburgh, King's Buildings, Edinburgh EH9 3JJ, United Kingdom.
authors.forename: S P
authors.initials: SP
authors.lastname: Webster
authors.forename: D
authors.initials: D
authors.lastname: Alexeev
authors.forename: D J
authors.initials: DJ
authors.lastname: Campopiano
authors.forename: R M
authors.initials: RM
authors.lastname: Watt
authors.forename: M
authors.initials: M
authors.lastname: Alexeeva
authors.forename: L
authors.initials: L
authors.lastname: Sawyer
authors.forename: R L
authors.initials: RL
authors.lastname: Baxter
mesh_terms: D000214:Acyl Coenzyme A / Q000378:metabolism; D000217:Acyltransferases / Q000737:chemistry* / Q000378:metabolism*; D000409:Alanine / Q000378:metabolism; D000595:Amino Acid Sequence; D001407:Bacillus / Q000201:enzymology; D001665:Binding Sites; D018360:Crystallography, X-Ray; D004926:Escherichia coli / Q000201:enzymology*; D006860:Hydrogen Bonding; D007700:Kinetics; D013058:Mass Spectrometry; D008958:Models, Molecular; D008969:Molecular Sequence Data; D011487:Protein Conformation; D013053:Spectrophotometry; D013379:Substrate Specificity
chemical_list: D000214:Acyl Coenzyme A; C019527:pimeloyl-coenzyme A; D000217:Acyltransferases; C021929:8-amino-7-oxononanoate synthase; D000409:Alanine