title: Affinity alkylation of the Trp-B4 residue of the beta -subunit of the glutaryl 7-aminocephalosporanic acid acylase of Pseudomonas sp. 130.
abstract: Glutaryl 7-aminocephalosporanic acid acylase of Pseudomonas sp. 130 (C130) was irreversibly inhibited in a time-dependent manner by two substrate analogs bearing side chains of variable length, namely 7beta-bromoacetyl aminocephalosporanic acid (BA-7-ACA) and 7beta-3-bromopropionyl aminocephalosporanic acid (BP-7-ACA). The inhibition of the enzyme with BA-7-ACA was attributable to reaction with a single amino acid residue within the beta-subunit proven by comparative matrix assisted laser desorption/ionization-time of flight mass spectrometry. Further mass spectrometric analysis demonstrated that the fourth tryptophan residue of the beta-subunit, Trp-B4, was alkylated by BA-7-ACA. By (1)H-(13)C HSQC spectroscopy of C130 labeled by BA-2-(13)C-7-ACA, it was shown that tryptophan residue(s) in the enzyme was alkylated, forming a carbon-carbon bond. Replacing Trp-B4 with other amino acid residues caused increases in K(m), decreases in k(cat), and instability of enzyme activity. None of the mutant enzymes except W-B4Y could be affinity-alkylated, but all were competitively inhibited by BA-7-ACA. Kinetic studies revealed that both BA-7-ACA and BP-7-ACA could specifically alkylate Trp-B4 of C130 as well as Tyr-B4 of the mutant W-B4Y. Because these alkylations were energy-requiring under physiological conditions, it is likely that the affinity labeling reactions were catalyzed by the C130 enzyme itself. The Trp-B4 residue is located in the middle of a characteristic alphabetabetaalpha sandwich structure. Therefore, a large conformational alteration during inhibitor binding and transition state formation is likely and suggests that a major conformational change is induced by substrate binding during the course of catalysis.
authors.affiliation: Laboratory of Microbial Molecular Physiology, Institute of Plant Physiology and Ecology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Shanghai 200032, China.
authors.forename: Xi
authors.initials: X
authors.lastname: Huang
authors.forename: Rong
authors.initials: R
authors.lastname: Zeng
authors.forename: Xiaoming
authors.initials: X
authors.lastname: Ding
authors.forename: Xiang
authors.initials: X
authors.lastname: Mao
authors.forename: Yi
authors.initials: Y
authors.lastname: Ding
authors.forename: Zihe
authors.initials: Z
authors.lastname: Rao
authors.forename: Yong
authors.initials: Y
authors.lastname: Xie
authors.forename: Weihong
authors.initials: W
authors.lastname: Jiang
authors.forename: Guoping
authors.initials: G
authors.lastname: Zhao
mesh_terms: D000581:Amidohydrolases / Q000737:chemistry*; D000595:Amino Acid Sequence; D001665:Binding Sites; D002384:Catalysis; D002511:Cephalosporins / Q000737:chemistry*; D002851:Chromatography, High Pressure Liquid; D006860:Hydrogen Bonding; D007700:Kinetics; D008024:Ligands; D009682:Magnetic Resonance Spectroscopy; D013058:Mass Spectrometry; D008956:Models, Chemical; D008958:Models, Molecular; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D009154:Mutation; D011485:Protein Binding; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D011549:Pseudomonas; D017386:Sequence Homology, Amino Acid; D019032:Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization; D013379:Substrate Specificity; D013997:Time Factors; D014364:Tryptophan / Q000737:chemistry*
chemical_list: C452854:7-bromoacetyl aminocephalosporanic acid; D002511:Cephalosporins; D008024:Ligands; C081063:glutaryl-7-aminocephalosporanic acid; D014364:Tryptophan; D000581:Amidohydrolases; C019665:amidase