Affinity alkylation of the Trp-B4 residue of the beta -subunit of the glutaryl 7-aminocephalosporanic acid acylase of Pseudomonas sp. 130.
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.