ISSN:
0887-3585
Keywords:
β-lactamase
;
active site
;
chemical modification
;
thiol modification
;
enzyme ammonium group
;
Chemistry
;
Biochemistry and Biotechnology
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Medicine
Notes:
The RTEM-1 thiol β-lactamase (Sigal, I. S., Harwood, B. G., Arentzen, R., Proc. Natl. Acad. Sci. USA. 79:7157-7160, 1982) is inactivated by thiol-selective reagents such as iodoacetamide, methyl methanethiosulfonate, and 4,4′-dipyridyldisulfide, which modify the active site thiol group. The pH-rate profiles of these inactivation reactions show that there are two nucleophilic forms of the enzyme, EH2 and EH, both of which, by analogy with the situation with cysteine proteinases, probably contain the active site nucleophile in the thiolate form. The pKa of the active site thiol is therefore shown by the data to be below 4.0. This low pKa is thought to reflect the presence of adjacent functionality which stabilizes the thiolate anion. The low nucleophilicity of the thiolate in both EH2 and EH, with respect to that of cysteine proteinases and model compounds, suggests that the thiolate of the thiol β-lactamase is stabilized by two hydrogen-bond donors. One of these, of pKa greater than 9.0, is suggested to be the conserved and essential Lys-73 ammonium group, of pKa around 6.7, is less clear, but may be the conserved Glu-166 carboxylic acid. β-Lactamase activity is associated with the EH2 form, and thus the β-lactamase active site is proposed to contain one basic or nucleophilic group (the thiolate in the thiol β-lactamase) and two acidic (hydrogen-bond donor) groups (one of which is likely to be the above-mentioned lysine ammonium group).
Additional Material:
2 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/prot.340060314
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