Chemical inactivation of bacterial GABA aminotransferase

• Published in: Biochemistry and molecular biology international 1998-09, Vol.46 (1), p.43-54
• Main Authors: Tunnicliff, G., Crites, G. J.
• Format: Article
• Language: English
• Subjects: 4-Aminobutyrate Transaminase - antagonists & inhibitors; 4-Aminobutyrate Transaminase - chemistry; 4-Aminobutyrate Transaminase - metabolism; Binding Sites; Enzyme Inhibitors - pharmacology; gamma-Aminobutyric Acid - analogs & derivatives; gamma-Aminobutyric Acid - metabolism; gamma-Aminobutyric Acid - pharmacology; Ketoglutaric Acids - metabolism; Ketoglutaric Acids - pharmacology; Kinetics; Mercuric Chloride - pharmacology; p-Chloromercuribenzoic Acid - pharmacology; Pyridoxal Phosphate - metabolism; Pyridoxal Phosphate - pharmacology
• ISSN: 1521-6543; 1039-9712; 1521-6551
• DOI: 10.1080/15216549800203542

The effects of three potential irreversible inhibitors of γ‐aminobutyrate aminotransferase from Pseudomonas fluorescens were studied in order to throw more light on the nature of the active site of the enzyme. The thiol group reagent mercuric chloride inactivated the enzyme in a concentration‐dependent manner. Inhibition kinetics were consistent with a simple bimolecular reaction. The second‐order rate constant was 4.2 × 103 ± 0.61 M‐1 see‐1. In contrast to either of the substrates, the cofactor pyridoxal 5′‐phosphate could protect the enzyme from the inhibition, suggesting cysteinyl residues are important for cofactor binding at the active site. p‐Chloromercuribenzoic acid produced a similar inactivation of the enzyme. 4‐Amino‐2‐fluorobutanoic acid also inhibited enzymic activity but in this case the inhibition was reversible and competitive with respect to γ‐aminobutyric acid (GABA). The inhibitor constant (Ki) was 0.83 ± 0.44 mM. We found no evidence that this fluorinated analogue of GABA could act as a substrate for the enzyme.