A MIXED DISULFIDE BOND IN BACTERIAL GLUTATHIONE TRANSFERASE - FUNCTIONAL AND EVOLUTIONARY IMPLICATIONS

Background: Glutathione S-transferases (GSTs) are a multifunctional gr oup of enzymes, widely distributed in aerobic organisms, that have a c ritical role in the cellular detoxification process. Unlike their mamm alian counterparts, bacterial GSTs often catalyze quite specific react ions, suggesting that their roles in bacteria might be different. The GST from Proteus mirabilis (PmGST B1-1) is known to bind certain antib iotics tightly and reduce the antimicrobial activity of beta-lactam dr ugs. Hence, bacterial GSTs may play a part in bacterial resistance tow ards antibiotics and are the subject of intense interest. Results: Her e we present the structure of a bacterial GST, PmGST B1-1, which has b een determined from two different crystal forms. The enzyme adopts the canonical GST fold although it shares less than 20% sequence identity with GSTs from higher organisms. The most surprising aspect of the st ructure is the observation that the substrate, glutathione, is covalen tly bound to Cys10 of the enzyme. In addition, the highly structurally conserved N-terminal domain is found to have an additional beta stran d. Conclusions: The crystal structure of PmGST B1-1 has highlighted th e importance of a cysteine residue in the catalytic cycle. Sequence an alyses suggest that a number of other GSTs share this property, leadin g us to propose a new class of GSTs - the beta class. The data suggest that the in vivo role of the beta class GSTs could be as metabolic or redox enzymes rather than conjugating enzymes. Compelling evidence is presented that the theta class of GSTs evolved from an ancestral memb er of the thioredoxin superfamily.