Significance of an unusually low K_m for glutathione in glutathione transferases of the α μ and π classes

Abstract

1. Interactions of glutathione transferases (GST) of the α, μ and π classes with glutathione (GSH) and glutathione conjugates (GS-X) are in contrast with those of a GST of the θ class (GST5-5).

2. GST 5-5 has a K_m for GSH of approx. 5 mM. Thus K_m/ambient [GSH] is approx. 1, within the range of K_m/ambient [s] of glycolytic enzymes. GSTs of the α, μ and π classes yield much lower values of K_m for GSH (approx. 0.1 mM) hence K_m/ambient [s] is significantly lower than those of most (non-GST) enzymes (p<0.025).

3. GSTs of the α, μ and π classes are sensitive to inhibition by GS-X (i.e. product) and GS-X analogues. GST 5-5 is not.

4. Rate enhancements up to 10¹⁰, similar to an average enzyme (10⁸-10¹²), are seen in catalysis by GST 5-5, but not in catalysis by GSTs of α, μ and π classes (> 10⁷).

5. Comparisons of primary structure indicate that θ class GSTs may have a decreased binding of the glu-α-amino- and gly-COO⁻-groups of GSH compared with GSTs of the other classes.

6. It is concluded that GSTs of α, μ and π classes have evolved towards increased product binding at the expense of catalytic efficiency. Thus GSH is uniquely utilized both as a nucleophile and a ‘tag’ which can be used to bind and sequester product particularly during GSH-depletion. This interpretation unifies the catalytic and binding properties of these GSTs and alters their perceived role in detoxication.