Structure and Function of YghU, a Nu-Class Glutathione Transferase Related to YfcG from Escherichia coli

The crystal structure (1.50 angstrom resolution) and biochemical properties of the GSH transferase homologue, YghU, from Escherichia coli reveal that the protein is unusual in that it binds two molecules of GSH in each active site. The crystallographic observation is consistent with biphasic equilibrium binding data that indicate one tight (K-d1 = 0.07 +/- 0.03 mM) and one weak (K-d2 = 1.3 +/- 0.2 mM) binding site for GSH. YghU exhibits little or no GSH transferase activity with most typical electrophilic substrates but does possess a modest catalytic activity toward several organic hydroperoxides. Most notably, the enzyme also exhibits disulfide-bond reductase activity toward 2-hydroxyethyl disulfide [k(cat) = 74 +/- 6 s(-1), and k(cat)/K-M(GSH) = (6.6 +/- 1.3) x 10(4) M-1 s(-1)] that is comparable to that previously determined for YfcG. A superposition of the structures of the YghU.2GSH and YfcG.GSSG complexes reveals a remarkable structural similarity of the active sites and the 2GSH and GSSG molecules in each. We conclude that the two structures represent reduced and oxidized forms of GSH-dependent disulfide-bond oxidoreductases that are distantly related to glutaredoxin 2. The structures and properties of YghU and YfcG indicate that they are members of the same, but previously unidentified, subfamily of GSH transferase homologues, which we suggest be called the nu-class GSH transferases.