Determination of the structure of Escherichia coli glyoxalase I suggests a structural basis for differential metal activation

The metalloenzyme glyoxalase I (GlxI) converts the nonenzymatically produced hemimercaptal of cytotoxic methylglyoxal and glutathione to nontoxic S-D-lactoylglutathione. Human GlxI, for which the structure is known, is active in the presence of Zn2+. Unexpectedly, the Escherichia coli enzyme is inactive in the presence of Zn2+ and is maximally active with Ni2+. To understand this difference in metal. activation and also to obtain a representative of the bacterial enzymes, the structure of E. coli Ni2+-GlxI has been determined. Structures have also been determined for the apo enzyme as well as complexes with Co2+, Cd2+, and Zn2+. It is found that each of the protein-metal complexes that is catalytically active has octahedral geometry. This includes the complexes of the E. coli enzyme with Ni2+, Co2+, and Cd2+, as well as the structures reported for the human Zn2+ enzyme. Conversely, the complex of the E. coli enzyme with Zn2+ has trigonal bipyramidal coordination and is inactive. This mode of coordination includes four protein ligands plus a single water molecule. In contrast, the coordination in the active forms of the enzyme includes two water molecules bound to the metal ion, suggesting that this may be a key feature of the catalytic mechanism. A comparison of the human and E. coli enzymes suggests that there are differences between the active sites that might be exploited for therapeutic use.