Role of the charged groups of glutathione disulfide in the catalysis of glutathione reductase: crystallographic and kinetic studies with synthetic analogues

Six analogues of glutathione disulfide were synthesized. All of them involved the abolishment of charges, either by amidation of carboxylates or by removal of amino groups. Four of these analogues could be bound to crystalline oxidized glutathione reductase, and their binding modes could be established by X-ray analyses at 2.4-A resolution. All six analogues were catalytically processed; the kinetic parameters were determined. The two analogues that did not bind in the crystals had by far the poorest catalytic efficiencies. Kinetic parameters together with X-ray data show the influence of each charged group on binding and catalytic rate. Data analysis indicates that the enzyme avoids processing of incorrect substrates in two ways: First, it reduces their binding strengths and/or enforces displacement of catalytically important substrate parts. Furthermore, it forms a fragile cluster of bound substrate and catalytically competent residues, which is unbalanced by incorrect parts of the substrate such that catalysis is prevented. A scouting microcalorimetric study using glutathione disulfide yielded a binding enthalpy of -103 (+/- 10) kJ/mol at 25 degrees C and a heat capacity change of -8 (+/- 1) kJ.mol-1.K-1. The study showed that it is feasible to measure these parameters as a function of substrate modification.