Transition structure and reactive complexes for hydride transfer in an isoalloxazine-nicotinamide complex. On the catalytic mechanism of glutathione reductase. An ab initio MO SCF study Original Research Article

An analysis is presented of the catalytic mechanism of glutathione reductase based upon a theoretically characterized saddle point of index one obtained for a model representing the active groups of the flavine and nicotinamide adenine dinucleotide phosphate, namely, an isoalloxazine and nicotinamide rings. The isoalloxazine rings appears deformed into a butterfly conformation in the saddle point of index one. The butterfly conformation is retained along the path leading to a reduced isoalloxazine (N5-H) forcing the transferred hydrogen to stick into the nicotinamide binding site, this geometric feature suggests the existence of a transposed hydride transfer path where the N5-proton goes back to a lysine residue leaving the electrons on FAD. This mechanism is discussed and proposed as an alternative catalytic pathway in glutathione reductase to the standard electron transfer one. These results help to understand the riddle created by the changing kinetic behavior of glutathione reductase when, for instance, 2,4,6-trinitro-benzene sulfonate is used to study in vitro kinetics or when specific site directed mutagenesis is performed.