Modelling the active site of glyceraldehyde-3 phosphate dehydrogenase with the LSCF formalism

In the framework of a theoretical approach to the relationship between structure and reactivity of the catalytic centers of enzymes, glyceraldehyde-3 phosphate dehydrogenase (GAPDH) has been chosen as a model enzyme. In GAPDH, the proximity of His(176) increases the reactivity of Cys(149) at neutral pH; however, its presence alone is not sufficient to explain the reactivity of the catalytic Cys. In order to determine which other interactions play an important role, a study of the geometric and electronic structure of the catalytic site has been made using a hybrid quantum mechanics/molecular mechanics local self-consistent field method. This allows the computation of the electronic properties of amino acid residues in subsystems influenced by other parts of the macromolecule. The quantum subsystem was centered on the Cys(149) residue of GAPDH. The structures of GAPDH taken from the crystallographic database did not include hydrogen atoms and these had to be added taking into account the fact that, in the active site, His(176) has three tautomeric forms: delta-His protonated, epsilon-His protonated and Hist. The results presented here suggest that the most stable His...Cys system in GAPDH is a strongly hydrogen-bonded Cys(149)(-)/His(176)(+) ion pair.