Laccase Redox Potentials: pH Dependence and Mutants, a QM/MM Study

2016-08-17T00:00:00Z (GMT) by Jan P. Götze Michael Bühl
We have studied the T. versicolor laccase T1 site redox potential (RP) at the M06/6-311++G**/SDD­(Cu) level of theory, employing QM/MM-optimized geometries (RI-BP86/def2-SVP/def2-TZVP­(Cu):CHARMM) of the whole protein system with electronic embedding. The oxidation state of the trinuclear cluster was found to affect the T1 site RP by about 0.2–0.3 V, depending on the protein protonation state. The computed laccase RP can be drastically lowered upon introduction of a protonation state corresponding to a neutral environment, by up to −1.37 V, which is likely an overestimation of the effect in vivo. The gradual change of the protonation state by single points without optimization or equilibration results in a change that is even larger, namely up to about −2.6 V. Thus, the preferred protein conformation supports a high redox potential, compensating for the RP-lowering effect of surface charges. The predicted change in RP on going to the F463M mutant, ca. −0.1 V, is consistent with observations for a related laccase. Based on our results, we also propose and test a D206N mutant but find it to be locked in a conformation with slightly lower RP.