EFFECTS OF TYR-83 NITROMODIFICATION AND PHE-82 PHENYL GROUP ROTATION ON ELECTRONIC TRANSMISSION COEFFICIENTS AT THE REMOTE ELECTRON-TRANSFER SITE OF HIGHER-PLANT PLASTOCYANINS

The blue single-copper plant protein plastocyanin (PC) has two electron-transfer (ET) paths, one via the exposed His-87 ligand, the other one through the protein and involving sites close to Tyr-83. We have investigated features of this pattern using ET theory and extended Huckel calculations. Prompted by previous observations that through-space Cu/Met-92/Phe-14/Phe-82 ET involving aromatic groups is unfavorable, the dynamics of this route was investigated. The Phe-82 ring was rotated from equilibrium almost perpendicular to the Phe-14 phenyl ring, to an almost parallel orientation. This leads to 50 times more facile ET, but the route remains highly unfavorable and conformational fluctuations appear unlikely to make it competitive. The dominating through-bond route is along Cu/Cys-84/Tyr-83 for which NO2 modification, most likely in the orthoposition, has provided strong indication. o-NO2-Tyr-83 modification was therefore investigated and found to lower the electronic transmission coefficient by 50%. This is in line with experimental observations that the transmission coefficients for PC(I)/cyt f(III) and PC(II)/cyt f(II) ET are both lowered by 25-50% on NO2 modification. In comparison, m-NO2 modification was found to increase the transmission coefficient by about 100%. Finally, the transmission coefficient for PC oxidation and reduction virtually coincide for both the Cu/His-87 and Cu/Cys-84/Tyr-83 routes. Electronic effects arising from the different Cu-site equilibrium configurations are therefore small.