DIRECT AND SUPEREXCHANGE ELECTRON-TUNNELING AT THE ADJACENT AND REMOTE SITES OF HIGHER-PLANT PLASTOCYANINS

The 42-45 and 59-61 residues of several higher plant plastocyanins are ionized at neutral pH. The resulting negative charges constitute a binding site for positively charged electron-transfer (ET) reactants even though the ET distances exceed the distance at the adjacent binding site close to the solvent-exposed His-87 ligand by 5-10 Å. We have calculated the exchange matrix elements, VDA. for ET between plastocyanin and the protonated pyridine anion radical as a probe for small reaction partners by superexchange and extended Hückel theory. Adjacent binding-site ET and four ET routes along different amino acid sequences from the copper atom toward the remote binding site through the protein are compared. One route, Cu/Met-92/Phe-14/Phe-82, is very inefficient (VDA = 1 × 10-9 cm-1) due to its through-space character and perpendicular phenyl group orientation. ET along two other routes, Cu/Met-92/Val-93/Gly-94 (VDA = 3 × 10-4 cm-1) and Cu/Cys-84/Tyr-83/Phe-82 (VDA = 6 × 10-4 cm-1), is much more facile, but only the Cu/Cys-84/Tyr-83 route (VDA = 6 × 10-2 cm-1) approaches the adjacent site efficiency (VDA = 0.4 cm-1), in line with many observations of a crucial role for Tyr-83. The results suggest that ET through the protein is dominated by selective through-bond pathways rather than simple geometric distance correlations and that remote site reactivity may rest on both favorable work terms and competitive electronic factors. © 1990, American Chemical Society. All rights reserved.