ELECTRON AND PROTON TRANSFER IN THE QUINONE-CYTOCHROME B/F REGION OF CHLOROPLASTS (Q-CYCLE)

MARK EDWARD GIRVIN, Purdue University

Abstract

The goal of this thesis research was to determine the pathway of electron transfer through the cytochrome b(,6)/f region of the chloroplast electron transport chain. NMR and UV/visible spectroscopy were used to identify the location and extent of transmembrane motion of quinones, the physiological donor to the cytochrome b(,6)/f complex, in membrane vesicles. The long chain quinones typically found in electron transfer chains are located in the center of the lipid bilayer, and do not approach the membrane surface. Their role in electron transport appears to be the transfer of reducing equivalents across only the hydrophobic core of the bilayer, and not from one side of the membrane to the other as previously thought. Measurements of the slow electrochromic phase and cytochrome b(,6) turnover under defined redox conditions showed that a Q cycle does not operate in chloroplasts. The slow electrochromic phase arises either from the Rieske FeS center acting as a redox proton pump, or from diffusion of plastosemiquinone across the membrane dielectric. The two hemes of the cytochrome b(,6) do not appear capable of transferring reducing equivalents between one another and seem to function independently. The outer heme functions in cyclic electron flow, transferring electrons from reduced ferredoxin to the PQ pool. The inner heme of the cytochrome may be a semiquinone dismutase.

Degree

Ph.D.

Subject Area

Biophysics

COinS