BIOINORGANIC CHEMISTRY A Short Course Second Edition

The following description of the electron transfer – proton transport scheme
is illustrated in Figure 7.26. First, an electron is transferred from doubly
reduced dihydroplastoquinone (PQH 2 ) to a high potential electron transfer
chain that consists of the Reiske iron – sulfur protein and the cytochrome
protein containing heme f. Rappaport, Lavergne and co - workers have reported
a midpoint potential at pH 7.0 of +355 mV for heme f.^76 These two centers
reside on the electropositive (lumen or p) side of the membrane, exterior to
the membrane. As a result, two protons are transferred to the aqueous lumen
phase. A second electron is transferred from PQH 2 sequentially to heme b p ,

Figure 7.26 Electron transfer and proton transport in cytochrome b(6)f: cofactors
(PDB: 1VF5). Visualized using The PyMOL Molecular Graphics System and
ChemDraw Ultra, version 10.0. (Printed with permission of Delano Scientifi c, LLC
and CambridgeSoft Corporation.) (See color plate)

H+

e-

e- e-

e-

heme f

heme bp

heme bn

chlorophyll a

heme x

TDS

Cytochrome b(6)f

PDB: 1VF5

Electron transfer

Proton transport

lumen (p) side

stromal (n) side

e-

to plastocyanin or cytochrome c 6

28.7 Å

29.1 Å

20.4 Å

β-carotene

TDS = tridecylstigmatellin

PQ = plastoquinone

PQH 2 = plastoquinol

PQH 2

2H+

PQ

2Fe-2S

CYTOCHROME b(6)f: A GREEN PLANT CYTOCHROME 385