LOW-TEMPERATURE ABSORBTION AND MAGNETIC CIRCULAR DICHROISM OF THE FOUR HAEMS OF THE CYTOCHROME b_6f COMPLEX

摘要

The cytochrome (cyt) b_6f complex is situated in the thylakoid membrane, where it mediates linear electron transfer between photosystems II and I and performs cyclic electron transfer. Both processes result in proton translocation across the membrane,creating an electrochemical gradient for ATP synthesis. Homologous to the mitochondrial cytochrome bc_1 complex, cyt b_6f incorporates cytochrome f (a c-type cytochrome) and an Fe_2S_2 protein, constituting the linear electron pathway from hydroplastoquinone to plastocyanin, and cytochrome b_6, containing the haem groups b_H and b_L involved in cyclic electron transfer. Unlike the mitochondrial complex, the b_6f complex also contains a chlorophyll (chl) a, a beta-carotene (car), and a fourth haem, haemc_i (reviewed by Allen (2004)). The presence of haem c_i at the stromal quinone binding site came as a surprise when the X-ray crystal structure was solved last year, for cyt b_6f purified from the unicellular green alga Chlamydomonas reinhardtii (Stroebel et al 2003) and from the thermophilic cyanobacterium Mastigodadus laminosus (Kurisu et al 2003), The Fe of haem c_i is penta-coordinate, with water (or hydroxide) the only axial ligand. This indicates a high-spin configuration of the haem, which would explain why it has not been predicted from previous spectroscopic studies.