An atypical haem in the cytochrome b6f complex.

Photosystems I and II (PSI and II) are reaction centres that capture light energy in order to drive oxygenic photosynthesis; however, they can only do so by interacting with the multisubunit cytochrome b₆f complex. This complex receives electrons from PSII and passes them to PSI, pumping protons across the membrane and powering the Q-cycle. Unlike the mitochondrial and bacterial homologue cytochrome bc₁, cytochrome b₆f can switch to a cyclic mode of electron transfer around PSI using an unknown pathway. Here we present the X-ray structure at 3.1?Å of cytochrome b₆f from the alga Chlamydomonas reinhardtii. The structure bears similarities to cytochrome bc₁ but also exhibits some unique features, such as binding chlorophyll, ß-carotene and an unexpected haem sharing a quinone site. This haem is atypical as it is covalently bound by one thioether linkage and has no axial amino acid ligand. This haem may be the missing link in oxygenic photosynthesis. [ABSTRACT FROM AUTHOR]