Molecular basis of plastoquinone reduction in plant cytochrome bf.

A multi-subunit enzyme, cytochrome bf (cytbf), provides the crucial link between photosystems I and II in the photosynthetic membranes of higher plants, transferring electrons between plastoquinone (PQ) and plastocyanin. The atomic structure of cytbf is known, but its detailed catalytic mechanism remains elusive. Here we present cryogenic electron microscopy structures of spinach cytbf at 1.

High-resolution cryo-EM structures of plant cytochrome bf at work.

Plants use solar energy to power cellular metabolism. The oxidation of plastoquinol and reduction of plastocyanin by cytochrome bf (Cyt bf) is known as one of the key steps of photosynthesis, but the catalytic mechanism in the plastoquinone oxidation site (Q) remains elusive. Here, we describe two high-resolution cryo-EM structures of the spinach Cyt bf homodimer with endogenous plastoquinones and in complex with plastocyanin.

Unexpected Heme Redox Potential Values Implicate an Uphill Step in Cytochrome .

Cytochromes , key enzymes of respiration and photosynthesis, contain a highly conserved two-heme motif supporting cross-membrane electron transport (ET) that connects the two catalytic quinone-binding sites (Q and Q). Typically, this ET occurs from the low- to high-potential heme , but in photosynthetic cytochrome , the redox midpoint potentials (s) of these hemes remain uncertain. Our systematic redox titration analysis based on three independent and comprehensive low-temperature spectroscopies (continuous wave and pulse electron paramagnetic resonance (EPR) and optical spectroscopies) allowed for unambiguous assignment of spectral components of hemes in cytochrome and revealed that of heme is unexpectedly low.