Characterization of the High-Spin Heme x in the Cytochrome b₆f Complex of Oxygenic Photosynthesis

Abstract

X-ray structures at 3.0-3.1 Å resolution of the cytochrome b 6 f complex from the cyanobacterium

The c i / b H moiety in the b 6 f complex studied by EPR: A pair of strongly interacting hemes

“…The spectrum contains contributions from the low-spin hemes f and b (g z peaks in the spectral region labeled 2) and from a small fraction of reduced Rieske center (spectral region 3) in addition to several strong lines in the g ϭ 6 region (region 1). The positions of the latter signals cover the range of those reported for the Mastigocladus enzyme in the untreated state and attributed to heme c i (4). High-spin hemes typically have superimposed g x and g y signals at g ϭ 6 (for axial symmetry) or two separate signals around g ϭ 6 (in the case of a rhombic center).…”

“…The paramagnetic properties of heme c i , by contrast, are not understood at present. EPR signals in the g ϭ 6 region have been interpreted to arise from heme c i on the basis of their sensitivity to potential exogenous sixth ligands of the putatively 5-coordinated heme (4). The number and field positions of observed lines, however, cannot be rationalized by assuming a single high-spin heme c i .…”

A semiquinone intermediate generated at the Q o site of the cytochrome bc 1 complex: Importance for the Q-cycle and superoxide production

“…The first electron carrier in the low potential chain is a b-type heme, termed cyt b L , which reduces the somewhat higher potential cyt b H , which in turn equilibrates with a Q/SQ couple bound at the quinone reductase (Q i ) site on the negatively charged side (i.e., the n-side) of the membrane, opposite the Q o site. [The cyt b 6 f ( [16][17][18] and likely the bc complexes of certain bacteria possess an additional c-type heme in the Q i site that may participate in reduction of Q.] After two rounds of the bifurcated reaction, two electrons are accumulated on the low potential chain, which fully reduce Q to QH 2 at the Q i site, with uptake of two protons from the n-side of the membrane.…”

Energy Conductance from Thylakoid Complexes to Stromal Reducing Equivalents

“…Since type 2 complexes are structurally much simpler than complex I (one subunit with a single flavin cofactor compared with at least 11 protein subunits, nine FeS clusters, and a flavin), Nda2 may be less efficient in energy balancing (Kramer & Evans, 2006). The Cyt b 6 f complex and FNR route uses the PQ reductase site of the Cyt b 6 f complex to reduce PQ (Zhang et al, 2004). Electron transfer to Q i probably involves the newly discovered heme c i , which allows electrons to flow from Fd or FNR to the bound PQ (Zhang et al, 2004).…”