Symmetry and Asymmetry

(i) Symmetry: the structure of the dimeric b6 f complex is symmetric between the two monomers, with space groups P6122 and I222, respectively, for the complexes from crystals in the native state and in the presence of the quinone-analog inhibitors, TDS and NQNO, of the cyanobacterium, M. laminosus [6] and for crystals grown in the presence of TDS for the b6 f complex from the green alga, C. reinhardtii [8];

7.1 Structure of the Cytochrome b6f Comples; Comparison with the Cytochrome bc, Comples | 159

Table 7.1 (A) Standard and (B, C) modified Q-cycle reactions in the cytochrome b6f complex; modifications in B, C arises from participation of heme cn; in (C), a modification entails stromal n-side electron donation by ferredoxin in a PSI-linked cyclic electron transport pathway.

A1. p-side quinol oxidation

PQH2 + FeS (o) ^ PQ- + FeS (r) + 2H+ PQ- + bp (o) ^ PQ + bp (r)

A2. high potential chain

cyt f (r) + Pc (ox) ^ cyt f (o) + Pc (r); in low Cu milieu, cyt c6 substitutes for Pc Pc (r) + P700 (ox) ^ Pc (o) + P700 (r)

A3. transmembrane p- to »-side electron transfer heme bp (red) + heme bn (ox) ^ bp (ox) + bn (red)

A4. »-side 2x1 electron reduction of PQ (as in bc1 complex)

bn (r) + PQ (o) ^ bn (o) + PQ- heme bn (r) + PQ- + 2 H+ ^ bn (o) + PQH2 (bn supplies second electron after it is reduced following second p-side turnover of PQH2)

B. »-side 2 electron reduction of bound PQ using two electrons from p-side; intermonomer pqh2

(i) bn (r) /cn (r) /PQ (o) + 2H+ ^ bn (o)/Cn (o) /PQH2

(bn/cn supply two electrons cooperatively to PQ after each heme is reduced by two lumenal p-side turnovers of PQH2- not shown)

(ii) bn (o)/cn (o) /PQH2 ^ bn (o)/c„ (o) + intermonomer PQH2

C. »-side 2 electron reduction of PQ by one electron from p-side, 1 from Fd (»-side)

(i) heme bp (red) + heme bn (ox) ^ bp (ox) + bn (red)

(iii) Fd (r) + bn(o)/c„(r)/PQ(o) ^ Fd(o) + bn(r)/c„ (r)/PQ(o)

(v) bn (o)/cn(o) /PQH2 ^ bn (o)/cn(o) + intermonomer PQH2

(ii) Asymmetry: The surface potentials in the M. laminosus complex that result from an asymmetric lumenal p- versus stromal n- side distribution of charged residues is -5.4kT (in M. laminosus) and +4.6kT, respectively, on the lumenal p- and stromal n-sides ofthe complex [1].

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