CYTOCHROME bc 1 : A BACTERIAL CYTOCHROME 395
Both the N - and C - termini are located in the matrix. The two hemes are
bound within the four helix bundle of helices A - B - C - D. The iron ion in heme
bL carries axial his83 (helix B, Fe – N e2 = 2.08 Å ) and his182 (helix D, Fe – N e2 =
2.06 Å ) ligands, while the iron ion in heme b H carries his97 (helix B, Fe – N e2 =
2.07 Å ) and his196 (helix D, Fe – N e2 = 2.05 Å ). These histidines are invariant in
sequences of cytochrome b as well as in cytochrome b 6 as described for cyto-
chrome b(6)f above.
The Iwata group^83 compared results for their bovine cytochrome bc 1 complex
structures (PDB: 1BE3 and 1BGY) with those completed previously for
bovine bc 1 (PDB: 1QCR)^79 and for chicken heart bc 1 (PDB: 1BCC and 3BCC).^84
The comparisons led the researchers to propose three positional states for the
iron – sulfur protein (ISP): (1) The “ c 1 ” state in which the ISP is closer to heme
c 1 and further away from heme b L ; (2) the “ int ” state in which the ISP is in an
intermediate position with respect to heme c 1 and heme b L ; and (3) the “ b ”
state in which the ISP is closer to heme b L and further away from heme c 1. In
Table 7.6 , X - ray crystallographic structures are classifi ed according to their ISP
position or “ state. ” However, see the qualifi ng statements in conclusions at the
end of Section 7.6.4.
7.6.4 The Cytochrome bc 1 Q Cycle,
The proton - motive Q - cycle model, put forward by Mitchell (references 80 and
81 ) and by Trumpower and co - workers,^77 is invoked in the following manner:
(1) One electron is transferred from ubiquinol (ubiquinol oxidized to ubisemi-
quinone; see Figure 7.27 ) to the Rieske [2Fe – 2S] center at the Q o site, the site
nearest the intermembrane space or p side; (2) this electron can leave the bc 1
complex via an attached cytochrome c or be transferred to cytochrome c 1 ; (3)
the reactive ubisemiquinone reduces the low - potential heme b L located closer
to the membrane ’ s intermembrane (p) side; (4) reduced heme b L quickly
transfers an electron to high - potential heme b H near the membrane ’ s matrix
side; and (5) ubiquinone or ubisemiquinone oxidizes the reduced b H at the Q i
site nearest the matrix or n side. Proton translocation results from the depro-
tonation of ubiquinol at the Q o site and protonation of ubisemiquinone at the
Qi site. Ubiquinol generated at the Q i site is reoxidized at the Q o site (see
Figure 7.27 ). Additional protons are transported across the membrane from
the matrix (see Figure 7.26 illustrating a similar process for cytochrome b(6)f).
The overall reaction can be written
QH 2 +− 2224 Fe++++^32 cytochromecc+→+−Hin Q Fe cytochrome +Hout (7.10)It should be noted that generation of the radical ubisemiquinone or any
semiquinone (QH − • ) can have dangerous physiological consequences, such as
producing the tissue - damaging superoxide ion, O 2 −. A. R. Crofts has written a
review recently that expands upon known cytochrome bc 1 structure – function
relationships.^87 The article reviews knowledge about genetic defects that lead
to mutations in respiratory chain proteins including bc 1 complex. These muta-