CYTOCHROME bc 1 : A BACTERIAL CYTOCHROME 397
value of 1.0, iron atoms in the other bc 1 subunits can be compared. For
instance, the presence of the inhibitor stigmatellin A in crystals increases the
stability (decreases the mobility) of the [2Fe – 2S] center in the ISP, whereas
the inhibitor myxothiazol decreases the stability (increases the mobility of the
ISP).^88 See Section 7.6.5 for a discussion of cytochrome bc 1 inhibitors. Other
experiments are needed to confi rm the movements of protein subunits in the
cytochrome bc 1 complex, especially the [2Fe – 2S] center, before the catalytic
mechanism described here is confi rmed.
A third, clearer explanation of the electron transfer, proton translocation
cycle is given by Saratse.^78 Each ubiquinol (QH 2 ) molecule can donate two
electrons. A fi rst QH 2 electron is transferred along a high - potential chain to
the [2Fe – 2S] center of the ISP and then to cytochrome c 1. From the cyto-
chrome c 1 site, the electron is delivered to the attached, soluble cytochrome c
in the intermembrane space. A second QH 2 electron is transferred to the Q i
site via the cytochrome b hemes, b L and b H. This is an electrogenic step driven
by the potential difference between the two b hemes. This step creates part of
the proton - motive force. After two QH 2 molecules are oxidized at the Q o site,
two electrons have been transferred to the Q i site (where one ubiquinone (Q 10 )
can now be reduced, requiring two protons to be translocated from the matrix
space). The net effect is a translocation of two protons for each electron trans-
ferred to cytochrome c. Each explanation of the cytochrome bc 1 Q cycle has
its merits and its proponents. The reader should consult the literature for
updates in this ongoing research area.
7.6.5 Cytochrome bc 1 Inhibitors,
Evidence in support of the Q cycle mechanism comes in many forms, including
oxidant - induced reduction of cytochrome b, existence of the antibiotic anti-
mycin - sensitive and - insensitive ubisemiquinone radicals, ejection of two
protons per electron transferred in bc 1 complexes, and binding of specifi c
inhibitors to sites Q i and Q o. We will concentrate on the effect of inhibitors in
this discussion. The understanding of inhibitor binding to cytochrome bc 1
complex is important to researchers who want to design new antifungal agents
as crop protection agents or wish to develop new disease - specifi c inhibitors
for use in animal or human populations. As part of the process, researchers
study the enzyme ’ s structure – function relationships or structure – activity rela-
tionships (SARs) as a means of understanding its mechanism of activity. The
process ’ s end game hopefully leads to design of new and better reagents for
disease treatment and cure.
Only a few of the many known cytochrome bc 1 complex inhibitors will be
mentioned in this chapter. Cytochrome bc 1 inhibitors that bind at the Q o site
(p side, near the intermembrane space) include myxothiazol (MYX) and stig-
matellin (SMA) (see Figure 7.29 ). MYX and SMA inhibit electron transfer
from ubiquinol (QH 2 ) to cytochrome c (outside the bc 1 complex) or to other
domains of the bc 1 complex. Antimycin A (see Figure 7.29 ), a fungicide, binds