402 IRON-CONTAINING PROTEINS AND ENZYMES
the group included the inhibitor myxothiazol (PDB: 1SQP, resolution 2.7 Å ).
Myxothiazol, MYX (see Figure 7.29 ), belongs to class Ia group of inhibi-
tors — that is, it binds in the Q o site and has chemical characteristics placing
it with others of the “ a ” subgroup that contain a β - methoxyacrylate or
similar entity. Table 1 of reference 88 includes descriptions of a variety of bc 1
inhibitors, classifi ed by the authors into categories based on bc 1 - binding site,
induction of ISP mobility by the inhibitor, and inhibitor structural character-
istics. Only two of these inhibitor – enzyme complexes, those with inhibitors
myxothiazol (Xia class P m — inducing mobility of the ISP) and stigmatellin
A (Xia class P f — inducing a fi xed ISP position), will be discussed here.
Pharmacologically, myxothiazol is an antifungal agent produced by myxobac-
teriumM. fulvus. It is widely used as an inhibitor of cellular respiration.
Chemically, it contains a methoxyacrylamide group (similar to the character-
isticβ - methoxyacrylate group often used in cytochrome bc 1 complex inhibitor
studies) and a dithiazole moiety. (See Figure 7.29 .) As stated previously, myxo-
thiazol, MYX, increases the mobility of the ISP protein (P m group). It prevents
electron transfer from ubiquinol to the ISP and causes red shifts in theα - and
β - bands of the reduced heme b L UV – visible spectrum. In the Q o pocket, myxo-
thiazol is held in place by aromatic – aromatic (Ar – Ar) stacking interactions
between a thiazole ring and the side chain of phe274. Additionally, there are
hydrogen bonds between the O 1 of the methoxyacrylamide moiety and the
backbone amide of glu271 (O 1 · · · N = 2.84 Å ) and the N 1 of the methoxyacryl-
amide moiety and the hydroxyl group of tyr273 (N 1 · · · O = 3.1 Å ). The glu271 –
MYX interaction is conserved in all class Ia inhibitors. Both glu271 and tyr273
belong to the PEWY sequence, highly conserved in cytochrome b (SU 4 of
cytochrome bc 1 ) proteins. This sequence is located between helices E and F in
the EF loop. (See Figure 7.28 .) In bovine bc 1 , the PEWY sequence is composed
of pro270 – glu271 – trp272 – tyr273. Mutations in the PEWY sequence are known
to block electron transfer and quinol binding at the Q o site. Resistance to
cytochrome bc 1 inhibitors may also be related to changes in the PEWY
sequence. The closest approach of MYX to a heme metal ion occurs between
the N 1 position of MYX and the iron ion in heme b L (N 1 · · · Fe = 10.1 Å ) and
the shortest distance between hem b L and any portion of MYX is 5.4 Å.
The Xia group also studied the binding of the inhibitor stigmatellin A
(SMA) (PDB: 1SQX, resolution 2.5 Å ), a class Ib inhibitor. Class Ib inhibitors
have a chromone ring system (see Figure 7.29 ), inhibit electron transfer from
cytochrome c 1 to the ISP, cause an increased redox potential in the ISP, decrease
the ISP mobility (P f group), and, like class Ia inhibitors, cause a red shift in
the reduced heme b L UV – vis spectrum. Stigmatellin (SMA) is a natural anti-
fungal agent produced by myxobacteriaS. aurantiaca. This inhibitor will not
bind to cytochrome bc 1 in the absence of the ISP. The protonated N ε 2 atom of
his161 in the ISP forms two hydrogen bonds with SMA ’ s methoxy oxygen
(O 5 · · · N ε 2 = 3.6 Å ) and its carbonyl group (O 4 · · · N ε 2 = 3.0 Å ). Part of the chro-
mone ring intercalates between pro270 and ile146. A strong hydrogen bond is