CYTOCHROME bc 1 : A BACTERIAL CYTOCHROME 389
as an electron transfer agent, and its reduced alcohol partner, ubiquinol,
coQH 2 , is a source of protons for proton transport through respiratory chain
components (see Figure 7.27 ). At the same time as electrons are transferred,
a proton gradient is generated across the mitochondrial membrane through a
reaction sequence such as
CoQH 2 +→++ 222 Fe -cytochrome bc+++^31 CoQ Fe -cytochrome bc^21 H (7.9)Two active sites within cytochrome bc 1 are required: (1) the Q o site where
ubiquinol is oxidized and protons are released to the intermembrane space
and (2) the Q i site where ubiquinone is reduced and protons are taken up from
the matrix side of the membrane. The mechanism requires that electrons be
transferred from the Q o site to the Q i site. The Q o site is located between the
iron – sulfur protein (ISP) and cytochrome b subunits of the cytochrome bc 1
complex, closer to the intermembrane space. The Q i site is located wholly
within the cytochrome b subunit, closer to the matrix side of the membrane.
Both sites communicate with aqueous phases using channels within the protein
complex. The ISP and cytochrome b subunits are described in detail below in
Sections 7.6.2 and 7.6.3. The translocation of protons across the mitochondrial
inner membrane from the matrix space (negative or n side) to the intermem-
brane space (positive or p side) contributes to the electrochemical proton
gradient that drives adenosine triphosphate (ATP) synthesis.^79 The coupling
of transmembrane electrochemical proton gradients to ATP synthesis was fi rst
proposed by Mitchell in his chemiosmotic hypothesis.^80 More recently, cyto-
chrome bc 1 ’ s electron transport coupled to proton translocation mechanism
has been cited as a version of Mitchell ’ s “ proton - motive Q cycle. ”^81 During
each iteration of the Q cycle, two electrons are transferred for every four
protons translocated. In addition to its respiratory function, multifunctional
cytochrome bc 1 is also involved in mitochondrial import protein processing.
This major mitochondrial processing activity removes presequences from
nuclear encoded precursor proteins.^82
7.6.2 Cytochrome bc 1 Structure,
Bovine heart cytochrome bc 1 (PDB: 1BE3 and PDB: 1BGY) as studied by
Iwata et al.^83 exists as a dimer in the asymmetric unit cell. Each monomer
consists of 11 different polypeptide subunits (SU) with a total of∼ 2165 amino
acid residues and a molecular mass of∼ 240 kDa. The protein subunits of the
complex occupy three separate regions: (1) the intermembrane space (p side)
occupied by cytochrome c 1 (subunit 4, SU4), the iron – sulfur protein (ISP, SU5)
and subunit 8; (2) the transmembrane region occupied by cytochrome b (SU3),
the transmembrane helices of cytochrome c 1 and the ISP, and subunits 7, 10,
and 11; and (3) the matrix space (n side) occupied by two large core proteins
(subunits 1 and 2) as well as subunits 6 and 9. Subunit 8 is often called the
“ hinge protein ” and is thought to be essential for proper complex formation
between cytochrome c (the exit point for some bc 1 complex electrons) and