BIOINORGANIC CHEMISTRY A Short Course Second Edition

388 IRON-CONTAINING PROTEINS AND ENZYMES

7.6 Cytochrome bc 1 : A Bacterial Cytochrome,

7.6.1 Introduction,

Cytochrome bc 1 is a multicomponent enzyme found in the inner mitochron-
drial membrane of eukaryotes and in the plasma membrane of bacteria.^77 The
cytochrome bc 1 complex functions as the middle component of the mitochon-
drial respiratory chain, coupling electron transfer between ubiquinone/
ubiquinol (see Figure 7.27 ) and cytochrome c.
Other terminology used in referring to cytochrome bc 1 includes complex
III of the respiratory chain, or ubiquinol:cytochrome c oxidoreductase.
Complex III is one of fi ve essential enzymes of the mitochondrial inner
membrane involved in the oxidative phosphorylation process. In this process,
carried out in the cellular mitochondria, electrons are passed along through a
series of molecules called the electron transport chain. Electrons are generated
from NADH (reduced nicotinamide adenine dinucleotide) and ultimately
transferred to molecular oxygen. The passage of electrons releases energy
stored as a proton gradient across the mitochondrial membrane. The energy
released is used by ATP synthase to make ATP (adenosine triphosphate) from
ADP. Knowledge about the fi ve respiratory enzyme complexes has been sum-
marized by M. Saratse in a 1999 Science magazine publication.^78 Figure 1 of
reference 78 is a helpful cartoon diagram illustrating the fi ve enzyme com-
plexes of this respiratory pathway: (1) Complex I, NADH - dehydrogenase
(NADH - DH); (2) Complex II, succinate dehydrogenase (SDH); (3) Complex
III, cytochrome bc 1 (bc1); (4) Complex IV, cytochrome oxidase (COX); and
(5) Complex V, ATP synthase. Cytochrome c and cytochrome c oxidase will
be described in Sections 7.7 and in 7.8 , respectively.
In cytochrome bc 1 , ubiquinone, also known as coenzymeQ, coQ, or Q 10 , is
a lipid - soluble compound that can move within the membrane. It is involved

Figure 7.27 The ubiquinol/ubiquinone reaction.

• 2H+, - 2 e- H 3 CO

H 3 CO

O

O

(CH 2 -CH=C(CH 3 )-CH 2 ) 10 -H

H 3 CO

H 3 CO

OH

OH

(CH 2 -CH=C(CH 3 )-CH 2 ) 10 -H

H 3 CO

H 3 CO

O

OH

(CH 2 -CH=C(CH 3 )-CH 2 ) 10 -H

ubiquinol, CoQH 2 ubiquinone,Q^10 , coenzyme Q, coQ

ubisemiquinone, QH, radical intermediate

• H+
  , - e
  
  
  • 
    
  
  
  
  

• H
  +, + e

• 
  


• H

+, - e

+ H

+, + e

(2,3-dimethoxy-5-methyl-6-polyisoprenyl-1,4-benzoquinone)