CLINICAL CASES
151
and passes the electron pair through the intervening carriers to complex IV,
which passes the electrons to one atom of molecular oxygen
(E'0 =
+0.82 V) to
form water with hydrogen ions (H+) from the medium.
The transport of electrons through the carriers is highly coupled to the for-
mation of ATP from ADP and P, through the formation and relaxation of the
proton gradient formed across the inner mitochondrial membrane by elec-
tron transport. Each time electrons are transported between complexes I
and III, between complexes III and IV, or between complex IV and oxygen,
protons are extruded from the mitochondrial matrix across the inner mem-
brane to the intermembrane space/cytosol. (The outer membrane poses no bar-
rier to proton passage.) In other words, the energy gained from these electron
transfers is used to pump protons from the mitochondrial matrix side to the
cytosol side. Because the mitochondrial membrane is impermeable to protons,
there is a gradient that develops with a higher concentration of protons outside
the matrix. The protons then come through the ATP synthase complex through
proton pores, and as they come back into the mitochondrial matrix, ADP is
phosphorylated to ATP. Thus, because the process of electron transport is
tightly coupled to ADP phosphorylation, ADP must be present for electron
transport to proceed, and therefore the ADP/ATP translocase must be able to
exchange a molecule of ADP in the cytosol for a molecule of ATP (newly
made) in the matrix of the mitochondria. When these various processes oper-
ate in concert the mitochondria are said to exhibit coupled respiration.
The components of the electron transport chain have various cofactors.
Complex I, NADH dehydrogenase, contains a flavin cofactor and iron sulfur
centers, whereas complex III, cytochrome reductase, contains cytochromes b
and c1. Complex IV, cytochrome oxidase, which transfers electrons to oxygen,
contains copper ions as well as cytochromes a and a3. The general structure of
the cytochrome cofactors is shown in Figure 16-2. Each of the cytochromes has
a heme cofactor but they vary slightly. The b-type cytochromes have proto-
porphyrin IX, which is identical to the heme in hemoglobin. The c-type
cytochromes are covalently bound to cysteine residue 10 in the protein. The a-type
Figure 16-2. Heme active center of cytochromes a, b, and c components of
electron transport chain.
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