10. ELECTRON TRANSPORT AND OXIDATIVE PHOSPHORYLATION
467
chromes a 3 , a, and c become oxidized while cytochrome b, the flavoprotein of the chain, and pyridine nucleotide become reduced. Thus, antimycin A is believed to be reacting either with reduced cytochrome b or
oxidized cytochrome c to prevent these two carriers from interacting.
In tightly coupled mitochondria, electron transfer becomes inhibited
when the supply of phosphate acceptor (ADP) is depleted. Under these
conditions, Chance and Williams have observed a cross over point between cytochrome c and cytochrome a. When low concentrations of
azide are present, another cross over point occurs between cytochromes
b and c. Finally, although there is extensive reduction of all the components between substrate and cytochrome in these mitochondria when
the supply of ADP becomes limiting, it is found that DPNH is more
than 99% reduced. Since this degree of reduction is far more than that of
the other carriers, Chance and Williams proposed that a third cross over
point exists between DPNH and flavoprotein. The cross over points are
considered sites of energy conservation or oxidative phosphorylation (see
Section III, G).
In the vast majority of the studies on phosphorylation associated with
the respiratory chain pyruvate, one of the substrates of the Krebs cycle,
cytochrome c, or DPNH has served as the hydrogen donor. In
addition ß-hydroxybutyrate (145, 146), glutamate (141, 146), proline
(141), and fatty acids (141, 150) when added to mitochondria will
support oxidative phosphorylation. The dehydrogenases for these reactions are known to be localized in the mitochondrion, and the dehydrogenases for /?-hydroxybutyrate, glutamate, and thioesters of ß-hydroxy
fatty acids are known to require pyridine nucleotides. In the case of the
acyl-CoA dehydrogenases which catalyze the oxidation of thioesters of
the fatty acids, the respiratory sequence presumably would be entered at
the flavoprotein level and two phosphorylations would occur. There is
no direct experimental evidence on this point, however.
The question of whether substrates other than these can give rise to
oxidative phosphorylation is, of course, of interest. If a dehydrogenase
for the substrate in question is localized on the mitochondria, the oxidation of the substrate may well yield high-energy phosphate. Moreover,
the results of previous work suggest that, under proper conditions,
soluble dehydrogenase systems may also support phosphorylation. The
number of reactions investigated is small, however, and only the triosephosphate dehydrogenase system functioned successfully in this regard.
G. MECHANISM OF OXIDATIVE PHOSPHORYLATION
Although the phosphorylations associated with the respiratory chain
have been extensively studied during the past decade, there is little
understanding of the precise mechanism accounting for the formation
467
chromes a 3 , a, and c become oxidized while cytochrome b, the flavoprotein of the chain, and pyridine nucleotide become reduced. Thus, antimycin A is believed to be reacting either with reduced cytochrome b or
oxidized cytochrome c to prevent these two carriers from interacting.
In tightly coupled mitochondria, electron transfer becomes inhibited
when the supply of phosphate acceptor (ADP) is depleted. Under these
conditions, Chance and Williams have observed a cross over point between cytochrome c and cytochrome a. When low concentrations of
azide are present, another cross over point occurs between cytochromes
b and c. Finally, although there is extensive reduction of all the components between substrate and cytochrome in these mitochondria when
the supply of ADP becomes limiting, it is found that DPNH is more
than 99% reduced. Since this degree of reduction is far more than that of
the other carriers, Chance and Williams proposed that a third cross over
point exists between DPNH and flavoprotein. The cross over points are
considered sites of energy conservation or oxidative phosphorylation (see
Section III, G).
In the vast majority of the studies on phosphorylation associated with
the respiratory chain pyruvate, one of the substrates of the Krebs cycle,
cytochrome c, or DPNH has served as the hydrogen donor. In
addition ß-hydroxybutyrate (145, 146), glutamate (141, 146), proline
(141), and fatty acids (141, 150) when added to mitochondria will
support oxidative phosphorylation. The dehydrogenases for these reactions are known to be localized in the mitochondrion, and the dehydrogenases for /?-hydroxybutyrate, glutamate, and thioesters of ß-hydroxy
fatty acids are known to require pyridine nucleotides. In the case of the
acyl-CoA dehydrogenases which catalyze the oxidation of thioesters of
the fatty acids, the respiratory sequence presumably would be entered at
the flavoprotein level and two phosphorylations would occur. There is
no direct experimental evidence on this point, however.
The question of whether substrates other than these can give rise to
oxidative phosphorylation is, of course, of interest. If a dehydrogenase
for the substrate in question is localized on the mitochondria, the oxidation of the substrate may well yield high-energy phosphate. Moreover,
the results of previous work suggest that, under proper conditions,
soluble dehydrogenase systems may also support phosphorylation. The
number of reactions investigated is small, however, and only the triosephosphate dehydrogenase system functioned successfully in this regard.
G. MECHANISM OF OXIDATIVE PHOSPHORYLATION
Although the phosphorylations associated with the respiratory chain
have been extensively studied during the past decade, there is little
understanding of the precise mechanism accounting for the formation