10. ELECTRON TRANSPORT AND OXIDATIVE PHOSPHORYLATION
447
of the pyridine nucleotides is to serve as the first step in the pathway of
hydrogen and electron transport from substrate to oxygen.
B. OXIDATION-REDUCTION POTENTIALS
Further understanding of the nature of the cytochrome respiratory
chain developed when oxidation-reduction potentials of some of the
components were first determined. The values for the old yellow enzyme (32) (E 0 = —0.06 volt, pH 7.0) and cytochrome c (33)
(Eo = +0.27 volt) placed these carriers intermediate in value between
those of most of the organic substrates (metabolites) and that of
0 2 (E 0 — +0.810 volt, pH 7.0). Moreover, the value for DPN
+
(34)
(E 0 = —0.28, pH 7.0) was appropriate for a stepwise transfer of electrons or hydrogen from DPN
+ to the cytochromes (Scheme I).
DPN
+
Substrate —> ΤΡΝ
+ "~
> Flavoprotein —> Cytochromes —► 0 2
(I)
The electron transfer sequence assigned to the cytochromes is based
on the oxidation-reduction potentials for cytochrome c, b, and a determined by Ball (35) in 1938. These values (E 0 ; pH 7.0) were —0.04
volt for cytochrome b, +0.27 volt for cytochrome c, and +0.29 volt
for cytochrome a. Thus the cytochromes in Scheme I can be elaborated
to give Scheme II.
Cytochrome b —> Cytochrome c —> Cytochrome a —* O2
(II)
Keilin and Hartree's (36) observations on the complexity of the absorption bands attributed to cytochrome a resulted in further expansion
to the sequence shown in Scheme III.
Cytochrome b —> Cytochrome c —- > Cytochrome a —- > Cytochrome &$
(III)
The values of the oxidation-reduction potentials for the different
components of the cytochrome respiratory chain were re-examined
whenever better methods or purified samples of the carriers became
available. Some of the values recently published are listed in Table I.*
It is interesting to note that the sequence postulated 20 years ago (37)
from values available at that time is the same which would be indicated
by the most recent values.
C. CELLULAR LOCALIZATION OF THE RESPIRATORY CHAIN
1. Non-phosphorylative
Particles
a. The Keilin and Hartree Heart Muscle Preparation. (1) Oxidation
of Succinate. In 1940 Keilin and Hartree (36) described a cell-free tissue preparation from heart muscle which has been extensively used in
* See p. 459.
447
of the pyridine nucleotides is to serve as the first step in the pathway of
hydrogen and electron transport from substrate to oxygen.
B. OXIDATION-REDUCTION POTENTIALS
Further understanding of the nature of the cytochrome respiratory
chain developed when oxidation-reduction potentials of some of the
components were first determined. The values for the old yellow enzyme (32) (E 0 = —0.06 volt, pH 7.0) and cytochrome c (33)
(Eo = +0.27 volt) placed these carriers intermediate in value between
those of most of the organic substrates (metabolites) and that of
0 2 (E 0 — +0.810 volt, pH 7.0). Moreover, the value for DPN
+
(34)
(E 0 = —0.28, pH 7.0) was appropriate for a stepwise transfer of electrons or hydrogen from DPN
+ to the cytochromes (Scheme I).
DPN
+
Substrate —> ΤΡΝ
+ "~
> Flavoprotein —> Cytochromes —► 0 2
(I)
The electron transfer sequence assigned to the cytochromes is based
on the oxidation-reduction potentials for cytochrome c, b, and a determined by Ball (35) in 1938. These values (E 0 ; pH 7.0) were —0.04
volt for cytochrome b, +0.27 volt for cytochrome c, and +0.29 volt
for cytochrome a. Thus the cytochromes in Scheme I can be elaborated
to give Scheme II.
Cytochrome b —> Cytochrome c —> Cytochrome a —* O2
(II)
Keilin and Hartree's (36) observations on the complexity of the absorption bands attributed to cytochrome a resulted in further expansion
to the sequence shown in Scheme III.
Cytochrome b —> Cytochrome c —- > Cytochrome a —- > Cytochrome &$
(III)
The values of the oxidation-reduction potentials for the different
components of the cytochrome respiratory chain were re-examined
whenever better methods or purified samples of the carriers became
available. Some of the values recently published are listed in Table I.*
It is interesting to note that the sequence postulated 20 years ago (37)
from values available at that time is the same which would be indicated
by the most recent values.
C. CELLULAR LOCALIZATION OF THE RESPIRATORY CHAIN
1. Non-phosphorylative
Particles
a. The Keilin and Hartree Heart Muscle Preparation. (1) Oxidation
of Succinate. In 1940 Keilin and Hartree (36) described a cell-free tissue preparation from heart muscle which has been extensively used in
* See p. 459.