10. ELECTRON TRANSPORT AND OXIDATIVE PHOSPHORYLATION
443
For example, the standard free energy change at pH 7.0 for the oxidation of DPNH by 0 2 is -52 kcal./mole (Eq. 4).
DPNH + H+ + i 0 2 -> DPN+ + H 2 0
(4)
The several ways in which the reduced carriers can be oxidized by
0 2 will be discussed. However, of greatest significance to the organism
are those pathways of oxidation in which free energy can be conserved
and made available for its use. When this consideration is taken into
account, predominant in interest is the electron transport pathway from
reduced nucleotide to 0 2 which involves flavin, the cytochromes, and
cytochrome oxidase. It is now well established that energy is conserved
by the formation of energy-rich phosphate bonds as electron transfer
occurs from reduced pyridine nucleotide to oxygen through the cytochrome chain. The comparative aspects of this process, which is termed
oxidative phosphorylation, will be discussed. Energy is also conserved
in the formation of acyl phosphate and thioester bonds during the oxi
dation of glyceraldehyde-3-phosphate and the α-keto acids pyruvate
and «-ketoglutarate. The mechanisms of these reactions are fairly well
understood but will not be discussed here, since the comparative biochemistry of glycolysis (2) and the Krebs cycle (3) are discussed elsewhere in this treatise.
At this point it is well to re-emphasize, as have Krebs and Kornberg,
the simplication inherent in the processes just described. Through these
processes, a large number of different organic compounds are first degraded to simpler units (hexose, amino acids, and fatty acids) and then
converted to three organic acids. Partly during this latter stage and
during the oxidation of the organic acids, a limited variety of reduced
carrier compounds (DPNH, TPNH, and reduced flavoprotein) are
formed. Finally, during the oxidation of these reduced carriers, free energy is released and made available to the organism in the form of energy-rich phosphate compounds. The means by which this last is accomplished is now discussed in some detail.
II. The Cytochrome Respiratory Chain
A. COMPONENTS OF THE CHAIN
1. Cytochromes
From the historical standpoint the first components of the cytochrome respiratory chain to be described were the cytochromes themselves, discovered and named "histohematins" by MacMunn (4) in
1886. In 1925 Keilin (5) confirmed MacMunn's discovery and showed
that at least three cellular pigments, "cytochromes," were widely dis-
443
For example, the standard free energy change at pH 7.0 for the oxidation of DPNH by 0 2 is -52 kcal./mole (Eq. 4).
DPNH + H+ + i 0 2 -> DPN+ + H 2 0
(4)
The several ways in which the reduced carriers can be oxidized by
0 2 will be discussed. However, of greatest significance to the organism
are those pathways of oxidation in which free energy can be conserved
and made available for its use. When this consideration is taken into
account, predominant in interest is the electron transport pathway from
reduced nucleotide to 0 2 which involves flavin, the cytochromes, and
cytochrome oxidase. It is now well established that energy is conserved
by the formation of energy-rich phosphate bonds as electron transfer
occurs from reduced pyridine nucleotide to oxygen through the cytochrome chain. The comparative aspects of this process, which is termed
oxidative phosphorylation, will be discussed. Energy is also conserved
in the formation of acyl phosphate and thioester bonds during the oxi
dation of glyceraldehyde-3-phosphate and the α-keto acids pyruvate
and «-ketoglutarate. The mechanisms of these reactions are fairly well
understood but will not be discussed here, since the comparative biochemistry of glycolysis (2) and the Krebs cycle (3) are discussed elsewhere in this treatise.
At this point it is well to re-emphasize, as have Krebs and Kornberg,
the simplication inherent in the processes just described. Through these
processes, a large number of different organic compounds are first degraded to simpler units (hexose, amino acids, and fatty acids) and then
converted to three organic acids. Partly during this latter stage and
during the oxidation of the organic acids, a limited variety of reduced
carrier compounds (DPNH, TPNH, and reduced flavoprotein) are
formed. Finally, during the oxidation of these reduced carriers, free energy is released and made available to the organism in the form of energy-rich phosphate compounds. The means by which this last is accomplished is now discussed in some detail.
II. The Cytochrome Respiratory Chain
A. COMPONENTS OF THE CHAIN
1. Cytochromes
From the historical standpoint the first components of the cytochrome respiratory chain to be described were the cytochromes themselves, discovered and named "histohematins" by MacMunn (4) in
1886. In 1925 Keilin (5) confirmed MacMunn's discovery and showed
that at least three cellular pigments, "cytochromes," were widely dis-