218
GEORGE G. LATIES
on the upper circle to the lower plane is directly proportional to the
energy released when two hydrogen atoms are removed from a substance
having the specified oxidation-reduction potential and are combined with
oxygen to form water. It is evident that with the exception of a-ketoglutarate oxidation the majority of the substrate transformations within
the tricarboxylic acid cycle occur with but small changes in energy, the
bulk of the energy of respiration being made available during the transport of hydrogen atoms from substrate to molecular oxygen. The number of high-energy phosphate bonds which from an energy point of view
may be comfortably made with the available energy of oxidation is indicated by the symbols "~ ph" in the path of each pair of hydrogen atoms.
Happily, this number agrees precisely with experimental observations.
Under normal conditions the relationship between the oxidative and
phosphorylative processes has been shown to be so intimate that oxidation stops in the absence of inorganic phosphate (Loomis and Lipmann,
1948; Rabinovitz et al, 1951; Lardy and Wellman, 1952). Oxidation is
likewise repressed in the absence of compounds which accept highenergy phosphate from the enzyme sites at which inorganic phosphate
is first transformed into the high-energy state (Lardy, 1952; Siekevitz
and Potter, 1953). Without these phosphate acceptors there is no regeneration of free enzyme sites, and the oxidative process is thereby
brought to a halt. The way in which phosphorylation takes place during
the oxidation of a-ketoglutarate to succinate, for example, is represented
in simplified form in Fig. 2.
Following the combination of substrate and enzyme, an oxidation
takes place which results in the original low-energy bond between substrate and enzyme being converted to a high-energy bond, the highenergy bond being represented as a curlicue in Fig. 2. The oxidized
product does not leave the enzyme surface, but rather is replaced by a
molecule of inorganic phosphate, the bond remaining a high-energy
bond. The high-energy phosphate molecule which arises in this way
is now transferred (in this case via an intermediate acceptor not shown;
Sanadi et al, 1954) to the most prevalent of phosphate acceptors, adenosine diphosphate (ADP), to yield adenosine triphosphate (ATP) along
with the enzyme in the original state. The oxidized product cannot be
removed from the enzyme without the intermediation of inorganic phosphate, and so the regeneration of the free enzyme is entirely dependent
upon, first, the presence of inorganic phosphate, and second, the presence
of a phosphate acceptor. In actual fact, one or more cofactors of oxidation may be intimately linked to the enzyme, and the substrate may
become attached to such cofactors. Furthermore, inorganic phosphate
may cause the removal of oxidized substrate from the enzyme by com-
GEORGE G. LATIES
on the upper circle to the lower plane is directly proportional to the
energy released when two hydrogen atoms are removed from a substance
having the specified oxidation-reduction potential and are combined with
oxygen to form water. It is evident that with the exception of a-ketoglutarate oxidation the majority of the substrate transformations within
the tricarboxylic acid cycle occur with but small changes in energy, the
bulk of the energy of respiration being made available during the transport of hydrogen atoms from substrate to molecular oxygen. The number of high-energy phosphate bonds which from an energy point of view
may be comfortably made with the available energy of oxidation is indicated by the symbols "~ ph" in the path of each pair of hydrogen atoms.
Happily, this number agrees precisely with experimental observations.
Under normal conditions the relationship between the oxidative and
phosphorylative processes has been shown to be so intimate that oxidation stops in the absence of inorganic phosphate (Loomis and Lipmann,
1948; Rabinovitz et al, 1951; Lardy and Wellman, 1952). Oxidation is
likewise repressed in the absence of compounds which accept highenergy phosphate from the enzyme sites at which inorganic phosphate
is first transformed into the high-energy state (Lardy, 1952; Siekevitz
and Potter, 1953). Without these phosphate acceptors there is no regeneration of free enzyme sites, and the oxidative process is thereby
brought to a halt. The way in which phosphorylation takes place during
the oxidation of a-ketoglutarate to succinate, for example, is represented
in simplified form in Fig. 2.
Following the combination of substrate and enzyme, an oxidation
takes place which results in the original low-energy bond between substrate and enzyme being converted to a high-energy bond, the highenergy bond being represented as a curlicue in Fig. 2. The oxidized
product does not leave the enzyme surface, but rather is replaced by a
molecule of inorganic phosphate, the bond remaining a high-energy
bond. The high-energy phosphate molecule which arises in this way
is now transferred (in this case via an intermediate acceptor not shown;
Sanadi et al, 1954) to the most prevalent of phosphate acceptors, adenosine diphosphate (ADP), to yield adenosine triphosphate (ATP) along
with the enzyme in the original state. The oxidized product cannot be
removed from the enzyme without the intermediation of inorganic phosphate, and so the regeneration of the free enzyme is entirely dependent
upon, first, the presence of inorganic phosphate, and second, the presence
of a phosphate acceptor. In actual fact, one or more cofactors of oxidation may be intimately linked to the enzyme, and the substrate may
become attached to such cofactors. Furthermore, inorganic phosphate
may cause the removal of oxidized substrate from the enzyme by com-