4. ENERGY-RICH COMPOUNDS
161
an imidazole residue is located at, or near, the active site of this enzyme (418-420). The possible role of imidazole in phosphate transfer
reactions has also been discussed (415).
To date, the net synthesis of acetyl imidazole has been demonstrated
only with an enzyme preparation from Clostridium kluyveri. It is of
interest that experiments with imidazole-containing compounds have
shown that only urocanate (48) can replace imidazole in Reaction 73.
From these considerations, it would appear that acyl imidazoles, like
acyl adenylates, probably function as intermediate carriers of the "energy-rich" linkage in enzymatic reactions, but that they do not serve as
major energy reservoirs in the cell.
C. SUMMARY: DISTRIBUTION OF "ENERGY-RICH" COMPOUNDS
The distribution of "energy-rich" compounds in microbial, plant, and
animal cells is so nearly universal that, with the exception of viruses
(421) and perhaps rickettsia, possession of these compounds can be
considered a basic attribute of all forms of life. Since the major biological processes (e.g., replication, growth, differentiation, maintenance, and aging) involve changes in energy, it is to be expected that
cells should contain compounds capable of acting as energy reservoirs
and enzyme systems for the utilization of these compounds. The "energy-rich" compounds may be categorized into: (a) energy reservoirs;
(b) primary phosphorylating agents; and (c) transient intermediates
in phosphorylating reactions, as shown in Fig. 13. Certain of the compounds which may serve to generate ATP (PEP and glyceryl phosphate) appear to be universally distributed, as is ATP, the primary
phosphorylating agent. Other compounds which act as storage forms
(inorganic poly phosphates, carboxyl phosphates, and amidine phosphates) appear to have a specialized distribution, while acyl thioesters,
acyl adenylates, S-phosphoryl-CoA and acyl imidazoles are probably
formed by most cells as intermediates in ATP-dependent reactions.
The relationships shown in Fig. 13 are consistent with the fact that
the AF° of hydrolysis for ATP is lower than that of most other "energyrich" compounds (cf. Table I). It follows from equilibrium considerations alone that there would be little tendency for ATP to interact with
the precursor of another "energy-rich" compound, unless this endergonic reaction could be coupled with a second reaction, exergonic in
nature. Thus, the energy of the system will tend to remain in the primary storage compound, ATP, until a specific demand for another
"energy-rich" compound makes possible the endergonic transfer.
The universal distribution of ATP is illustrated by its isolation from
the following diverse sources: higher plants (422, 423) algae (303),
161
an imidazole residue is located at, or near, the active site of this enzyme (418-420). The possible role of imidazole in phosphate transfer
reactions has also been discussed (415).
To date, the net synthesis of acetyl imidazole has been demonstrated
only with an enzyme preparation from Clostridium kluyveri. It is of
interest that experiments with imidazole-containing compounds have
shown that only urocanate (48) can replace imidazole in Reaction 73.
From these considerations, it would appear that acyl imidazoles, like
acyl adenylates, probably function as intermediate carriers of the "energy-rich" linkage in enzymatic reactions, but that they do not serve as
major energy reservoirs in the cell.
C. SUMMARY: DISTRIBUTION OF "ENERGY-RICH" COMPOUNDS
The distribution of "energy-rich" compounds in microbial, plant, and
animal cells is so nearly universal that, with the exception of viruses
(421) and perhaps rickettsia, possession of these compounds can be
considered a basic attribute of all forms of life. Since the major biological processes (e.g., replication, growth, differentiation, maintenance, and aging) involve changes in energy, it is to be expected that
cells should contain compounds capable of acting as energy reservoirs
and enzyme systems for the utilization of these compounds. The "energy-rich" compounds may be categorized into: (a) energy reservoirs;
(b) primary phosphorylating agents; and (c) transient intermediates
in phosphorylating reactions, as shown in Fig. 13. Certain of the compounds which may serve to generate ATP (PEP and glyceryl phosphate) appear to be universally distributed, as is ATP, the primary
phosphorylating agent. Other compounds which act as storage forms
(inorganic poly phosphates, carboxyl phosphates, and amidine phosphates) appear to have a specialized distribution, while acyl thioesters,
acyl adenylates, S-phosphoryl-CoA and acyl imidazoles are probably
formed by most cells as intermediates in ATP-dependent reactions.
The relationships shown in Fig. 13 are consistent with the fact that
the AF° of hydrolysis for ATP is lower than that of most other "energyrich" compounds (cf. Table I). It follows from equilibrium considerations alone that there would be little tendency for ATP to interact with
the precursor of another "energy-rich" compound, unless this endergonic reaction could be coupled with a second reaction, exergonic in
nature. Thus, the energy of the system will tend to remain in the primary storage compound, ATP, until a specific demand for another
"energy-rich" compound makes possible the endergonic transfer.
The universal distribution of ATP is illustrated by its isolation from
the following diverse sources: higher plants (422, 423) algae (303),