138
F. M. HUENNEKENS AND H. R. WHITELEY
gested to explain the formation of acetyl phosphate from the scission of
xylulose-5-phosphate (198); and could also be invoked for pyruvate
degradations not requiring lipoic acid. In the latter instance, phosphorolysis or transfer reactions would follow oxidation of the enzymebound "acetaldehyde-diphosphothiamine" complex, yielding acetyl phosphate or acetyl-CoA.
(3). Function. Acetyl phosphate has two major functions: it is a
reservoir of "energy-rich" phosphate, and is a precursor of acetyl-CoA.
The "energy-rich" bond is made available for ATP synthesis by reversal of Reaction 44. Especially in bacteria, acetyl phosphate and the
acetokinase system may be an important energy reserve for ATP synthesis.
Phosphotransacetylase facilitates an isoenergetic interconversion of
acetyl phosphate and acetyl-CoA. The latter compound is required for
numerous synthetic reactions, as will be discussed in Section ΙΙΙ,Β,Ι. It
is obvious also that Reaction 45 represents not only a mechanism for
conserving the "energy-rich" bond of acetyl-CoA, but also for releasing
CoA for further reactions.
In addition to generating ATP directly, acetyl phosphate can replace ATP as the phosphorylating agent in certain metabolic reactions
(199), as illustrated by the example in Eq. 46.
Acetyl phosphate + Glycerol ^=± Glycerol phosphate + Acetate
(46)
Transfer reactions of this type are frequently carried out by phosphatases [reviewed by Axelrod (200)].
In bacterial systems, the phosphorolytic cleavage of acyl-enzyme
complexes conserves the energy derived from an oxidative step. In
mammalian tissues, which lack both acetokinase and phosphotransacetylase, acetyl phosphate cannot be used to generate ATP. Furthermore, acetyl phosphate is rapidly hydrolyzed in mammalian tissues by
an acyl phosphatase (177, 201-202a), which, unlike the enzymes mediating Reaction 46, is incapable of transferring the phosphate group to
any acceptor other than water. However, thiolysis of acyl-enzyme complexes by CoA (for example, in the degradation of pyruvate by bacterial or animal systems) conserves the energy of oxidation.
Other carboxyl phosphates participate, or have been postulated as
intermediates, in various reactions: carbamyl phosphate* in the synthe* Carbamyl phosphate (CP) is synthesized in mammalian and frog liver by the
reaction (164a):
acetyl glutamate
2 ATP + NH, + C0 2 «
CP + 2 ADP + P t
(46a)
while in bacteria (164b) a slightly different system is found:
ATP + NH 3 + C0 2 ^ CP + ADP
(46b)
F. M. HUENNEKENS AND H. R. WHITELEY
gested to explain the formation of acetyl phosphate from the scission of
xylulose-5-phosphate (198); and could also be invoked for pyruvate
degradations not requiring lipoic acid. In the latter instance, phosphorolysis or transfer reactions would follow oxidation of the enzymebound "acetaldehyde-diphosphothiamine" complex, yielding acetyl phosphate or acetyl-CoA.
(3). Function. Acetyl phosphate has two major functions: it is a
reservoir of "energy-rich" phosphate, and is a precursor of acetyl-CoA.
The "energy-rich" bond is made available for ATP synthesis by reversal of Reaction 44. Especially in bacteria, acetyl phosphate and the
acetokinase system may be an important energy reserve for ATP synthesis.
Phosphotransacetylase facilitates an isoenergetic interconversion of
acetyl phosphate and acetyl-CoA. The latter compound is required for
numerous synthetic reactions, as will be discussed in Section ΙΙΙ,Β,Ι. It
is obvious also that Reaction 45 represents not only a mechanism for
conserving the "energy-rich" bond of acetyl-CoA, but also for releasing
CoA for further reactions.
In addition to generating ATP directly, acetyl phosphate can replace ATP as the phosphorylating agent in certain metabolic reactions
(199), as illustrated by the example in Eq. 46.
Acetyl phosphate + Glycerol ^=± Glycerol phosphate + Acetate
(46)
Transfer reactions of this type are frequently carried out by phosphatases [reviewed by Axelrod (200)].
In bacterial systems, the phosphorolytic cleavage of acyl-enzyme
complexes conserves the energy derived from an oxidative step. In
mammalian tissues, which lack both acetokinase and phosphotransacetylase, acetyl phosphate cannot be used to generate ATP. Furthermore, acetyl phosphate is rapidly hydrolyzed in mammalian tissues by
an acyl phosphatase (177, 201-202a), which, unlike the enzymes mediating Reaction 46, is incapable of transferring the phosphate group to
any acceptor other than water. However, thiolysis of acyl-enzyme complexes by CoA (for example, in the degradation of pyruvate by bacterial or animal systems) conserves the energy of oxidation.
Other carboxyl phosphates participate, or have been postulated as
intermediates, in various reactions: carbamyl phosphate* in the synthe* Carbamyl phosphate (CP) is synthesized in mammalian and frog liver by the
reaction (164a):
acetyl glutamate
2 ATP + NH, + C0 2 «
CP + 2 ADP + P t
(46a)
while in bacteria (164b) a slightly different system is found:
ATP + NH 3 + C0 2 ^ CP + ADP
(46b)