80
P. K. STUMPF AND G. A. BARBER
ketoglutaric acid, organisms with TCA cycles potentially can channel
the thioester bond energy from succinyl-CoA to the ß-keto acid series
which then by thiolytic cleavage can yield acetyl-CoA and an activated
acyl derivative. At pH 7 in the presence of Mg
2+
JT
^ouuumatc; ^»uctuta^cuji-vvuxxy
O CQ V 1 (\— 3
(succinate) (acetoacetyl-CoA)
(succinyl-CoA) (acetoacetic)
The enzyme has been isolated from pig heart, dog skeletal muscle, and
ox adrenal glands, but is absent in pigeon breast muscle, rabbit skeletal
muscle, and pigeon brain.
In Clostridium kluyveri, phosphotransacetylase (12) catalyzes the
reaction:
Acetyl phosphate + CoA ^ Acetyl CoA + Phosphate
(9)
The enzyme is quite specific since the rates of reaction of higher
homologs of acetyl phosphate fall off very rapidly.
Ke
=
(acetyl-CoA)(P0 4 )
= ?4
eq
(acetyl phosphate) (CoA)
which favors trapping the energy of the phosphoryl carboxylate anhydride bond into the more useful thioester bond. The direct biosynthesis of acetyl phosphate is by means of the acetokinase reaction (13):
Acetate + ATP ^=± Acetyl phosphate + ADP
(10)
The enzyme is highly specific for acetate. The K eq at pH 7 is approximately 0.006 and favors the formation of ATP from acetyl phosphate
and ADP. By combining Reactions 9 and 10, the K' eq will be 0.44,
which is quite adequate for biosynthetic purposes.
Other sources of acetyl phosphate are:
(a) pyruvic oxidase (14)—Lactobacillus
delbrueckii
Pyruvic acid + P» + 0 2 —> Acetyl phosphate + C0 2 + H 2 0 2
(b) phosphoroclastic split (15)—Escherichia coli
Pyruvate + P t ^± Formate + Acetyl phosphate
(c) phosphoketolase (16)—Lactobacillus
pentosus
Xylulose 5-phosphate + P t - —> Acetyl phosphate + Triose phosphate
(d) phosphoroclastic split (17)—Clostridium
butylicum
Pyruvate + P» ^± Acetyl phosphate + H 2 + C0 2
With these systems available for the generation of acetyl phosphate,
an organism with phosphotransacetylase can readily transfer the energy of high-energy phosphate to a thioester bond. Acetyl phosphate,
P. K. STUMPF AND G. A. BARBER
ketoglutaric acid, organisms with TCA cycles potentially can channel
the thioester bond energy from succinyl-CoA to the ß-keto acid series
which then by thiolytic cleavage can yield acetyl-CoA and an activated
acyl derivative. At pH 7 in the presence of Mg
2+
JT
^ouuumatc; ^»uctuta^cuji-vvuxxy
O CQ V 1 (\— 3
(succinate) (acetoacetyl-CoA)
(succinyl-CoA) (acetoacetic)
The enzyme has been isolated from pig heart, dog skeletal muscle, and
ox adrenal glands, but is absent in pigeon breast muscle, rabbit skeletal
muscle, and pigeon brain.
In Clostridium kluyveri, phosphotransacetylase (12) catalyzes the
reaction:
Acetyl phosphate + CoA ^ Acetyl CoA + Phosphate
(9)
The enzyme is quite specific since the rates of reaction of higher
homologs of acetyl phosphate fall off very rapidly.
Ke
=
(acetyl-CoA)(P0 4 )
= ?4
eq
(acetyl phosphate) (CoA)
which favors trapping the energy of the phosphoryl carboxylate anhydride bond into the more useful thioester bond. The direct biosynthesis of acetyl phosphate is by means of the acetokinase reaction (13):
Acetate + ATP ^=± Acetyl phosphate + ADP
(10)
The enzyme is highly specific for acetate. The K eq at pH 7 is approximately 0.006 and favors the formation of ATP from acetyl phosphate
and ADP. By combining Reactions 9 and 10, the K' eq will be 0.44,
which is quite adequate for biosynthetic purposes.
Other sources of acetyl phosphate are:
(a) pyruvic oxidase (14)—Lactobacillus
delbrueckii
Pyruvic acid + P» + 0 2 —> Acetyl phosphate + C0 2 + H 2 0 2
(b) phosphoroclastic split (15)—Escherichia coli
Pyruvate + P t ^± Formate + Acetyl phosphate
(c) phosphoketolase (16)—Lactobacillus
pentosus
Xylulose 5-phosphate + P t - —> Acetyl phosphate + Triose phosphate
(d) phosphoroclastic split (17)—Clostridium
butylicum
Pyruvate + P» ^± Acetyl phosphate + H 2 + C0 2
With these systems available for the generation of acetyl phosphate,
an organism with phosphotransacetylase can readily transfer the energy of high-energy phosphate to a thioester bond. Acetyl phosphate,