3. MECHANISMS FOR FATTY ACID OXIDATION
79
This complex then transfers the acyl group to suitable acceptors such
as CoA to form the acylated CoA derivatives.
Acyl adenylates are readily synthetized by reacting acyl anhydrides
with AMP in the presence of pyridine as catalyst (8). They are relatively stable at acid pH at 0° but are rapidly split at pH 2 at 100°. At
pH 10 and above, rapid hydrolysis occurs even at 0°. Acyl adenylates
react rather slowly with GSH and CoA to form the acylated thioesters
and with amino acids to form acylated amino acids, but the rate of
transfer is greatly accelerated by low concentrations of imidazole (9).
Apparently, acylated AMP transfers its acyl group nonenzymatically
to imidazole to form acetyl imidazole and AMP. The catalytic effect of
imidazole in the transfer reactions is depicted as
AMP-acylate + Imidazole —> AMP + Acyl-imidazole
(5)
Acyl-imidazole + Acceptor —> Imidazole + Acyl-acceptor
(6)
At pH 8, the anhydride bond of AMP-acylate has been estimated to
have a AF° of hydrolysis of about 15,000 cal., indicating that the K of
reaction 2 or 4 must be of a low order of magnitude. Since the difference of free energies of hydrolysis between ATP and AMP-acylate is
about 8000 cal., the amount of AMP-acylate complex formed would
probably be very low. Since the over-all K eq of 2 and 3 or
(acetyl-CoA) (PP) (AMP)
(acetate) (ATP) (CoA)
is about unity, the tendency to transfer acyl groups from the Enz-AMPacylate complex to CoA would be great.
Although the thiokinase reaction is probably the main route for
acylate activation, there are several indirect pathways which serve an
important function in some organisms.
Found in considerable concentration in extracts of Clostridiurn
kluyveri (10), CoA transphorase catalyzes the reaction:
Acetyl-CoA + Butyrate ^ Butyryl-CoA + Acetate
(7)
The CoA moiety of acetyl-CoA can be transferred to formate, acetate,
propionate, butyrate, valerate, n-caproate, n-caprylate, vinylacetate, and
lactate. The K is about unity. Since the enzyme has not been greatly
purified, no specificity can be assigned to a single protein.
A similar transphorase (11) has been described for the reaction:
Succinyl-CoA + Acetoacetate ^=± Acetoacetyl-CoA + Succinate
(8)
Succinate is the specific dicarboxylic acid which participates, but several ß-keto acids may also be activated. Saturated and unsaturated
acids are inert. Since succinyl-CoA is formed in the oxidation of a-
79
This complex then transfers the acyl group to suitable acceptors such
as CoA to form the acylated CoA derivatives.
Acyl adenylates are readily synthetized by reacting acyl anhydrides
with AMP in the presence of pyridine as catalyst (8). They are relatively stable at acid pH at 0° but are rapidly split at pH 2 at 100°. At
pH 10 and above, rapid hydrolysis occurs even at 0°. Acyl adenylates
react rather slowly with GSH and CoA to form the acylated thioesters
and with amino acids to form acylated amino acids, but the rate of
transfer is greatly accelerated by low concentrations of imidazole (9).
Apparently, acylated AMP transfers its acyl group nonenzymatically
to imidazole to form acetyl imidazole and AMP. The catalytic effect of
imidazole in the transfer reactions is depicted as
AMP-acylate + Imidazole —> AMP + Acyl-imidazole
(5)
Acyl-imidazole + Acceptor —> Imidazole + Acyl-acceptor
(6)
At pH 8, the anhydride bond of AMP-acylate has been estimated to
have a AF° of hydrolysis of about 15,000 cal., indicating that the K of
reaction 2 or 4 must be of a low order of magnitude. Since the difference of free energies of hydrolysis between ATP and AMP-acylate is
about 8000 cal., the amount of AMP-acylate complex formed would
probably be very low. Since the over-all K eq of 2 and 3 or
(acetyl-CoA) (PP) (AMP)
(acetate) (ATP) (CoA)
is about unity, the tendency to transfer acyl groups from the Enz-AMPacylate complex to CoA would be great.
Although the thiokinase reaction is probably the main route for
acylate activation, there are several indirect pathways which serve an
important function in some organisms.
Found in considerable concentration in extracts of Clostridiurn
kluyveri (10), CoA transphorase catalyzes the reaction:
Acetyl-CoA + Butyrate ^ Butyryl-CoA + Acetate
(7)
The CoA moiety of acetyl-CoA can be transferred to formate, acetate,
propionate, butyrate, valerate, n-caproate, n-caprylate, vinylacetate, and
lactate. The K is about unity. Since the enzyme has not been greatly
purified, no specificity can be assigned to a single protein.
A similar transphorase (11) has been described for the reaction:
Succinyl-CoA + Acetoacetate ^=± Acetoacetyl-CoA + Succinate
(8)
Succinate is the specific dicarboxylic acid which participates, but several ß-keto acids may also be activated. Saturated and unsaturated
acids are inert. Since succinyl-CoA is formed in the oxidation of a-