234
G. L. CANTONI
Only Reaction 27 is significant for the purpose of this discussion and it
is clear from it that the ribosyl pyrophosphate bond is utilized for the
synthesis of the onium bond of DPN.
2. Biosynthesis of S-Adenosylmethionine
The biosynthesis of S-adenosylmethionine has been investigated
rather thoroughly particularly with the aim of elucidating the part
played by ATP in the reaction and the mechanism underlying the energy
utilization of the energy of a pyrophosphate bond for the synthesis of
a sulfonium bond. The studies of Cantoni and Durell (173), and Mudd
and Cantoni (124) on the methionine-activating enzyme of liver and
yeast have revealed that the reaction proceeds as indicated by the
equation
Mg2+
AR—P—P—P
> £-Adenosylmethionine + Pyrophosphate + I\·
(30)
a ß y
Furthermore it has become clear that the terminal or γ-phosphate of
ATP is mineralized while the a- and ^-phosphates give rise to pyrophosphate moiety. These results are of interest since they reveal an entirely
new mechanism for the utilization of ATP in biological systems. Heretofore ATP has been known to act (a) as a phosphate donor with the
concomitant formation of ADP, (b) as a pyrophosphate donor with the
concomitant formation of AMP, and (c) as an adenylate donor with the
resultant mineralization of its terminal pyrophosphate moiety. While the
intimate mechanism of the reaction studied here is not yet fully understood, it is clear that this reaction cannot be grouped in any of the
three categories mentioned above, and furthermore it is the first demonstration of the fact that ATP functions as an adenosine donor. The finding that pyrophosphate can be formed from the a- and β-phosphates of
ATP is also new and indicates that the ribose-phosphate bond in ATP
is not as inert as had been thought until now.
From the viewpoint of comparative biochemistry it is worthwhile to
emphasize both the similarities and differences between methionineactivating enzyme of liver and yeast. Both enzymes require an unusually
high concentration of Mg
2+ for optimal activity. In addition, both proteins require a monovalent cation and the activity pattern is similar: NH 4
+ , K
+
, and Rb
+ are almost equally effective while Li
+ ,
Cs
+ , Na
+ , and tris* are much less active or completely ineffective. Many
enzymes which require a monovalent cation behave in a similar way,
although differences exist in the detailed order of activities (203-211).
The most striking difference between the two enzymes is the fact that
* Tris (hydroxymethyl) aminomethane.
G. L. CANTONI
Only Reaction 27 is significant for the purpose of this discussion and it
is clear from it that the ribosyl pyrophosphate bond is utilized for the
synthesis of the onium bond of DPN.
2. Biosynthesis of S-Adenosylmethionine
The biosynthesis of S-adenosylmethionine has been investigated
rather thoroughly particularly with the aim of elucidating the part
played by ATP in the reaction and the mechanism underlying the energy
utilization of the energy of a pyrophosphate bond for the synthesis of
a sulfonium bond. The studies of Cantoni and Durell (173), and Mudd
and Cantoni (124) on the methionine-activating enzyme of liver and
yeast have revealed that the reaction proceeds as indicated by the
equation
Mg2+
AR—P—P—P
> £-Adenosylmethionine + Pyrophosphate + I\·
(30)
a ß y
Furthermore it has become clear that the terminal or γ-phosphate of
ATP is mineralized while the a- and ^-phosphates give rise to pyrophosphate moiety. These results are of interest since they reveal an entirely
new mechanism for the utilization of ATP in biological systems. Heretofore ATP has been known to act (a) as a phosphate donor with the
concomitant formation of ADP, (b) as a pyrophosphate donor with the
concomitant formation of AMP, and (c) as an adenylate donor with the
resultant mineralization of its terminal pyrophosphate moiety. While the
intimate mechanism of the reaction studied here is not yet fully understood, it is clear that this reaction cannot be grouped in any of the
three categories mentioned above, and furthermore it is the first demonstration of the fact that ATP functions as an adenosine donor. The finding that pyrophosphate can be formed from the a- and β-phosphates of
ATP is also new and indicates that the ribose-phosphate bond in ATP
is not as inert as had been thought until now.
From the viewpoint of comparative biochemistry it is worthwhile to
emphasize both the similarities and differences between methionineactivating enzyme of liver and yeast. Both enzymes require an unusually
high concentration of Mg
2+ for optimal activity. In addition, both proteins require a monovalent cation and the activity pattern is similar: NH 4
+ , K
+
, and Rb
+ are almost equally effective while Li
+ ,
Cs
+ , Na
+ , and tris* are much less active or completely ineffective. Many
enzymes which require a monovalent cation behave in a similar way,
although differences exist in the detailed order of activities (203-211).
The most striking difference between the two enzymes is the fact that
* Tris (hydroxymethyl) aminomethane.