Ill 



ATP AND RELATED NUCLEOTIDES 



43 



enzyme in Lactobacillus arabinosus. When the bacteria are harvested and then exposed to 

 malate, much less enzyme is formed if the microorganisms have been grown in a biotin 

 deficient medium. The evidence seems to suggest that the biotin is concerned with the 

 formation of the mahc apoenzyme and that it does not directly function as a coenzyme 

 in CO: fixation. 



Preparations of propionyl-CoA carboxylase from biotin deficient rats are also much less 

 effective than those from normal rats in catalyzing the formation of succinate. The meta- 

 bolic defect is repaired by biotin. Tissues from biotin deficient animals also manifest an 

 impaired capacity to fix CO2 into citrulline or purines, (Lardy and Peansky, 1953). 



Isovaleryl-CoA is an intermediate in the biosynthesis of terpenes and steroids. 

 The carboxylation of isovaleryl-CoA, one of the steps in the cataboHsm of leucine 

 is another COj fixation reaction which requires ATP. 



i) ATP + [3-hydroxyisovaleryl-CoA ^ CO2 



[3-hydroxy-[i-methyl-glutaryl-CoA + 



PP + AMP 



Enzymes for reaction i) have been found in heart and Hver (Bachhawat et al., 

 1956a). It now appears that reaction i) may be resolved into two steps (Robinson 

 et al., 1955; Bachhawat et al., 1956b): 



2) ATP + CO2 -^ AMP ~ CO2 + pyrophosphate 



3) AMP ~ CO2 + [B-hydroxyisovaleryl-CoA ~> AMP ^ |3-hydroxy-p-methylglutaryl-CoA 



The primary reaction may consist of the formation of the new "active CO2" com- 

 pound, adenyl-COj. An enzyme purified 60 fold from heart tissue liberates pyro- 

 phosphate from ATP but does not accomplish the carboxylation of hydroxyiso- 

 valeryl-CoA unless a preparation of hydroxyisovaleryl carboxylase is added. 

 Recently Bachhawat and coworkers (1954, 1956a) have synthesized a compound 

 believed to be adenosine phosphoryl-carbonate. This compound is capable of 

 carboxylating hydroxyisovaleryl-CoA in the presence of carboxylase and in the 

 absence of the activating enzyme. 



It is also appropriate to mention at this point a key carboxylation reaction of photo- 

 synthesis. The work of Calvin and coworkers had established that in algae 3-phospho- 

 glycerate is an early product of the "dark" carboxylation reaction (Wilson and Calvin, 



1) Ribose -5- phosphate 



isomerase 



Ribulose -5- phosphate 



2) Ribulose -5-phosphate kinase — ^ Ribulose - 1,5 - diphosphate + ADP 



ATP 



3) CHsOPOaHj 



COH 



II o 



II 11+ _ 



COH +C— O 



HCOH 



CH2OPO3H2 



Ribulose diphosphate 

 (enol form) 



AJSO 



H CHjOPO,H, 

 1 ^ 



I I 

 0=C-C-OH 



c=o 



HC— OH 

 CHjOPOaHj 



OH" 



Fig. 1 7. Carboxylation of ribulose diphosphate. 



COOH 



I 

 CHOH 



I 

 CHzOPOaHz 



P-glyceric 



Literature p. 124 



