The Enzymatic Reactions of Purine Synthesis

Since ribose phosphate compounds are involved at some stage in purine synthesis, knowledge of the mechanism of ribotide formation has become a fundamental aspect of the purine problem. The exact enzymatic step mediating the coupling of ribose phosphate to a purine precursor is not known, although the addition is thought to occur before the AIC skeleton is completed (in view of the negative bank experiments discussed above). Since AIC and hypoxanthine are both converted to their respe< -tive ribotides, it was hoped that a study of hypoxanthine conversion to inosinic acid would reveal some of the important aspects of the fundamental reactions of ribotide synthesis. 

The new reaction of formate mentioned above did not take place in all experiments. In several instances, the ratio of formate glycine incorporation approached 2 1, and the formate label was approximately equally distributed between C2 and 0. It was observed in these latter experiments that purine synthesis de novo (as measured by the incorporation of NH2CH2C"00H into C4) took place much more rapidly than in the experiments demonstrating unequal formate incorporation. It was believed that rapid synthesis de novo of the purine ring was masking the enzymatic exchange reaction of formate with inosinic acid. 

It is of course surprising that amino acids can be obtained via the Strecker synthesis, purines from the condensation of HCN, pyrimidines from the reaction of cyanoacetilene with urea, and sugars from the autocatalytic condensation of formaldehyde. The synthesis of chemical constiments of contemporary organisms by non-enzymatic processes under laboratory conditions does not necessarily imply that they were either essential for the origin of life or available in the primitive environment. However, the significance of prebiotic simulation experiments is... 

The active form of folate is the tetrahydro-derivative that is formed through reduction by dihydrofolate reductase. This enzymatic reaction (Figure 29.5) is inhibited by trimethoprim, leading to a decrease in the folate coenzymes for purine, pyrimidine, and amino acid synthesis. Bacterial reductase has a much stronger affinity for trimethoprim than does the mammalian enzyme, which accounts for the drug s selective toxicity. [Note Examples of other folate reductase inhibitors include pyrimethamine, which is used with sulfonamides in parasitic infections (see p. 353), and methotrexate, which is used in cancer chemotherapy (see p. 378).]... 

Compound 25 (Fig. 18.9), a prodrug of 9-P-D-arabinofuranosyl guanine (26), was developed for the potential treatment of leukemia. Compound 24 is poorly soluble in water and its synthesis by conventional techniques is difficult. An enzymatic demethoxylation process was developed using adenosine deaminase (Mahmoudian et al., 1999, 2001). Compound 25 was enzymatically prepared from 6-methoxyguanine (27) and ara-uracil (28) using uridine phosphorylase and purine nucleotide phosphorylase. Each protein was cloned and overexpressed in independent Escherichia coli strains. Fermentation conditions were optimized for production of both enzymes and a co-immobilized enzyme preparation was used in the biotransformation process at 200 g/L substrate input. Enzyme was recovered at the end of the reaction by filtration and reused in several cycles. A more water soluble 5 -acetate ester of compound 26 was subsequently prepared by an enzymatic acylation process using immobilized Candida antarctica lipase in 1,4-dioxane (100 g/L substrate) with vinyl acetate as the acyl donor (Krenitsky et al., 1992). 

P. arabinosum and the bacteria that live in the gut of humans and animals synthesize mostly the purine-containing cobamides. If DMB is added to the feed, then the synthesis of cobalamins in the stomach of animals increases (Rickard et al., 1975). The finding of incomplete corrinoids in various bacteria and algae (Neujahr and Frires, 1966) shows that their biosynthesis may proceed with difficulty. Cobinamide, pseudo-vitamin B, factor A and factor III can act as growth factors for microorganisms if they contain a nucleotide, they are also active in enzymatic reactions in animals. 

Aim of this research was to increase efficiency of producing T. thermophilus PyrNPase in cells of genetically engineered E. coli strain by optimizing the structure of the respective translated mRNA and to investigate enzymatic synthesis of purine 3 -fluoro-3 -deoxy- and 3 -fluoro-2, 3 -dideoxynucleosides possessing antiviral and cytostatic activities from the available pyrimidine nucleosides engaging tandem reactions in the presence of recombinant nucleoside phosphorylases [8],... 

The N-5 position is considerably more basic than the N-10 position, and this basicity is one of several factors that control certain preferences in the course of reactions involving tetrahydrofolate. Thus, for-mylation occurs more readily at N-10 while alkylation occurs more readily at N-5. Benkovic and Bullard (1973) have reviewed evidence for an iminium cation at N-5 as the active donor in formaldehyde oxidation-level transfers. Recently, Barrows et al. (1976) have further studied such a mechanism for folic acid. The interconversion of these forms of folate coenzymes by enzymatic means has been reviewed by Stokstad and Koch (1967), and the reader is directed there for further details. Folate coenzymes are involved in a wide variety of biochemical reactions. These include purine and pyrimidine synthesis, conversion of glycine to serine, and utilization and generation of formate. In addition, the catabolism of histidine, with the formation of formiminoglu-tamic acid (FIGLU), is an important cellular reaction involving folate. 

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