Inosine from formate

Fig. 7-1. Inosinate, the true end product of purine biosynthesis de novo in pigeon liver extracts incubated with substrates and C-formate. (A) Total radioactivity of products formed (B) specific activity of products formed. HX = hypoxanthine IMP-5 = inosinate. From (8). Reproduced with permission.

Fig. 8. The efiect of P5C on inosine monophosphate formation. Erythrocytes were incubated with 25 /iM [ C]hypoxanthine for 1 hour and nucleotides were separated using high-pressure liquid chromatography. The concentration of P5C was 0.5 mM. IMP pools in P5C-treated ( ) and control (O) cells as well as the incorporation of [ C]hypoxanthine into IMP in P5C-treated (A) and control (A) cells are shown. Taken from ref. (118).

One of the steps in the biosynthesis of a nucleotide called inosine monophosphate is the formation of aminoimidazole ribonucleotide from formyjglycin-amidine ribonucleotide. Propose a mechanism. 

The synthesis of inosinic acid (123) from AIR (106) using soluble avian liver enzymes has been shown to proceed in several steps. The first step involves the formation of C-AIR (107) by carboxylation of the aminoimid-azole (106) (Scheme 15) (57JA1511). 

Far less data are available from other diaqua Pt(II) compounds. Comparison of the diaqua derivatives of cis- and trans-DDP has shown that their complexation with inosine derivatives is mechanistically similar, but the rate parameters for various steps show considerable differences [40,41], For example, for isomeric [Pt(NH3)2(H20)2]2+ ions kx cis) = 10 k trans), whereas for the [Pt(0H)(NH3)2(H20)]+ ions the difference is k cis) 6 kyitrans) in the formation of 1 1 complexes. The ability of isomeric 1 1 complexes to bind the second nucleobase is, however, very similar in both cases, also by taking proton transfer formally from inosine... 

In the second control mechanism, exerted at a later stage, an excess of GMP in the cell inhibits formation of xanthylate from inosinate by IMP dehydrogenase, without affecting the formation of AMP (Fig. 22-35). Conversely, an accumulation of adenylate inhibits formation of adenylosuccinate by adenylosuccinate synthetase, without affecting the biosynthesis of GMP. In the third mechanism, GTP is required in the conversion of IMP to AMP (Fig. 22-34, step (T)), whereas ATP is required for conversion of IMP to GMP (step (4)), a reciprocal arrangement that tends to balance the synthesis of the two ribonucleotides. 

The first surprise was that these molecules are much longer than seems necessary for the formation of adapters. In addition, 10-20% of their bases are modified greatly from their original form.171 Another surprise was that the anticodons are not all made up of "standard" bases. Thus, hypoxanthine (whose nucleoside is inosine) occurs in some anticodons. Conventional "cloverleaf" representations of tRNA, which display their secondary structures, are shown in Figs. 5-30 and 29-7. However, the molecules usually have an L shape rather than a cloverleaf form (Figs. 5-31 and 29-6),172 and the L form is essential for functioning in protein synthesis as indicated by X-ray and other data.173 Three-dimensional structures, now determined for several different tRNAs,174 175 are all very similar. Structures in solution are also thought to be... 

All biosynthetic pathways are under regulatory control by key allosteric enzymes that are influenced by the end products of the pathways. For example, the first step in the pathway for purine biosynthesis is inhibited in a concerted fashion by nucleotides of either adenine or guanine. In addition, the nucleoside monophosphate of each of these bases inhibits its own formation from inosine monophosphate (IMP). On the other hand, adenine nucleotides stimulate the conversion of IMP into GMP, and GTP is needed for AMP formation. 

In a similar way, enzymes from Azotobacter vinelandii catalyze the transfer of phosphate from adenosine 5-triphosphoric acid and inosine 5-tri-phosphoric acid to the 2-deoxypentonucleoside monophosphates, with the formation of the pyrophosphates and triphosphates. ... 

Figure 10.7 Schematic representation of the formation and fate of IMP. Formation of IMP is catalyzed by the enzyme hypoxanthine/guanine phosphoribosyl tranfer-ase (1) from the substrate hypoxanthine (Hypo) and phosphoribosyl pyrophosphate (PRibPP). IMP is shown undergoing several reactions the first (2) is catalyzed by 5 -nucleotidase to form inosine (INO) and orthophosphate (Pj) the other (3) is a two-step reaction catalyzed by sAMP synthetase to form adenylosuccinate (sAMP) and (4) by the enzyme sAMP lyase to convert sAMP to AMP and fumarate. Finally, (3) the deamination of AMP to IMP and NHa is catalyzed by AMP deaminase.

Figure 25.8. Inosinate Formation. The removal of fumarate, the addition of a second formyl group from N lO-formyltetrahydrofolate, and cyclization completes the synthesis of inosinate (IMP), a purine nucleotide.

Remaining issues. We developed a new route to the practical synthesis of FddA from inosine in nine steps and 36% overall yield. During the course of this study, we greatly improved the fluorination of 3 -deoxyriboside, which had been very difficult and the bottleneck in FddA synthesis. However, even with this process, formation of the elimination byproduct was inevitable. To further improve the yield, studies are still needed to fix the 3 -deoxy riboside to the 2 -endo conformation, which does not easily give the elimination product. 

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