Biosynthesis aminoimidazole

The pathways for thiamine biosynthesis have been elucidated only partiy. Thiamine pyrophosphate is made universally from the precursors 4-amino-5-hydroxymethyl-2-methylpytimidinepyrophosphate [841-01-0] (47) and 4-methyl-5-(2-hydroxyethyl)thiazolephosphate [3269-79-2] (48), but there appear to be different pathways ia the eadier steps. In bacteria, the early steps of the pyrimidine biosynthesis are same as those of purine nucleotide biosynthesis, 5-Aminoimidazole ribotide [41535-66-4] (AIR) (49) appears to be the sole and last common iatermediate ultimately the elements are suppHed by glycine, formate, and ribose. AIR is rearranged in a complex manner to the pyrimidine by an as-yet undetermined mechanism. In yeasts, the pathway to the pyrimidine is less well understood and maybe different (74—83) (Fig. 9). 

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. 

Shaw and co-workers during studies into the de novo biosynthesis of purine nucleotides demonstrated that 4(5)-aminoimidazole (25 R = H) on treatment with a saturated aqueous solution of potassium bicarbonate at 70°C for 15 min gave 4-aminoimidazole-5-carboxylic acid (38) in an estimated yield of 40% [71JCS(C)1501]. This and related reactions are discussed in more detail in Section V,B,4. 

In order to synthesize biologically relevant phosphonylimidazole 73, bromoimidazole 72 was derived from radical-initiated bromination of methyl l-p-methoxybenzyl-2-thiomethyl-5-imidazolylcarboxylate (71) [56]. The thiomethyl group served to block the C(2) position, which would otherwise undergo preferential halogenation under these conditions. As expected, a variety of Arbusov-Michaelis reaction conditions failed even under forcing conditions. On the other hand, Pd-catalyzed phosphorylation of 72 with diethyl phosphite led to methyl-4-diethylphosphonyl-l-p-methoxybenzyl-2-thiomethyl-5-imidazolylcarboxylate (73). After further manipulations, the desired phosphonic acid-linked aminoimidazoles, which resembled intermediates formed during purine biosynthesis, were accessed. 

The effect of 6-mercaptopurine on the incorporation of a number of C-labelled compounds into soluble purine nucleotides and into RNA and DNA has been studied in leukemia L1210, Ehrlich ascites carcinoma, and solid sarcoma 180. At a level of 6-mercaptopurine that markedly inhibited the incorporation of formate and glycine, the utilization of adenine or 2-aminoadenine was not affected. There was no inhibition of the incorporation of 5(or 4)-aminoimidazole-4(5)-carboxamide (AIC) into adenine derivatives and no marked or consistent inhibition of its incorporation into guanine derivatives. The conversion of AIC to purines in ascites cells was not inhibited at levels of 6-mercaptopurine 8-20 times those that produced 50 per cent or greater inhibition of de novo synthesis [292]. Furthermore, AIC reverses the inhibition of growth of S180 cells (AH/5) in culture by 6-mercaptopurine [293]. These results suggest that in all these systems, in vitro and in vivo, the principal site at which 6-mercaptopurine inhibits nucleic acid biosynthesis is prior to the formation of AIC, and that the interconversion of purine ribonucleotides (see below) is not the primary site of action [292]. Presumably, this early step is the conversion of PRPP to 5-phosphoribosylamine inhibited allosterically by 6-mercaptopurine ribonucleotide (feedback inhibition is not observed in cells that cannot convert 6-mercaptopurine to its ribonucleotide [244]. 

The biosynthesis of histidine. The 5-aminoimidazole-4-carboxamide ribotide formed during the course of histidine biosynthesis is also an intermediate in purine nucleotide biosynthesis. Therefore it can be readily regenerated to an ATP, thus replenishing the ATP consumed in the first step in the histidine biosynthetic pathway (see fig. 23.13). 

Another group of inhibitors prevents nucleotide biosynthesis indirectly by depleting the level of intracellular tetrahydrofolate derivatives. Sulfonamides are structural analogs of p-aminobenzoic acid (fig. 23.19), and they competitively inhibit the bacterial biosynthesis of folic acid at a step in which p-aminobenzoic acid is incorporated into folic acid. Sulfonamides are widely used in medicine because they inhibit growth of many bacteria. When cultures of susceptible bacteria are treated with sulfonamides, they accumulate 4-carboxamide-5-aminoimidazole in the medium, because of a lack of 10-formyltetrahydrofolate for the penultimate step in the pathway to IMP (see fig. 23.10). Methotrexate, and a number of related compounds inhibit the reduction of dihydrofolate to tetrahydrofolate, a reaction catalyzed by dihydrofolate reductase. These inhibitors are structural analogs of folic acid (see fig. 23.19) and bind at the catalytic site of dihydrofolate reductase, an enzyme catalyzing one of the steps in the cycle of reactions involved in thymidylate synthesis (see fig. 23.16). These inhibitors therefore prevent synthesis of thymidylate in replicating... 

In the biosynthesis pathways of histidine and tryptophan, the enzymes HisA (N -[(5 -phosphoribosyl)formimino]-5-aminoimidazole-4-carboxamide ribonucleotide isomerase) and TrpF (PRAI,phosphoribosylanthranilate isomerase) (Figure 16.13), both of which are ()3a)8-barrels, both catalyze Amadori rearrangements of a ther-molabile aminoaldose into the corresponding aminoketose (Figure 16.13). 

All the carbon atoms of the purine ring were supposedly provided by HCN molecules through a complex step-by-step condensation process. In particular, oligomers of HCN, such as the HCN-trimer aminomaleonitrile (AMN) and the HCN-tetramer diaminomaleonitrile (DAMN), were found to be intermediates in this transformation (Scheme 1) [43,44]. In accordance with the present-day biosynthesis of purines in the cell, two 4,5-di-substituted imidazole derivatives, 4-aminoimidazole-5-carbonitrile (AICN) and 4-aminoimidazole-5-carboxamide (AICA) were successively formed from AMN and DAMN by chemical or, most probably, photochemical reactions [45-47]. Finally, a ring-closure process of AICA and HCN yielded adenine 1. 

Aimi,J., Qiu, H., Williams, J., Zalkin, H., and Dixon, J. E. (1990). De novo purine nucleotide biosynthesis cloning of human and avian cDNAs encoding the trifunctional glycinam-ide ribonucleotide synthetase-aminoimidazole ribonucleotide synthetase-glycinamide ribonucleotide transformylase by functional complementation in E. colt. Nucleic Acids Res., 18, 6665-6672. 

A new intermediate in purine nucleotide de novo biosynthesis in E. coli, namely 7V(5)-carboxy-aminoimidazole ribotide (35) has been identified together with two new enzymatic activities involving the carboxylation of the 5-amino group of 5-aminoimidazole ribotide (AIR) and the rearrangement of the /V-carboxy derivative to the C-carboxy derivative (CAIR) (Scheme 21) <94B2269>. 

In bacteria, the pyrimidine precursor 38 is derived from 5-aminoimidazole ribotide (37), an intermediate of the basic branch of purine biosynthesis, which supplies all carbon atoms for 38 by a complex rearrangement reaction (the fate of the individual carbon atoms is indicated by Greek letters in Fig. 4). In yeasts, a totally unrelated reaction sequence uses carbon atoms from vitamin Be (39) that are indicated by roman letters in Fig. 4 for the assembly of the thiamine precursor 38,... 

In plants, little is known about the basic building blocks and the reactions involved in thiamine biosynthesis. An early study with chloroplasts of spinach indicated that 1-deoxy-D-xylulose 5-phosphate, tyrosine, and cysteine act as precursors of the thiazole moiety in analogy to the pathway in E. coli. More recently, it has been shown that a homolog of the THIC protein that converts 5-aminoimidazole ribotide into 38 is essential (25). These results suggest that the plant pathway is similar to the pathway in prokaryotes but not to that in yeast. 

Figure 4 Biosynthesis of thiamine (vitamin ). 37, aminoimidazole ribotide 38, 2-methyl-4-amino-5-hydroxymethyl-pyrimidine phosphate 39, pyridoxal 5 -phosphate 40, histidine 41, 2-methyl-4-amino-5-hydroxymethyl-pyrimidine pyrophosphate 42, 4-methyl-5-p-hydroxyethylthiazole phosphate 43,1 -deoxy-D-xylulose 5-phosphate 44, 5-ADP-D-ribulose 45, thiamine phosphate 46, thiamine pyrophosphate.

The biosynthetic pathway is outlined in Figure 1. The thiazole 4 is formed by an oxidative condensation of glycine (1), deoxy-D-xylulose-5-phosphate (DXP, 2), and a sulfide carrier protein with a thiocarboxylate at its carboxy terminus (ThiS-COSH, 3). The pyrimidine phosphate 8 is formed by a deep-seated rearrangement of aminoimidazole ribotide (AIR, 7). It is then pyrophosphorylated and used to alkylate the thiazole to give 10. A final phosphorylation completes the biosynthesis. The entire pathway from glycine, DXP, cysteine, AIR, and ATP has now been reconstituted using purified enzymes. 

Another enzyme recently shown [62] to be competitively inhibited by lAHQ is aminoimidazole-4-carboxamide ribonucleotide transferase (AICAR TFase), which catalyzes the final step in the biosynthesis of purine nucleo-... 

AIR. /-(S-O-Phosphono-d-D-ribofuranosyl)-lH-imidazol-5-amine 5-amino-I-ribofuranosylimidazole S -phosphate 5-aminoimidazole ribonucleotide 5-aminoimida-zole ribotide 5 -amino -1 -(5 -phosphofuranoribosy] )imida -zole. C H14N307P mol wt 295-19. C 32.55%. H 4.78%, N 14.23%, O 37.94%, P 10.49%. An intermediate in the biosynthesis of purines. Synthesis from formylglycinamide ribonucleotide Levenberg, Buchanan, J. Biol Chem. 224, 1005 (1957) Lukens, Buchanan, ibid. 234, 1791 (1959). 

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