Adenylosuccinate synthase

A common property of all Bacillus purine production strains is their aux-otrophy for adenine caused by a dysfunctional adenylosuccinate synthase gene (purA) of the AMP-specific branch of the purine pathway. Furthermore, the... 

Neither the nature of the repressor substance nor the metabolic corepressor has yet been determined for any of the enzymes. Attempts to determine the true corepressor have been frustrated by interconversion reactions. In mutants where interconversions are blocked, a guanine derivative has been implicated as the corepressor for IMP dehydrogenase, an adenine derivative for adenylosuccinate synthase, and either for the early enzymes [20,22]. It was suggested that different operons were controlled by different repressors with varying affinities for a common corepressor [20,23]. The eventual resolution of the problem will require isolation and analysis of regulatory mutants. One such mutant has been obtained which appears to have a specific constitutive effect on the purE operon [25]. Others from Bacillus subtilis show less specificity [23], and another from Salmonella [26] shows nonrepressibility of five different enzymes and its current state of analysis implies that it is blocked in the formation of a common corepressor. 

Subsequent investigations of the adenylosuccinate synthase of L. donovani> have demonstrated that HPPR-MP, in the concentrations in which it is found within the parasite, can serve as a substrate for this enzyme. Although the kinetic constant for HPPR-MP is very high (K = 340 uM V = 1%), it is well within the concentration... 

The conversion of IMP to AMP requires amination at C-6 of the purine system, and the nitrogen for this process is derived from aspartate (Asp, D) (adenylosuccinate synthase, EC 6.3.4.4). It appears that the driving force for the loss of water in this process that yields N -l, 2-dicarboxyl adenosine monophosphate (adenosine 5 -phosphate, AMP) is the conversion of guanosine triphosphate (GTP) to guano-sine diphosphate (GDP). Adenosine monophosphate (adenosine 5 -phosphate, AMP) is formed from the N -derivative by loss of fumarate (catalyzed by adenylosuccinate lyase, EC 4.3.2.2). 

It was shown in Scheme 12.97 (which is partially repeated here as Scheme 14.2) that inosine 5 -phosphate (IMP) underwent amination by the addition of aspartate (Asp, D) to the carbon of the carbonyl to produce N -(5)-l,2-dicarboxyethyl adenosine monophosphate. The reaction was aided by adenylosuccinate synthase (EC 6.3.4.4).The phosphate was derived from guanosine triphosphate (GTP).Then, after the elimination of fumarate (adenylosuccinate lyase, EC 4.3.2.2), AMP resulted. 

Table 7.1.4 Concentration range of purine and pyrimidine metabolites in urine (pmol/mmol creatinine) from patients. ADA Adenosine deaminase, APRT adenine phosphoribosyltransferase, ASA adenylosuccinate lyase, DHP dihydropyrimidinase, DPD dihydropyrimidine dehydrogenase, HGPRT hypoxanthine-guanine phosphoribosyltransferase, PNP purine nucleoside phosphorylase, TP thymidine phosphorylase, UMPS uridine monophosphate synthase, / -UP fi-ureidopropionase... 

A few steps convert inosinate into either AMP or GMP (Figure 25,9). Adenylate is synthesized from inosinate by the substitution of an amino group for the carbonyl oxygen atom at C-6. Again, the addition of aspartate followed by the elimination of fumarate contributes the amino group. GXP, rather than ATP, is the phosphoryl-group donor in the synthesis of the adenylosuccinate intermediate from inosinate and aspartate. In accord with the use of GTP, the enzyme that promotes this conversion, adenylsuccinate synthase, is structurally related to the G-protein family and does not contain an ATP-grasp domain. The same enzyme catalyzes the removal of fumarate from adenylosuccinate in the synthesis of adenylate and from 5-aminoimidazole-4-jV-succinocarboxamide ribonucleotide in the synthesis of inosinate. 

Figure 3. Compartmentalization of the purine salvage pathway of Leishmania. Abbreviations are as follows AAH, adenine aminohydrolase XPRT, xanthine phosphoribosyltransferase HGPRT, hypoxanthine-guaninephosphoribosyltransferase ADSS, adenylosuccinate synthetase ASL, adenylosuccinate lyase IMPDH, inosine monophosphate dehydrogenase GMPS, gua-nosine monophosphate synthase GDA, guanine deaminase AMPDA, adenosine monophosphate deaminase GMPR, guanosine monophosphate reductase APRT, adenine phosphoribosyltransferase AK, adenosine kinase. Enzymes that have been localized are shown in black and those that are predicted to be in the denoted locations are depicted in gray.

Fig. 173. Interconversion of inosine, adenosine and guanosine monophosphates 1 Inosine monophosphate dehydrogenase 2 guanosine monophosphate synthase 3 guanosine monophosphate reductase 4 adenylosuccinate synthetase 5 adenylosuccinate lyase 6 adenosine (phosphate) deaminase...

Phosphorylation at the carboxylate (ATP ADP) followed by reaction with aspartate (Asp,D) in the presence of phosphoribosylaminoimidazolesuccinocarbox-amide synthase (EC 6.S.2.6) produces the corresponding (S)-succinate derivative. This is followed by loss of fumarate (adenylosuccinate lyase, EC 4.S.2.2) to the corresponding amide (last seen in the biosynthesis of histidine (His, H) (Scheme 12.26). Then, as shown in Scheme 12.97, N-formylation (as in Scheme 12.87) on the C-5 amino group is effected with IO-CHO-H4 folate (EC 2.1.2.3) to yield the N-formyl derivative (5-formamido-l-(5 -phosphoribosyl) imidazole-4-carboxyamide). Loss of water accompanies cyclization (EC 3.5.2.10) to yield inosine 5 -phosphate (IMP). 

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