IMP synthase

SAICAR synthetase SAICAR lyase AICAR transformylase (jj) IMP synthase... 

A bifunctional enzyme, comprising the activities of AIR carboxylase and SAICAR synthetase, catalyzes reactions 6 and 7 of the purine pathway (AIR—> CAIR— SAICAR Fig. 15-16). A second bifunctional enzyme, IMP synthase, containing the activities of AICAR transformylase and IMP cyclohydrolase, catalyzes reactions 9 and 10 of the pathway (AICAR — FAICAR— IMP Fig. 15-16). Human IMP synthase has a subunit molecular weight of 62.1 kDa and associates as a dimer. A... 

AIR carboxylase A -CAIR mutase SAICAR synthetase (9) SAICAR lyase ) AICAR transformylase (Q IMP synthase... 

Human IMP synthase has a subunit molecular weight of 62.1 kDa and associates as a dimer. A trifunctional enzyme, Cj-THF synthase, containing (V °-methenyl tetrahydrofolate (5,10-CH-THF) cyclohydrolase and A °-formyl tetrahydrofolate (lO-CHO-THF) synthetase, catalyzes the reactions 5,10-CH2-THF 5,10-CH-THF and THF —> lO-CHO-THF. The A °-CH-THF produced is a substrate for GAR and AICAR transformylases catalyzing reactions 3 and 9 of the pathway. In higher eukaryotes, the dehydrogenase and cyclohydrolase activities are located in one domain of the protein, which is fused to a larger synthetase domain, forming a trifunctional enzyme. 

Since biosynthesis of IMP consumes glycine, glutamine, tetrahydrofolate derivatives, aspartate, and ATP, it is advantageous to regulate purine biosynthesis. The major determinant of the rate of de novo purine nucleotide biosynthesis is the concentration of PRPP, whose pool size depends on its rates of synthesis, utilization, and degradation. The rate of PRPP synthesis depends on the availabihty of ribose 5-phosphate and on the activity of PRPP synthase, an enzyme sensitive to feedback inhibition by AMP, ADP, GMP, and GDP. 

Fig. 14.1 Cellular pathway of methotrexate. ABCBl, ABCCl-4, ABC transporters ADA, adenosine deaminase ADP, adenosine diphosphate AICAR, aminoimidazole carboxamide ribonucleotide AMP, adenosine monophosphate ATIC, AICAR transformylase ATP, adenosine triphosphate SjlO-CH -THF, 5,10-methylene tetrahydrofolate 5-CHj-THF, 5-methyl tetrahydro-folate DHFR, dihydrofolate reductase dTMP, deoxythymidine monophosphate dUMP, deoxy-uridine monophosphate FAICAR, 10-formyl AICAR FH, dihydrofolate FPGS, folylpolyglutamyl synthase GGH, y-glutamyl hydrolase IMP, inosine monophosphate MTHFR, methylene tetrahydrofolate reductase MTR, methyl tetrahydrofolate reductase MTX-PG, methotrexate polyglutamate RFCl, reduced folate carrier 1 TYMS, thymidylate synthase. Italicized genes have been targets of pharmacogenetic analyses in studies published so far. (Reproduced from ref. 73 by permission of John Wiley and Sons Inc.)...

Figure 1. Inositol synthesis and catabolism. Inositol is synthesized from glucose-6-phosphate (Glucose 6-P) by the action of L-wyo-inositol 1-phosphate synthase (MIPS) and wyo-inositol monophosphatase (IMP). IMPase is also required for the last step of inositol (1,4,5)P3 second messenger breakdown. The first step in inositol catabolism utilizes the wyo-inositol oxygenase (MIOX) enzyme, which cleaves the inositol ring to form D-glucuronic acid.

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. 

The glutamine-dependent CP synthase has been extensively purified in E. coli and shown to be a biotin-containing enzyme with marked allosteric properties [74,75]. Homotropic interaction is evident from the sigmoidal kinetics obtained with ATP as substrate. Negative effectors are UMP, UDP and UTP, and purine nucleotides, particularly IMP, act as positive effectors. 

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). 

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. 

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