GAR transformylase

There are four multifunctional enzymes in the pathway A trifunctional enzyme comprising GAR synthetase, GAR transformylase and AIR synthetase catalyzes reactions 2, 3 and 5 (PRA—> GAR— FGAR, FGAM > AIR Fig. 15-16), respectively. The GAR synthetase and GAR transformylase domains may be separated by digestion of the trifunctional enzyme with the protease, chymotrypsin. 

Fig. 15-16 The de novo purine biosynthetic pathway. Rib-5-P, ribose 5-phosphate P-Rib-PP, 5-phosphoribosyl 1-pyrophosphate PRA, 5-phosphoribosylamine IO-CHO-FH4, /Vl0-formyl tetrahydrofolate GAR, glycineamide ribotide FGAR. /V-formylglycineamide ribotide FGAM, /V-formylglycineamidine ribotide AIR, 5-aminoimidazole ribotide CAIR, 4-carboxy-5-aminoimidazole ribotide SAICAR, iV-succino-5-aminoimidazole-4-carboxamide ribotide AICAR, 5-aminoimidazole-4-carboxamide ribotide FAICAR, 5-formamidoimidazole-4-carboxamide ribotide sAMP, /V-succino-AMP. Enzymes (1) amido phosphoribosyltransferase (2) GAR synthetase (3) GAR transformylase (4) FGAM synthetase (5) AIR synthetase (6) AIR carboxylase (7) SAICAR synthetase (8) adenylosuecinase (9) AICAR transformylase (10) IMP cyclohydrolase (11) sAMP synthetase (12) adenylosuecinasc (13) IMP dehydrogenase (14) GMP synthetase.

While providing valuable insight towards a better understanding of protein evolution, the bisection of proteins into functional heterodimers has also found practical applications to study protein-protein interactions. Heterodimeric variants of dihydrofolate reductase (DHFR), green fluorescent protein (GFP), GAR transformylase, and phosphotransferases were constructed to work in two-hybrid systems [64]. 

The possibility that the iV °-formyl derivative (III.163) might serve as a substrate for two important enzymes of the de novo purine pathway that utilize reduced folates as their natural substrates, namely 5-aminoimidazole-4-car-boxamide ribonucleotide (AICAR) transformylase and glycinamide ribonucleotide (GAR) transformylase, was examined [61]. While the affinity of (III. 163) (AT, (app) = 29/xM) for AICAR transformylase appeared to be greater than that of the natural substrate 10-formyltetrahydrofolate (A (app) = 68 /xM), the reaction was slow, resulting in a 750-fold lower Frei/Am(app) ratio for the quinazoline. The affinity of (III. 163) (/frn(app) = 1.9 /xM) for GAR transformylase was likewise several times greater than that of the natural substrate, in this case 5,10-methenyltetra-hydrofolate (/if,n(app) = 8.9/xM). However, (III. 163) was also used rather efficiently in the reaction by GAR transformylase, resulting in a 4-fold higher V.J Ai, (app) for the quinazoline than for 5,10-methenyltetrahydrofolate. The authors concluded from these results that the 5,10-methenyl structure is not needed for GAR transformylase activity. 

The pathway outlining the normal synthesis of AMP and GMP is provided in Figure 42.31. It is important to recognize that the rate-limiting step is the first of the pathway if that step is inhibited, no other step can proceed. Also, note that the rate-limiting transferase enzyme works on a phosphorylated ribose substrate. Because phosphorylated ribose is a component of every intermediate in the pathway, no enzyme in the sequence will function without its presence. Finally, note the reaction in the pathway catalyzed by GAR transformylase, which requires the methyl-donating 10-formyltetrahydrofolate. As previously mentioned, this step is inhibited by methotrexate. 

Glycinamide ribonucleotide (GAR) transformylase is an enzyme that requires a THF-coenzyme. The formyl group that is given to the substrate eventually ends up... 

Fig. 3. The pathway of de novo purine ribonucleotide biosynthesis. The pathway includes the synthesis of PRPP, which is also used in the synthesis of pyrimidines, pyridine nucleotides, histidine, and tryptophan in plants. The enzymes catalyzing the numbered reactions are (1) PRPP synthetase, (2) PRPP amidotransferase, (3) GAR synthetase, (4) GAR transformylase, (5) FGAR amidotransferase, (6) AIR synthetase, (7) AIR carboxylase, (8) succino-AICAR synthetase, (9) adenylosuccinase, (10) AICAR transformylase, and (11) IMP cyclohydrolase.

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