Substrate specificity of glycinamide ribonucleotide transformylase from chicken liver

Stereoselective Synthesis of β-Glycinamide Ribonucleotide

A diastereoselective synthesis of the β-anomer of glycinamide ribonucleotide (β-GAR) has been developed. The synthesis was accomplished in nine steps from D-ribose and occurred in 5% overall yield. The route provided material on the multi-milligram scale. The synthetic β-GAR formed was remarkably resistant to anomerization both in solution and as a solid.

Substrate specificity of human glycinamide ribonucleotide transformylase

The nucleotide substrate specificity of human glycinamide ribonucleotide transformylase, a chemotherapeutic target, has been examined. The enzyme accepts the sarcosyl analog of glycinamide ribonucleotide, carbocyclic glycinamide ribonucleotide, and two phosphonate derivatives of carbocyclic glycinamide ribonucleotide with V/K values, relative to that obtained for beta-glycinamide ribonucleotide, of 1, 27, 1.4, and 2.9%, respectively. Several other analogs of carbocyclic glycinamide ribonucleotide, namely a truncated phosphonate and 2',3'-dideoxy- and 2',3'-dideoxy-2',3'-didehydro-carbocyclic glycinamide ribonucleotide, were inhibitors of the enzyme, competitive against glycinamide ribonucleotide, with Ki values approximately 100 times higher than the Km for -glycinamide ribonucleotide. Although the results of the present study parallel those obtained previously with the avian enzyme (V. D. Antle, D. Liu, B. R. McKellar, C. A. Caperelli, M. Hua, and R. Vince (1996) J. Biol. Chem. 271, 6045-6049), quantitative differences between the two enzyme species have been uncovered.

Synthesis of chiral carbocyclic ribonucleotides

The carbocyclic analogs of CMP, UMP, GMP, IMP, and ribo-TMP, of the same absolute configuration as the naturally occurring beta-D-ribofuranose-based ribonucleoside monophosphates, have been synthesized. The synthetic route employed Mitsunobu coupling of the heterocycles, appropriately protected where necessary, with a differentially protected, chiral carbocyclic core.

The human trifunctional enzyme of de novo purine biosynthesis: heterologous expression, purification, and preliminary characterization

The cDNA for the human trifunctional enzyme of de novo purine biosynthesis, which encodes glycinamide ribonucleotide synthetase, aminoimidazole ribonucleotide synthetase, and glycinamide ribonucleotide trans-formylase, has been overexpressed in Escherichia coli and its protein product has been purified to homogeneity. The glycinamide ribonucleotide transformylase activity, which constitutes the C-terminal domain of the trifunctional enzyme, has been characterized with respect to its kinetic constants, Vmax = 3.03 +/- 0.15 micromol/min-mg and Km values for beta-glycinamide ribonucleotide and 10-formyl-5,8-dideazafolate of 0.94 +/- 0.21 and 1.58 +/- 0.25 microM, respectively, and its kinetic mechanism, which is ordered-sequential with the folate substrate binding first. The correspondence of these data to those obtained for the glycinamide ribonucleotide transformylase activity of the mammalian trifunctional enzyme indicates that the recombinant enzyme is fully functional.

Tight binding of folate substrates and inhibitors to recombinant mouse glycinamide ribonucleotide formyltransferase

The binding of the prototypical folate inhibitor of de novo purine synthesis, 5,10-dideazatetrahydrofolate (DDATHF), and its hexaglutamate to recombinant trifunctional mouse glycinamide ribonucleotide formyltransferase (rmGARFT) was studied by equilibrium dialysis and by steady-state kinetics using sensitive assays that allowed initial rate calculations. rmGARFT was expressed in insect cells infected with a recombinant baculovirus and purified by a two-step procedure that allowed production of about 25 mg of pure protein/L of culture. The binding of DDATHF to GARFT was approximately 50-fold tighter than previously reported, with Kd and Ki values of 2-9 nM, making the parent form of this antifolate a tight-binding inhibitor. The binding of the hexaglutamate of DDATHF to rmGARFT had Kd and Ki values of 0.1-0.3 nM, consistent with the view that polyglutamation enhances binding of antifolates to GARFT. Kinetic analyses using either mono- or hexaglutamate substrate did not yield different values for the Ki for the hexaglutamate form of DDATHF, in contradiction with previous reports. Both the folate substrate commonly used to study GARFT, 10-formyl-5,8-dideazafolate, and its hexaglutamate were found to have very low Km values, namely, 75 and 7.4 nM, respectively, and the folate reaction products for these substrates were equally potent inhibitors, results which modify the interpretation of previous kinetic experiments. The product analog DDATHF and beta-glycinamide ribonucleotide bound to enzyme equally well in the presence and absence of the other, an observation at variance with the concept that GARFT obeys an ordered sequential binding of the substrates. We conclude that the kinetics of mouse GARFT are most consistent with a random order of substrate binding, that both the inhibitor DDATHF and the folate substrate are tight-binding ligands, and that polyglutamate forms enhance the affinity of both substrate and inhibitor by an order of magnitude.

The human glycinamide ribonucleotide transformylase domain: purification, characterization, and kinetic mechanism

Glycinamide ribonucleotide transformylase catalyzes the third reaction of de novo purine biosynthesis, namely, the conversion of glycinamide ribonucleotide to N-formylglycinamide ribonucleotide, with concomitant conversion of 10-formyltetrahydrofolate to tetrahydrofolate. This activity has been shown to be a target for cancer chemotherapy, which has generated renewed interest in both the enzyme and the pathway. Moreover, in higher eukaryotes this activity constitutes the C-terminal domain of a monomeric protein which also catalyzes two additional reactions of de novo purine biosynthesis. In this study, the human glycinamide ribonucleotide transformylase domain has been expressed to high levels in Escherichia coli and purified to homogeneity. Our improved expression-purification system produces the desired activity exclusively in a soluble form and in higher abundance than previously achieved. The kinetic constants have been determined and the kinetic mechanism has been established as ordered-sequential, with the folate substrate binding first. The correspondence of these data to those obtained for the glycinamide ribonucleotide transformylase activity of the mammalian trifunctional enzyme indicates that the recombinant enzyme is fully functional.