Studies on models for tetrahydrofolic acid. 3. Hydrolytic interconversions of the tetrahydroquinoxaline analogs at the formate level of oxidation

From Bioorganic Models to Cells

I endeavor to share how various choices-some deliberate, some unconscious-and the unmistakable influence of many others shaped my scientific pursuits. I am fascinated by how two long-term, major streams of my research, DNA replication and purine biosynthesis, have merged with unexpected interconnections. If I have imparted to many of the talented individuals who have passed through my lab a degree of my passion for uncloaking the mysteries hidden in scientific research and an understanding of the honesty and rigor it demands and its impact on the world community, then my mentorship has been successful.

Simulation of Food Folate Digestion and Bioavailability of an Oxidation Product of 5-Methyltetrahydrofolate

Generating bioavailability data from in vivo studies is time-consuming and expensive. In vitro simulation can help to investigate factors influencing bioavailability or facilitate quantifying the impact of such factors. For folates, an efficient deconjugation of polyglutamates to the corresponding monoglutamates is crucial for bioavailability and highly dependent on the food matrix. Therefore, the bioaccessibility of folates of different foodstuffs was examined using a simulated digestion model with respect to folate stability and the efficiency of deconjugation. For realistic simulated deconjugation, porcine brush border membrane was used during the phase of the simulated digestion in the small intestine. For a better understanding of folate behaviour during digestion, single folate monoglutamates were also investigated with this in vitro digestion model. The results for bioaccessibility were compared with data from a human bioavailability study. They support the idea that both stability and deconjugation have an influence on bioaccessibility and thus on bioavailability. Tetrahydrofolate is probably lost completely or at least to a high extent and the stability of 5-methyltetrahydrofolate depends on the food matrix. Additionally, 5-methyltetrahydrofolate can be oxidised to a pyrazino-s-triazine (MeFox), whose absorption in the human intestinal tract was shown tentatively.

Cloning and characterization of methenyltetrahydrofolate synthetase from Saccharomyces cerevisiae

The folate derivative 5-formyltetrahydrofolate (folinic acid; 5-CHO-THF) was discovered over 40 years ago, but its role in metabolism remains poorly understood. Only one enzyme is known that utilizes 5-CHO-THF as a substrate: 5,10-methenyltetrahydrofolate synthetase (MTHFS). A BLAST search of the yeast genome using the human MTHFS sequence revealed a 211-amino acid open reading frame (YER183c) with significant homology. The yeast enzyme was expressed in Escherichia coli, and the purified recombinant enzyme exhibited kinetics similar to previously purified MTHFS. No new phenotype was observed in strains disrupted at MTHFS or in strains additionally disrupted at the genes encoding one or both serine hydroxymethyltransferases (SHMT) or at the genes encoding one or both methylenetetrahydrofolate reductases. However, when the MTHFS gene was disrupted in a strain lacking the de novo folate biosynthesis pathway, folinic acid (5-CHO-THF) could no longer support the folate requirement. We have thus named the yeast gene encoding methenyltetrahydrofolate synthetase FAU1 (folinic acid utilization). Disruption of the FAU1 gene in a strain lacking both 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR) transformylase isozymes (ADE16 and ADE17) resulted in a growth deficiency that was alleviated by methionine. Genetic analysis suggested that intracellular accumulation of the purine intermediate AICAR interferes with a step in methionine biosynthesis. Intracellular levels of 5-CHO-THF were determined in yeast disrupted at FAU1 and other genes encoding folate-dependent enzymes. In fau1 disruptants, 5-CHO-THF was elevated 4-fold over wild-type yeast. In yeast lacking MTHFS along with both AICAR transformylases, 5-CHO-THF was elevated 12-fold over wild type. 5-CHO-THF was undetectable in strains lacking SHMT activity, confirming SHMT as the in vivo source of 5-CHO-THF. Taken together, these results indicate that S. cerevisiae harbors a single, nonessential, MTHFS activity. Growth phenotypes of multiply disrupted strains are consistent with a regulatory role for 5-CHO-THF in one-carbon metabolism and additionally suggest a metabolic interaction between the purine and methionine pathways.

The crystal structure of methenyltetrahydromethanopterin cyclohydrolase from the hyperthermophilic archaeon Methanopyrus kandleri

BACKGROUND

The reduction of carbon dioxide to methane in methanogenic archaea involves the tetrahydrofolate analogue tetrahydromethanopterin (H(4)MPT) as a C(1) unit carrier. In the third step of this reaction sequence, N(5)-formyl-H(4)MPT is converted to methenyl-H(4)MPT(+) by the enzyme methenyltetrahydromethanopterin cyclohydrolase. The cyclohydrolase from the hyperthermophilic archaeon Methanopyrus kandleri (Mch) is extremely thermostable and adapted to a high intracellular concentration of lyotropic salts.

RESULTS

Mch was crystallized and its structure solved at 2.0 A resolution using a combination of the single isomorphous replacement (SIR) and multiple anomalous dispersion (MAD) techniques. The structure of the homotrimeric enzyme reveals a new alpha/beta fold that is composed of two domains forming a large sequence-conserved pocket between them. Two phosphate ions were found in and adjacent to this pocket, respectively; the latter is displaced by the phosphate moiety of the substrate formyl-H(4)MPT according to a hypothetical model of the substrate binding.

CONCLUSIONS

Although the exact position of the substrate is not yet known, the residues lining the active site of Mch could be tentatively assigned. Comparison of Mch with the tetrahydrofolate-specific cyclohydrolase/dehydrogenase reveals similarities in domain arrangement and in some active-site residues, whereas the fold appears to be different. The adaptation of Mch to high salt concentrations and high temperatures is reflected by the excess of acidic residues at the trimer surface and by the higher oligomerization state of Mch compared with its mesophtic counterparts.

Mechanism for the coupling of ATP hydrolysis to the conversion of 5-formyltetrahydrofolate to 5,10-methenyltetrahydrofolate

5,10-Methenyltetrahydrofolate synthetase catalyzes the irreversible conversion of 5-formyl-tetrahydropteroylpolyglutamates (5-CHO-H4PteGlu(n)) to 5,10-methenyltetrahydropteroylpolyglutamates (5, 10-CH(+)-H4PteGlu(n)). The equilibrium of the nonenzymatic reaction, which equilibrates slowly in the absence of enzyme, greatly favors 5-CHO-H4PteGlu(n). The enzyme couples the reaction to the hydrolysis of ATP shifting the equilibrium to favor 5,10-CH(+)-H4PteGlu(n). Substrate-dependent non-equilibrium isotope exchange of [3H]ADP into ATP was observed, suggesting the formation of a phosphorylated intermediate of 5-CHO-H4PteGlu(n) during the enzyme-catalyzed reaction. The competitive inhibitor 5-formyltetrahydrohomofolate also supported the ADP to ATP exchange, suggesting that this molecule could also form a phosphorylated intermediate. The initial rates of the ADP-ATP exchange with saturating ADP were about 70 s-1 for both compounds, while the kcat values for product formation were 5 s-1 for 5-CHO-H4PteGlu(n) and 0.005 s-1 for 5-formyltetrahydrohomofolate. Starting with 5(-)[18O]CHO-H4PteGlu(n), it was shown by 31P NMR that the formyl oxygen of the substrate was transferred to the product phosphate during the reaction. This further supports the existence of a phosphorylated intermediate. The formyl group of 5-CHO-H4PteGlu(n) is known to be an equilibrium mixture of two rotamers. Stopped-flow analysis of the enzymatic reaction showed that only one of the rotamers serves as a substrate for the enzyme.

Evidence for the accumulation of a stable intermediate in the nonenzymatic hydrolysis of 5,10-methenyltetrahydropteroylglutamate to 5-formyltetrahydropteroylglutamate

Solutions of 5,10-methenyltetrahydropteroylglutamate can be converted to a stable hydrated adduct by heating solutions at 50 degrees C at pH values of 3-5 for several hours. The adduct is stable at pH values from 4 to 9 for hours, but at pH values below 2 it is converted to 5,10-methenyltetrahydropteroylglutamate and at pH values above 8 it is converted to 5-formyltetrahydropteroylglutamate. Arguments are presented that the adduct is (11R)-5,10-hydroxymethylenetetrahydropteroylglutamate formed from (11S)-5,10-hydroxymethylenetetrahydropteroylglutamate by formation of an ylide at C-11 which undergoes inversion of the electron pair to form the (11R) isomer. The (11R) hydrated adducted is believed to be the isomer of 5,10-methenyltetrahydropteroylglutamate referred to as anhydroleucovorin B by Cosulich et al. [Cosulich, D. C., Roth, B., Smith, J. M., Hultquist, M. E., & Parker, R. P. (1952) J. Am. Chem. Soc. 74, 3252-3263]. In addition, a new mechanism for the formation of 5-formyltetrahydropteroylglutamate from either 5,10-methenyltetrahydropteroylglutamate or 10-formyltetrahydropteroylglutamate via (11R)-5,10-hydroxymethylenetetrahydropteroylglutamate is proposed. A requirement for this pathway is that the formyl proton of 10-formyltetrahydropteroylglutamate exchange with solvent protons. The exchange of this formyl proton was observed at all pH values from 5.5 to 11.5 at a rate which exceeded by more than an order of magnitude the rate of formation of 5-formyltetrahydropteroylglutamate.

A new chemical synthesis of 2-amino-(N-D-ribofuranosyl)acetamide 5'-phosphate

2-Amino-(N-D-ribosyl)acetamide 5'-phosphate (GAR,10) has been prepared in a readily scaled-up synthesis from 2,3-O-isopropylidene-D-ribofuranosylamine p-toluenesulfonate (3) by condensation with the mixed anhydride of N-)benzyloxy-carbonyl)glycine, followed by phosphorylation with 2-cyanoethyl phosphate and removal of the protecting groups. By varying the conditions, the alpha-4 and the beta-5 anomers could be obtained and separated from each other. Anomerization occurs upon removing the O-isopropylidene group from either 7 or 8, so that the final compound (10) is an equimolar mixture of both anomers, of which only one is active toward GAR-transformylase.