7-8-dihydrobiopterin and dihydrofolate

Nuclear magnetic resonance (NMR) spectra for [2-amino,3-15N2]folate and [2-13C]folate complexed with human dihydrofolate reductase, and for complexes of similarly labeled dihydrofolate, show that the N-3 proton of bound folate or dihydrofolate exchanges slowly with solvent and that the bound substrates are in the imino-keto tautomeric form. Previously proposed schemes for substrate protonation that require bound substrate to be in the enolic tautomer are therefore unlikely. The NMR spectra for bound folate are unchanged by raising the pH from 7 to 9.5, whereas those for free folate show marked changes due to ionization for the N-3 proton. The fraction of bound folate with the N-3 proton ionized at pH 9.5 is therefore very small, and the rate constant for the dissociation of the ionized species must be at least 320 times faster than for the protonated species. Comparison of NMR spectra over the pH range 5 to 7 gives no indication of a change in ionization state of the Glu30 carboxyl group over this pH range. This raises doubts about whether the apparent pKa of approximately 6 that describes pH dependence of hydride transfer is due to ionization of this carboxyl group.

Topics: Amino Acid Sequence; Binding Sites; Biopterins; Carbon Isotopes; Folic Acid; Glutamates; Glutamic Acid; Humans; Hydrogen-Ion Concentration; Kinetics; Ligands; Magnetic Resonance Spectroscopy; Models, Theoretical; Nitrogen Isotopes; Protons; Recombinant Proteins; Tetrahydrofolate Dehydrogenase

The dissociation constants (pKa) for the pteridine ring system of dihydrofolate (H2folate) have been redetermined, and those for dihydrobiopterin (H2biopterin) have been determined. Determination of the pKa for N5 of H2folate is complicated by the low solubility and instability of H2folate at pH 2-4, and other complicating factors. The initial rate of absorbance change due to degradation is a maximum at pH 2.5, and the products depend on the oxygen concentration: under aerobic conditions, (p-aminobenzoyl)glutamic acid and 7,8-dihydropterin-6-carboxaldehyde are major products. H2Biopterin is much more soluble and more stable at low pH. For protonation of N5, the pKa is 2.56 +/- 0.01 for H2biopterin and 2.59 +/- 0.03 for H2folic acid. Spectrophotometric determination of the pKa for the N3-O4 amide group of H2folate is subject to serious errors when a wavelength between 220 and 235 nm is used. These errors arise from the pH-dependent absorbance of mercaptoethanol often present in the preparation. The amide group has a pKa of 10.41 +/- 0.04 in H2biopterin and 10.85 +/- 0.04 in H2folate. The redetermined value for the pKa of N5 of H2folate has implications for mechanistic models for dihydrofolate reductase, and revised kinetic constants have been calculated for one model.

Topics: Biopterins; Folic Acid; Hydrogen-Ion Concentration; Kinetics; Mercaptoethanol; Models, Chemical; Osmolar Concentration; Solubility; Spectrophotometry, Ultraviolet; Tetrahydrofolate Dehydrogenase

The substrate specificity of dihydrofolate reductase from cells of different origin has been thought to be quite narrow, and unconjugated dihydropterins such as 6-methyl-dihydropterin are known to be very poor substrates. We have reinvestigated the substrate specificity of several dihydropterins and, in addition, have observed that in a new series of unconjugated dihydropterins of the general structure 6-CH2O(CH2)nCH3 several compounds are excellent substrates for the bovine liver enzyme, but none of them bind as well as dihydrofolate. The substrate activity (apparent Vmax) of these compounds increases from 17 to 110% that of the natural substrate, dihydrofolate, as n is increased from 0 to 3. In contrast, these unconjugated dihydropterins are very poor substrates for the Escherichia coli enzyme.

Topics: Animals; Biopterins; Cattle; Folic Acid; Kinetics; Liver; Pteridines; Pterins; Substrate Specificity; Tetrahydrofolate Dehydrogenase