Pterinechemistry. Part 84. A new, regiospecific synthesis ofL-biopterin

Making Moco: A Personal History

This contribution describes the path of my nearly forty-year quest to understand the special ligand coordinated to molybdenum and tungsten ions in their respective enzymes. Through this quest, I aimed to discover why nature did not simply use a methyl group on the dithiolene that chelates Mo and W but instead chose a complicated pyranopterin. My journey sought answers through the synthesis of model Mo compounds that allowed systematic investigations of the interactions between molybdenum and pterin and molybdenum and pterin-dithiolene and revealed special features of the pyranopterin dithiolene chelate bound to molybdenum.

A Regioselective Synthesis of the Dephospho DIthiolene Protected Molybdopterin

The molybdopterin (MPT) is a complex molecule, made out of three distinctly different components. A retrosynthetic analysis provides a possible route for its synthesis that utilizes the coupling of a diamine with an osone analog. A regioselective condensation of the diamine with an osone affords the dephospho MPT, which has been characterized by NMR and IR spectroscopies, as well as high-resolution mass spectrometry.

Pterin chemistry and its relationship to the molybdenum cofactor

The molybdenum cofactor is composed of a molybdenum coordinated by one or two rather complicated ligands known as either molybdopterin or pyranopterin. Pterin is one of a large family of bicyclic N-heterocycles called pteridines. Such molecules are widely found in Nature, having various forms to perform a variety of biological functions. This article describes the basic nomenclature of pterin, their biological roles, structure, chemical synthesis and redox reactivity. In addition, the biosynthesis of pterins and current models of the molybdenum cofactor are discussed.

Redox reactions of the pyranopterin system of the molybdenum cofactor

This work provides the first extensive study of the redox reactivity of the pyranopterin system that is a component of the catalytic site of all molybdenum and tungsten enzymes possessing molybdopterin. The pyranopterin system possesses certain characteristics typical of tetrahydropterins, such as a reduced pyrazine ring; however, it behaves as a dihydropterin in redox reactions with oxidants. Titrations using ferricyanide and dichloroindophenol (DCIP) prove a 2e(-)/2H(+) stoichiometry for pyranopterin oxidations. Oxidations of pyranopterin by Fe(CN)(6)(3-) or DCIP are slower than tetrahydropterin oxidation under a variety of conditions, but are considerably faster than observed for oxidations of dihydropterin. The rate of pyranopterin oxidation by DCIP was studied in a variety of media. In aqueous buffered solution the pyranopterin oxidation rate has minimal pH dependence, whereas the rate of tetrahydropterin oxidation decreases 100-fold over the pH range 7.4-8.5. Although pyranopterin reacts as a dihydropterin with oxidants, it resists further reduction to a tetrahydropterin. No reduction was achieved by catalytic hydrogenation, even after several days. The reducing ability of the commonly used biological reductants dithionite and methyl viologen radical cation was investigated, but experiments showed no evidence of pyranopterin reduction by any of these reducing agents. This study illustrates the dual personalities of pyranopterin and underscores the unique place that the pyranopterin system holds in the spectrum of pterin redox reactions. The work presented here has important implications for understanding the biosynthesis and reaction chemistry of the pyranopterin cofactor in molybdenum and tungsten enzymes.

Human pterin-4 alpha-carbinolamine dehydratase/dimerization cofactor of hepatocyte nuclear factor-1 alpha. Characterization and kinetic analysis of wild-type and mutant enzymes

Pterin-4a-carbinolamine dehydratase/dimerization cofactor for hepatocyte nuclear factor-1 alpha is a protein with two different functions. We have overexpressed and purified the human wild-type protein, and its Cys81Ser and Cys81Arg mutants. The Cys81Arg mutant has been proposed to be causative in a hyperphenylalaninaemic patient [Citron, B. A., Kaufman, S., Milstien, S., Naylor, E. W., Greene, C. L. & Davis, M. D. (1993) Am. J. Hum. Genet. 53, 768-774]. The dehydratase behaves as a tetramer on gel filtration, while cross-linking experiments showed mono-, di-, tri-, and tetrameric forms, irrespective of the presence of the single Cys81. Sulfhydryl-modifying reagents did not affect the activity, but rather showed that Cys81 is exposed. Various pterins bind and quench the tryptophan fluorescence suggesting the presence of a specific binding site. The fluorescence is destroyed upon light irradiation. Wild-type and the Cys81Ser protein enhance the rate of the phenylalanine hydroxylase assay approximately 10-fold, a value similar to that of native dehydratase from rat liver; the Cys81Arg mutant, in contrast, has significantly lower activity. This is compatible with the hypothesis that the dehydratase is a rate-limiting factor for the in vivo phenylalanine hydroxylase reaction. The three proteins enhance the spontaneous dehydration of the synthetic substrate 6,6-dimethyl-7,8-dihydropterin-4a-carbinolamine approximately 50-70-fold at 4 degrees C and pH 8.5. The results are discussed in view of the recently solved three-dimensional structure of the enzyme [Ficner, R., Sauer, U. W., Stier, G. & Suck, D. (1995) EMBO J. 14, 2032-2042].