Pterin tautomerization

Pterins belong to a family of nitrogen heterocyclic compounds and consist of 2-amino-4-hydroxypteridine. Due to keto-enol tautomerism (Eq. 1), pterin exists generally as the 4-keto, i.e. amido, form that is illustrated as 2-aminopteridin-4(3H)one (4) rather than the enol form (3). Various pterin derivatives have been unexceptionally isolated from almost all kinds of living organisms and almost all such pterin derivatives have carbon substituents on the C(6) position. 

Figure 3. A general scheme illustrating oxidation and reductions reactions of the pterin ring system, including tautomeric forms of the semireduced states.

Six-six condensed heterocyclic systems without a tautomeric functional group(s) [for instance, pteridine 135 (R = R2 = R4 = H)], do not exhibit prototropic tautomerism. The introduction of an oxo(thioxo)- and/or amino group(s) into the pteridine system, and the appearance of at least one NH group in the ring, leads to functional and/or annular tautomerism. Pteridine is formally the parent of three groups of compounds of particular interest because of their biological importance pterins (136), lumazines (137), and flavins (138). 

In summary, a 6-substituted pterin was first identified as a structural component of the molybdenum cofactor from sulfite oxidase, xanthine oxidase and nitrate reductase in 1980 (24). Subsequent studies provided good evidence that these enzymes possessed the same unstable molyb-dopterin (1), and it seemed likely that 1 was a constituent of all of the enzymes of Table I. It now appears that there is a family of closely related 6-substituted pterins that may differ in the oxidation state of the pterin ring, the stereochemistry of the dihydropterin ring, the tautomeric form of the side chain, and the presence and nature of a dinucleotide in the side chain. In some ways the variations that are being discovered for the pterin units of molybdenum enzymes are beginning to parallel the known complexity of naturally occurring porphyrins, which may have several possible side chains, various isomers of such side chains, and a partially reduced porphyrin skeleton (46). 

DHFR catalyzes the reduction of 7,8-dihydrofolate (H2F) to 5,6,7,8-tetrahydrofolate (H4F) using nicotinamide adenine dinucleotide phosphate (NADPH) as a cofactor (Fig. 17.1). Specifically, the pro-R hydride of NADPH is transferred stereospecifi-cally to the C6 of the pterin nucleus with concurrent protonation at the N5 position [1]. Structural studies of DHFR bound with substrates or substrate analogs have revealed the location and orientation of H2F, NADPH and the mechanistically important side chains [2]. Proper alignment of H2F and NADPH is crucial in enhancing the rate of the chemical step (hydride transfer). Ab initio, mixed quantum mechanical/molecular mechanical (QM/MM), and molecular dynamics computational studies have modeled the hydride transfer process and have deduced optimal geometries for the reaction [3-6]. The optimal C-C distance between the C4 of NADPH and C6 of H2F was calculated to be 2.7A [5, 6], which is significantly smaller than the initial distance of 3.34 A inferred from X-ray crystallography [2]. One proposed chemical mechanism involves a keto-enol tautomerization (Fig. 

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