Pterins tetrahydro- from

BH4 is essential for the AAHs to carry out their respective catalytic reactions and, at least for PAH, the prereductive activation, which appears to produce dihydrobiopterin quinonoid (g-BH2) directly (20). After the PAH catalytic cycle an oxygen atom is incorporated into the cofactor, providing 4a-OH-BH4 which dissociates from the active site. In order to regenerate the functional tetrahydro form of BH4 pterin carbinolamine dehydratase catalyzes the dehydration of 4-OH-BH4 to g-BH2, which is reduced back to by dihydropteridine reductase (Scheme 2). g-BH2 can also be converted to 7,8-dihydropterin (BH2) which can be regenerated to BH4 by dihydrofolate reductase (DHFR). 

A pteridine glycoside closely related to (463) was isolated from Chlorobium limicola f. thio-sulfatophilum NCIB 8327 and identified, by spectral means and chemical degradations, as l-(bio-pterin-2 -yl)-Af-acetyl-/J-D-glucosamine <95HCA600>. The native form of limipterin (464) is the 5,6,7,8-tetrahydro derivative. 

Application of fluorescence-detected circular dichroism (FDCD) to the determination of the major pterin (L-monapterin) from Escherichia coli has been published <01H(54)283>. Identification of (6/ )-5,6,7,8-tetrahydro-D-monapterin as the native pleridine in Tetrahymena pyriformis has been reported <01HCA918>. A novel modified pterin has been isolated from... 

The fully oxidized state, the semi-redueed or dihydro state and the fully reduced or tetrahydro state (Seheme 2.2a-e) are the three main redox states of pterins interconverted by 2e , 2H reaetions. The complexity of pterin redox chemistry results from many tautomeric forms of the semi-redueed state (Scheme 2.2d-h), whieh, unless highly substituted, will eventually rearrange to the most thermodynamieally stable form, ca. the 7,8-dihydropterin (Scheme 2.2b)... 

Pterin appears to cycle between the tetrahydro (BPH4) and dihydro forms, providing the two electrons necessary for reduction of dioxygen as shown in Scheme 6. One oxygen atom is incorporated into substrate and another into water as shown in the general equation of monooxygenase-catalyzed reaction [see eq. (1)]. Interesting information from the side of chemistry has been obtained recently in the studies of PAH and TH, but not TPH. 

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