Structural insights into the dual substrate specificities of mammalian and Dictyostelium dihydropteridine reductases toward two stereoisomers of quinonoid dihydrobiopterin.

Up to now, d-threo-tetrahydrobiopterin (DH(4), dictyopterin) was detected only in Dictyostelium discoideum, while the isomer L-erythro-tetrahydrobioterin (BH(4)) is common in mammals. To elucidate the mechanism of DH(4) regeneration by D. discoideum dihydropteridine reductase (DicDHPR), we have determined the crystal structure of DicDHPR complexed with NAD(+) at 2.16 Å resolution. Significant structural differences from mammalian DHPRs are found around the coenzyme binding site, resulting in a higher K(m) value for NADH (K(m)=46.51±0.4 μM) than mammals. In addition, we have found that rat DHPR as well as DicDHPR could bind to both substrates quinonoid-BH(2) and quinonoid-DH(2) by docking calculations and have confirmed their catalytic activity by in vitro assay.