Isotope, pulse-chase, stopped-flow, and rapid quench studies on the kinetic mechanism of bovine dihydropteridine reductase

Inhibition kinetics and emodin cocrystal structure of a type II polyketide ketoreductase

Type II polyketides are a class of natural products that include pharmaceutically important aromatic compounds such as the antibiotic tetracycline and antitumor compound doxorubicin. The type II polyketide synthase (PKS) is a complex consisting of 5-10 standalone domains homologous to fatty acid synthase (FAS). Polyketide ketoreductase (KR) provides regio- and stereochemical diversity during the reduction. How the type II polyketide KR specifically reduces only the C9 carbonyl group is not well understood. The cocrystal structures of actinorhodin polyketide ketoreductase (actKR) bound with NADPH or NADP+ and the inhibitor emodin were solved with the wild type and P94L mutant of actKR, revealing the first observation of a bent p-quinone in an enzyme active site. Molecular dynamics simulation help explain the origin of the bent geometry. Extensive screening for in vitro substrates shows that unlike FAS KR, the actKR prefers bicyclic substrates. Inhibition kinetics indicate that actKR follows an ordered Bi Bi mechanism. Together with docking simulations that identified a potential phosphopantetheine binding groove, the structural and functional studies reveal that the C9 specificity is a result of active site geometry and substrate ring constraints. The results lay the foundation for the design of novel aromatic polyketide natural products with different reduction patterns.

Purification and characterization of a novel mannitol dehydrogenase from a newly isolated strain of Candida magnoliae

Mannitol biosynthesis in Candida magnoliae HH-01 (KCCM-10252), a yeast strain that is currently used for the industrial production of mannitol, is catalyzed by mannitol dehydrogenase (MDH) (EC 1.1.1.138). In this study, NAD(P)H-dependent MDH was purified to homogeneity from C. magnoliae HH-01 by ion-exchange chromatography, hydrophobic interaction chromatography, and affinity chromatography. The relative molecular masses of C. magnoliae MDH, as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and size-exclusion chromatography, were 35 and 142 kDa, respectively, indicating that the enzyme is a tetramer. This enzyme catalyzed both fructose reduction and mannitol oxidation. The pH and temperature optima for fructose reduction and mannitol oxidation were 7.5 and 37 degrees C and 10.0 and 40 degrees C, respectively. C. magnoliae MDH showed high substrate specificity and high catalytic efficiency (k(cat) = 823 s(-1), K(m) = 28.0 mM, and k(cat)/K(m) = 29.4 mM(-1) s(-1)) for fructose, which may explain the high mannitol production observed in this strain. Initial velocity and product inhibition studies suggest that the reaction proceeds via a sequential ordered Bi Bi mechanism, and C. magnoliae MDH is specific for transferring the 4-pro-S hydrogen of NADPH, which is typical of a short-chain dehydrogenase reductase (SDR). The internal amino acid sequences of C. magnoliae MDH showed a significant homology with SDRs from various sources, indicating that the C. magnoliae MDH is an NAD(P)H-dependent tetrameric SDR. Although MDHs have been purified and characterized from several other sources, C. magnoliae MDH is distinguished from other MDHs by its high substrate specificity and catalytic efficiency for fructose only, which makes C. magnoliae MDH the ideal choice for industrial applications, including enzymatic synthesis of mannitol and salt-tolerant plants.

Temperature dependence of dihydropteridine reductase activity

The kinetic parameters Km and kcat of dihydropteridine reductase with a mixture of 6R and 6S quinonoid 7,8(6H)-dihydrobiopterin were determined at several temperatures in the range 20-37 degrees C. Both Km and kcat increased with temperature. Thermodynamic activation parameters were calculated and compared with those for the non-enzymic reduction of quinonoid 7,8(6H)-dihydrobiopterin by NADH. The temperature coefficients of the enzyme catalysed and uncatalyzed reactions are 3.3 and 1.67 respectively. The results are consistent with an ordered bi-bi enzyme mechanism, in which the rate-determining step is an isomerisation of the ternary complex. This isomerisation involves a positive entropy of activation, which overcomes an enthalpy of activation that is significantly higher for the enzymic than for the non-enzymic reaction.

New pteridine substrates for dihydropteridine reductase and horseradish peroxidase

The oxidation of 4,5-diaminopyrimidin-6(1H)-one, 5,6,7,8-tetrahydropteridin-4(3H)-one, its 6-methyl and cis-6,7-dimethyl derivatives, and 6-methyl- and cis-6-7-dimethyl-5,6,7,8-tetrahydropterins, by horseradish peroxidase/H2O2 is enzymic and follows Michaelis-Menten kinetics, and its Km and kcat. values were determined. This oxidation of 5,6,7,8-tetrahydropterins produces quinonoid dihydropterins of established structure, and they are known to be specific substrates for dihydropteridine reductase. By analogy the peroxidase/H2O2 oxidation of the 5,6,7,8-tetrahydropteridin-4(3H)-ones should produce similar quinonoid dihydro species. The quinonoid species derived from 5,6,7,8-tetrahydropteridin-4(3H)-one and its 6-methyl and cis-6,7-dimethyl derivatives are shown to be viable substrates for human brain dihydropteridine reductase, and apparent Km and Vmax. values are reported.