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.