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°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 uncatalysed 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.