Recombinant wild-type and mutant complexes of ferredoxin and ferredoxin:NADP+ reductase studied by isothermal titration calorimetry.

Abstract

The interaction of spinach ferredoxin:NADP+ reductase (FNR) with ferredoxin (Fd) is driven by a favorable change of entropy and shows almost no change in enthalpy. The change in heat capacity between the free proteins and the complex is -0.47 +/- 0.1 kJ mol(-1) K(-1), a value indicating a relatively small surface area buried in the complex. A single proton is taken up from the environment when the ferredoxin:FNR complex forms. In the complex, the protonated residue(s) is (are) probably located in the vicinity of E92 of Fd because charge reversal in Fd(E92K) quenches protonation. Substitution of K88 by Q in FNR(K88Q) destabilizes the complex by a 7 kJ mol(-1) reduction in binding entropy, which indicates that dehydration of the complex interface contributes to stability.