RECOMBINANT WILD-TYPE AND MUTANT COMPLEXES OF FERREDOXIN AND FERREDOXIN-NADP(+) REDUCTASE STUDIED BY ISOTHERMAL TITRATION CALORIMETRY

The interaction of spinach ferredoxin:NADP(+) reductase (FNR) with fer redoxin (Fd) is driven by a favorable change of entropy and shows almo st 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 ind icating a relatively small surface area buried in the complex. A singl e proton is taken up from the environment when the ferredoxir:FNR comp lex forms. In the complex, the protonated residue(s) is (are) probably located in the vicinity of E92 of Fd because charge reversal in Fd(E9 2K) quenches protonation. Substitution of K88 by Q in FNR(K88Q) destab ilizes the complex by a 7 kJ mol(-1) reduction in binding entropy, whi ch indicates that dehydration of the complex interface contributes to stability.