EFFECTS OF NO2-MODIFICATION OF TYR83 ON THE REACTIVITY OF SPINACH PLASTOCYANIN WITH CYTOCHROME-F

We have investigated the electron transfer (ET) reactions between turnip cytochrome f, and the native and NO2-Tyr83-modified forms of spinach plastocyanin (PCu) at 10.0-degrees-C and ionic strength 0.200 M(NaCl), in both directions as a function of pH. The PCu(II)/cytochrome f(II) rate constants in the pH-range 4-6.8 reflect active and remote binding site protonation. At higher pH, NO2-Tyr83 and positively charged residues on cytochrome f are deprotonated, and both native and NO2-modified PCu exhibit a composite rate constant variation in this pH range. When framed by ET theory this pattern is fully understandable in terms of variations in reduction potentials and electrostatic interactions, caused by the protonation equilibria. The rate constant ratio k(nitro)/k(native) is, however, only 1.04 for the PCu(II)/cytochrome f(II) reactions in spite of a 18 mV higher reduction potential for NO2-Tyr83-modified PCu. This is much lower than the value of 1.42 expected from ET theory solely on the basis of such a reduction potential effect. A similar effect is seen for PCu(I)/cytochrome f(III) for which the low-pH k(nitro)/k(native) ratio is 0.51. Notable but smaller effects are also observed for the small reaction partners [Fe(CN)6]3-/4- and [Co(phen)]3+/2+. The effect of NO2-modification in addition to the reduction potential effect can be resolved into a small reorganization energy increase around the copper atom and a smaller electronic transmission coefficient for ET through the Cu/Cys84/Tyr83 sequence. The former effect dominates in the reactions with the small reaction partners, while the electronic effects contribute significantly for PCu/cytochrome f, supporting the concept that the PCu/cytochrome f ET is at the remote PCu binding site.