Because metalloproteins act as electron carriers in essential biological processes, such as respiration, photosynthesis, and metabolism, mechanisms of their electron-transfer reactions are being studied vigorously. The goal for many years has been to understand how protein structure and the structure of the redox center itself influence the rate constants for electron transfer between proteins. Toward this goal, we strive to understand how protein conformation modulates electron-transfer reactivity in heme proteins. We investigated photoinduced electron-transfer reactions of folded, partially unfolded, and completely unfolded zinc cytochrome c (Zncyt) with various inorganic and protein electron acceptors.;In our studies of Zncyt in the folded conformation, photoinduced reactions of the excited triplet state of Zncyt (3Zncyt) with iron(III) cytochrome c, iron(II) cytochrome c, metal-free porphyrin cytochrome c, and heme-free apocytochrome c revealed the mechanism of quenching. Electron transfer from 3Zncyt to iron(III) cytochrome c is a more efficient quenching mechanism than energy transfer and enhanced radiationless decay at high ionic strength. Iron-free porphyrin cytochrome c and iron(II) cytochrome c quenches 3Zncyt by energy transfer.;To study the effect of protein conformation on reactivity, we compared the electron-transfer properties of 3Zncyt in the folded, molten-globule, and completely unfolded forms toward the following four oxidative quenchers: Fe(CN)63-, Co(acac) 3, Co(phen)33+, and iron(III) cytochrome c. The observed bimolecular rate constants show electron-transfer reactivity depends mostly on electrostatic interactions and the degree of porphyrin exposure as the protein unfolds. Ionic strength and pH are chosen in some cases to bring out, but in most cases to minimize, effects of electrostatic interactions between Zncyt and the oxidative quenchers, so that effects of conformation on reactivity become discernible. Using the electroneutral complex Co(acac)3 as a quencher, we eliminated the electrostatic effects and assessed only the consequences of porphyrin exposure upon partial and complete unfolding of the protein for the electron-transfer reactivity. The bimolecular rate constant for the reaction of 3Zncyt and Co(acac) 3 increases 10-fold upon partial unfolding into the molten-globule form and approximately 50-fold upon complete unfolding of Zncyt. Electroneutral inorganic complexes are sensitive probes of the amount of partial and complete unfolding in zinc-substituted heme proteins.