Phosphomimetic Substitution of Cytochrome c Tyrosine 48 Decreases Respiration and Binding to Cardiolipin and Abolishes Ability to Trigger Downstream Caspase Activation

Mammalian cytochrome c (Cytc) transfers electrons from the bc1 complex to cytochrome c oxidase (CcO) as part of the mitochondrial electron transport chain, and it also participates in type II apoptosis. Our recent discovery of two tyrosine phosphorylation sites in Cytc, Tyr97 in bovine heart and Tyr48 in bovine liver, indicates that Cytc functions are regulated through cell signaling. To characterize the role of Cytc tyrosine phosphorylation in detail using an independent approach, we here overexpressed and purified a Tyr48Glu mutant Cytc, mimicking the in vivo Tyr48 phosphorylation found in cow liver, along with wild-type and Tyr48Phe variants as controls. The midpoint redox potential of the phosphomimetic mutant was decreased by 45 mV compared to control (192 vs 237 mV). Similar to Tyr48 in vivo phosphorylated Cytc, direct kinetic analysis of the Cytc reaction with isolated CcO revealed decreased Vmax for the Tyr48Glu mutant by 30% compared to wild type or the Tyr48Phe variants. Moreover, the phosphomim...