Cytochrome c

An electron transfer protein

One unique property of metals is their redox chemistry, they are stable in multiple oxidation states and thus can be used to shuttle electrons from site to site. Heme proteins have two very stable oxidation states, FeIII and FeII, and many different electron transfer cytochromes use heme cofactors to carry the charge. The term cytochrome means cell's color, and indicates the protein has a highly colored heme group. There are literally hundreds of different cytochromes which have been isolated and characterized.

One of the most studied of these is the small cytochrome c, 12kD, a component of the mitochondrial electron transport cycle involved in respiration. It lives between the membrane walls in the mitochondria, shuttling electrons between different membrane-bound redox sites. It also functions as a warning signal of damage to the mitochondria; its presence in the cytosol induces apoptosis, programmed cell death! The cell commits suicide rather than unleash the ROS generating machinery of the mitochondria on other healthy cells!

The cytochrome c heme is actually not a cofactor, it is covalently linked to the protein through two cystiene bridges. This is a post-translational modification-- in other words, a modification of the protein AFTER its biosynthesis. The Fe is bound by two axial ligands, a histidine and a methionine, in BOTH oxidation states. This means the actual structure of both states are similar, with only a small reorganizational barrier to electron transfer. So electron transfer can be very fast and efficient. This is illustrated in the cartoon below.