Cytochromes are a group of red or brown heme-containing proteins (Figure 15.6) having distinctive visible-light spectra (Figure 15.5). The major respiratory cytochromes are classified as b, c, or a, depending on the wavelengths of the spectral absorption peaks. Within each class (b, c, or a), the cytochromes are distinguished by smaller spectral differences. For example, cytochrome c1 has a spectrum similar to that of cytochrome c, but the and absorption peaks are shifted slightly toward the red.

Among the respiratory electron carriers are three b-type cytochromes, cytochromes c and c1, and cytochromes a and a3. Cytochromes b, c, and c1 all contain the same heme found in hemoglobin and myoglobin - iron complexed with protoporphyrin IX (Figure 7.4). In cytochromes c and c1, but not b, this heme is linked covalently to the protein component via thioether bonds formed between two of the vinyl side chains and two cysteine residues (Figure 15.6a).

Cytochromes a and a3 contain heme A, in which two of the side chains are modified (Figure 15.6b). Cytochromes a and a3 evidently represent two identical heme A moieties, attached to the same polypeptide chain. They are within different environments in the inner membrane, however, so they have different reduction potentials. Each of the hemes in cytochromes a and a3 is associated with a copper ion, located close to the heme iron. Cytochromes undergo oxidoreduction through the complexed metal, which cycles between +2 and +3 states for the heme iron and +1 and +2 states for the copper in cytochromes a and a3. Thus, the cytochromes are one-electron carriers. Cytochromes a and a3 form part of mitochondrial complex IV (Figures 15.2 and 15.3).

Cytochrome c is a small protein (Mr = 13,000). It is associated with the inner membrane of the mitochondria, but is readily extracted in soluble form. Because it is small and relatively abundant, detailed structural studies have been carried out with this protein. The amino acid sequence of the protein has been highly conserved in evolution, with nearly 50% identity between residues at corresponding positions of cytochromes c in organisms as diverse as yeast and human. The other cytochromes are integral membrane proteins and are exceedingly difficult to dissociate from the membrane. As a result, less is known about their structure.