Homologous proteins in the electron transport chains of mitochondria and chloroplasts

(a) Cytochrome b6, a subunit of the cytochrome b6-f complex found in chloroplasts, and (b) cytochrome b, a subunit of the cytochrome b-c1 complex found in mitochondria, are homologous proteins that play similar roles in their respective electron transport chains. The inset shows the three-dimensional structure of cytochrome b, which was determined by X-ray crystallography. It is an integral membrane protein with several transmembrane ? helices and two heme groups, which are prosthetic groups involved in electron transfer. The structure of cytochrome b6 has also been determined and found to be very similar. (Note: The orientation of cytochrome b in the inset is such that it would pump H+ out of the matrix. The orientation of cytochrome b6 is oriented so it would pump H+ out of the stroma.)