The cytochromes are highly colored components of all aerobic cells (the word cytochrome comes from words meaning cell and color). They are moderate size proteins, typically consisting of about one hundred to a few hundred amino acid units. The active site or prosthetic group of all the cytochromes is an iron containing porphyrin, or heme. In a number of cytochromes, the heme unit is the same as the one in the oxygen carrying proteins hemoglobin and myoglobin, called iron-protoporphyrin IX. In other cytochromes there are different organic groups around the periphery of the porphyrin ring. The biological origin of the heme unit in all cytochromes is the same, porphobiligen, a five membered ring with one nitrogen atom, that undergoes a series of reactions to form the porphyrin ring which has four of the nitrogen atom containing five membered rings connected to one another by an intervening carbon atom...