electron transfer activity obsolete cytochrome d true OBSOLETE. A cytochrome in which the prosthetic group is a tetrapyrrolic chelate of iron in which the degree of conjugation of double bonds is less than in porphyrin. This definition would appear to include siroheme proteins (e.g. nitrite and sulfite reductases), but these are not cytochromes. cytochrome d molecular_function GO:0009462 This term was made obsolete because it represents a gene product.