Biochemistry - The Chemical Reactions of Living Cells, Volume 2 - D. Metzler 1980

How electrons meet oxygen, how ATP is formed in the process, and some related phenomena
Hemoproteins
Hemes

It should be noted that Porphyrins contain a fully conjugated system of double bonds and possess two central hydrogen atoms bound to two nitrogen atoms. In reality, these two hydrogen atoms can freely migrate to other central nitrogen atoms with a corresponding redistribution of double bonds. Thus, the heme ring exhibits both Tautomerism and Resonance. The two central hydrogens can be replaced by various Metal Ions to form highly stable chelates. Complexes with Fe(II) are referred to as hemes, and the protoporphyrin IX complex with Fe(II) is called protoheme. These compounds may also be termed ferrohemes, whereas the Fe(III) compounds formed upon their oxidation are called ferrihemes. Since iron is characterized by a coordination number of six, two open coordination sites remain on either side of the heme, which can be occupied by other iron ligands. When these are nitrogenous ligands, such as pyridine or imidazole, the resulting compounds exhibit characteristic Absorption Spectra and are designated as hemochromes. Certain heme-containing Proteins, notably cytochrome b₅, which possesses two imidazole groups at these positions, display characteristic hemochromic spectra and can be regarded as hemochromes.

Several modifications of protoheme are known, as indicated in Fig. 10-1. To determine which specific heme is present in a given protein, it is usually cleaved by Treatment with acetone and Hydrochloric acid, and then, upon The addition of pyridine, converted into a pyridine hemochrome, which is analyzed spectrophotometrically. This approach has established that Hemoglobin, Myoglobin, Cytochromes b, o, and P-450, as well as catalases and peroxidases, contain protoheme. Cytochromes a and a3 contain heme a [3], whereas the terminal oxidase system of many Bacteria contains heme d (originally designated as heme a₂). In heme c (present in Cytochromes c, b₄, and f), the protein —SH groups are linked to the vinyl groups of protoheme. Recently, siroheme (Fig. 14-16) has been discovered, along with a formyl-group-containing heme in human erythrocyte n [3a].