Heme-bound iron

The most conspicuous use of iron in biological systems is in our blood, where the erythrocytes are filled with the oxygen-binding protein hemoglobin. The red color of blood is due to the iron atom bound to the heme group in hemoglobin. Similar heme-bound iron atoms are present in a number of proteins involved in electron-transfer reactions, notably cytochromes. A chemically more sophisticated use of iron is found in an enzyme, ribo nucleotide reductase, that catalyzes the conversion of ribonucleotides to deoxyribonucleotides, an important step in the synthesis of the building blocks of DNA. 

Except for the two histidines associated with the heme-bound iron atom, all the polar side chains are well positioned for hydrogen bonding with solvent molecules (water). In contrast, all the nonpolar side chains adopt interior positions, avoiding contact with polar solvent molecules. 

It is well known and confirmed that heme-bound iron from animal-based foods is absorbed differently from iron in plant-based foods [111]. This requires intrinsic labeling of the diet/test meal if iron absorption from meat needs to be assessed. A separate pathway of iron absorption has been suggested for ferritin-bound iron [112], but it appears to be of minor relevance to iron nutrition. Ferritin is the body s most important iron storage protein and can trap iron in a hollow protein sphere, but it is effectively released during digestion [113]. Therefore, it should enter the same common chemical pool from which all other non-heme iron is absorbed. 

The following data were obtained for cytochrome c and cytochrome C55, two proteins in which heme-bound iron ions shuttle between the oxidation states Fe(II) and Fe(III) ... 

One very active NADHj-cytochrome c reductase from mitochondria recently was separated into two components, a NADHr-ubiquinone reductase and an ubihydroquinone-cytochrome c reductase (see below under Quinone Catalysis ). In addition, non-heme-bound iron was found (about 15 moles/mole of flavin). Martius prepared a highly purified phylloquinone reductase, which contains flavin-adenine dinuoleotide (FAD) and which reduces ubiquinone besides phylloquinone (= vitamin K). The hydroquinone is assumed to be reoxidized to the quinone by cytochrome b. 

The protein from D. desulfuricans has been characterized by Mbss-bauer and EPR spectroscopy 224). The enzyme has a molecular mass of approximately 150 kDa (three different subunits 88, 29, and 16 kDa) and contains three different types of redox-active centers four c-type hemes, nonheme iron arranged as two [4Fe-4S] centers, and a molybdopterin site (Mo-bound to two MGD). Selenium was also chemically detected. The enzyme specific activity is 78 units per mg of protein. 

However, the latter residue is in no sense equivalent to the Tyr 25 of the P. pantotrophus enzyme. The Tyr 10, which is not an essential residue (19), is provided by the other subunit to that in which it is positioned close to the di heme iron (Fig. 6). In other words, there is a crossing over of the domains. A reduced state structure of the P. aeruginosa enzyme has only been obtained with nitric oxide bound to the d heme iron (20) (Fig. 6). As expected, the heme c domain is unaltered by the reduction, but the Tyr 10 has moved away from the heme d iron, and clearly the hydroxide ligand to the d heme has dissociated so as to allow the binding of the nitric oxide (Fig. 6). This form of the enzyme was prepared by first reducing with ascorbate and then adding nitrite. 

A large number of iron-containing proteins form nitrosyl complexes. Heme proteins, iron-sulfur proteins, and other iron proteins such as nonheme iron dioxygenases all form characteristic nitrosyl complexes. In enzymes in which the metal center has an open coordination position, NO often can be bound without severe disruption of the site. This introduces the possibility of reversibility of inhibition. 

In the resting (oxidized) state (1) the iron atoms of the binuclear fes-Fes site are antiferromagnetically coupled via a single strong Fe-O-Fe bond. The heme bs iron is five-coordinate in the oxidized state (1) upon reduction (2) the heme Fe is still five-coordinate, but bound to the proximal histidine. Intermediate (2) is ready to bind the two molecules of NO. Kmnita et a/. attempt to answer the question whether both NO molecules bind to Fes (the cis mechanism dzs or one to bs, the other to Fes as depicted in (3) of Figure 5(a) (the trans mechanism " ). 

Illustrative examples for such a possibility are found with the cytochromes. The name of these proteins comes from the Greek words meaning colored substances in the cell. Cytochromes are intensely red-colored redox enzjunes containing a heme prosthetic group as their dominant chromophore. Hemes are iron complexes of protoporph5uin IX derivatives (10,26). One of the most frequently studied metalloproteins of this family is cytochrome c (27). The ribbon structure of a cytochrome c enzyme together with the protein-bound heme c cofactor 6 is shown in Fig. 4. 

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