Iron and Oxygen Transport

The hemoglobin molecule is a tetramer composed of two similar globins (polypeptide chain) of unequal length. In the center of the protein lies a prosthetic group made of a porphyrin nucleus. This acts as a tetradentate ligand for the iron ion. The porphyrin-iron complex is called a heme group. The association of a protein molecule with a heme is referred to as a hemo-protein. 

Hemoproteins, ubiquitous among plants and animals, perform at least four important functions related to oxygen and the production of energy (1) oxygen transport to tissue (2) catalytic oxidation of organic compounds (3) decomposition of hydrogen peroxide and (4) electron transfer. 

It should be realized that the ability to bind oxygen reversibly is a unique property found in nature only in iron-porphyrin proteins, iron proteins, and copper proteins. However, other small molecules such as CO, CO2, or CN can also interact with these metalloproteins. In fact, CO binds to hemoglobin even more avidly than oxygen, producing a cellular oxygen deficiency, which is sometimes identified as carbon monoxide poisoning.  

Of course, the current direction is to synthesize complexes which will more closely mimic hemoglobin and myoglobin and to develop oxygenactivating species in general. In 1973 and 1974, several model systems were devised. We will thus be concerned here with the interaction of molecular oxygen (dioxygen) and carbon monoxide with metalloporphyrin complexes as models of biologically important hemoproteins. 

Baldwin and co-workers (235) at M.LT. (now at Oxford) first synthesized a porphyrin-hke structure surrounding an Fe(II) atom that reversibly bound oxygen in solution. However, this occurred only at — 85°C. Later he developed a simple method to prepare a cagelike or a capped porphyrin molecule. This was done in the hope that upon O2 binding the irreversible Fe(II) Fe(III) process will not occur and O2 will bind reversibly to the model molecule at room temperature (236). 

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Although not related to iron and oxygen transport, it should be mentioned that synthetic biomimetic models of special pair bacteriochlorophyll a have also been prepared (247). The reason is that, in the molecular organization of chlorophyll in the photoreaction centers of both green plants and photosynthetic bacteria, it is believed that special pairs of chlorophyll molecules are oxidized in the primary light conversion event in photosynthesis. Dimeric chlorophyll derivatives such as the one in Fig. 6.6 in which the porphyrin... 

NOTE Probably the most important junction of oxygen scavengers is, in reality, the ability to passivate boiler steel. In recognition of this, today most novel oxygen scavenger trials try to identify, not merely comparative oxygen reaction rates, but more importantly, the reduction in iron and copper transport rates through the boiler system. In other words, they seek to optimize the passivation of boiler surfaces and other system components. 

Of the metals that were just listed as the major participants in biochemical functions, some (such as Mg and Zn) rarely change oxidation states. Therefore, metals such as these are involved in processes in which there is no redox chemistry taking place. These metals function in some other way. On the other hand, metals such as Fe, Mn, Mo, and Cu can change oxidation states more easily, so they are the metals that participate in redox reactions. For example, the role of iron in oxygen transport requires it to bond to oxygen and thereby, at least formally, to become oxidized in the process. There are other instances of this type of behavior. As mentioned earlier, the list of metals that are involved in the vast majority of biochemical processes is not a particularly long one. 

Hemoglobin is the iron-containing oxygen-transport metalloprotein in red blood cells (RBCs) of the blood in humans and animals. 

Mecfianism of Action A trace element that is an essential component in the formation of Hgb. It s necessary for effective erythropoiesis and oxygen transport capacity of blood, and transport and utilization of oxygen, and serves as cofactor of several essential enzymes. Therapeutic Effect Replenishes body iron stores in patients who have iron deficiency anemia. 

Model Compounds for Iron in Biological Systems.—There have been a number of publications concerned mainly with the nature of iron-sulphur-containing proteins, nitrogen fixation, and oxygen transport. Most of these are concerned with studies on cluster compounds of the type [Fe4S4(SR)4]2. ... 

Transition elements have some biological functions for chemistry of living systems. Metals, such as iron, cobalt, copper, and molybdenum have functions in living systems, whereas protein contains iron, which helps in electron transfer and oxygen transport. Most transition metal elements plays similar major roles in various biological living systems and for these reasons we focused on the transition metals. 

Similarly, when catalyzed the reaction rate decreases significantly as a function of pH level. The optimum reaction pH level is approximately 9.5 to 10.5. Iron, and especially copper, in the boiler may act as adventitious catalysts. However, as metal transport polymers are frequently employed, iron, copper, or cobalt may be transported away from contact with sulfite, and thus are not available for catalysis. (This may be a serious problem in high-pressure units employing combinations of organic oxygen scavengers and metal ion catalysts.)... 

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