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Bsc Chemistry Subject Chemistry 11. Inorganic Chemistry –III (Metal-Complexes and Metal Paper No and Title Clusters) Module 16 :Transport and storage of oxygen in animals by Module No and Title dioxygen complexes: Activation of dioxygen molecules in transition metal dioxygen complexes. Module Tag CHE_P11_M16 CHEMISTRY PAPER No. : 11. Inorganic Chemistry –III (Metal-Complexes and Metal Clusters) 18: Transport and storage of oxygen in animals… TABLE OF CONTENTS 1. Learning Outcomes 2. Introduction 3. Myoglobin and Hemoglobin 4. Hemerythrin 5. Hemocyanin 6. Enzymes Involved in Oxygen Activation 7. Summary CHEMISTRY PAPER No. : 11. Inorganic Chemistry –III (Metal-Complexes and Metal Clusters) 18: Transport and storage of oxygen in animals… 1. Learning Outcomes After studying this module, you shall be able to learn about: the natural proteins that carry dioxygen in various animals. the role of myoglobin and hemoglobin in carrying dioxygen in mammals. the role of hemerythrin in dioxygen transport in marine invertebrates. various enzymes involved in dioxygen activation. 2. Introduction Most organisms require molecular oxygen for their survival. For some small animals and for plants, where the surface-to-volume ratio is large, an adequate supply of dioxygen can be obtained from simple diffusion across cell membranes. The dioxygen may be extracted from air or water and for plants that produce dioxygen in photosynthesis it is also available endogenously. For other organisms, particularly those with non-passive lifestyles, from scorpions to whales, diffusion does not supply sufficient dioxygen for respiration. Therefore, in these organisms specialized molecules for the transport and storage of oxygen are necessary. These functions are carried out by a number of well-known iron and copper containing species which occur in the blood. These are listed in Table 1. Color Mole ratio Color Protein Source Metal Function (oxy (O2/metal) (deoxy form) form) mammals, Haemoglobin birds, fish, Fe(II) 1/1 Carrier red-purple red (Hb) reptiles, some insects mammals, Myoglobin birds, fish, Fe(II) 1/1 Storage (Mb) reptiles, some insects CHEMISTRY PAPER No. : 11. Inorganic Chemistry –III (Metal-Complexes and Metal Clusters) 18: Transport and storage of oxygen in animals… Color Mole ratio Color Protein Source Metal Function (oxy (O2/metal) (deoxy form) form) marine hemerythrin Fe(II) 1/2 Storage colorless red worms mollusks, hemocyanin crustaceans, Cu(I) 1/2 Storage colorless blue spiders A good oxygen carrier should bind O2 at high pressure, should not oxidise cellular components and give up oxygen on demand. Both heme and non-heme iron proteins are involved in oxygen transport and storage. Heme or haem is a cofactor consisting of a Fe²⁺ ion contained in the centre of a large heterocyclic organic ring called a porphyrin, made up of four pyrrolic groups joined together by methine bridges. Heme O2 carriers are responsible for red colour of human blood, absorb one mole of oxygen per mole of iron(II), while the blue pigment of crab blood, hemocyanin, absorbs one mole of oxygen for every two moles of metal ions. Certain marine worms (e.g. Golfingia elongata) have a violet colour, which is due to the presence of non-heme iron protein hemerythrein. Thus, myoglobin, hemoglobin, hemerythrin, and hemocyanin all use a transition-metal complex to transport/store oxygen. In the following sections each of these and two enzymes, dioxygenase and methane monooxygenase which are involved in the activation of oxygen for incorporation in biological molecules, will be discussed. 3. Myoglobin and Hemglobin Myoglobin is a relatively small protein that contains 150 amino acids. The functional unit of myoglobin is an iron–porphyrin complex that is embedded in the protein (Fig. 1). In myoglobin, the heme iron is five-coordinate, with only a single histidine imidazole ligand from the protein (called the proximal histidine because it is near the iron) in addition to the four nitrogen atoms of the porphyrin. A second histidine imidazole (the distal histidine because it is more distant from the iron) is located on the CHEMISTRY PAPER No. : 11. Inorganic Chemistry –III (Metal-Complexes and Metal Clusters) 18: Transport and storage of oxygen in animals… other side of the heme group, too far from the iron to be bonded to it. Consequently, the iron atom has a vacant coordination site, which is where O2 binds. In the ferrous form (deoxymyoglobin), the iron is five-coordinate and high spin. Because high-spin Fe2+ is too large to fit into the “hole” in the center of the porphyrin, it is about 60 pm above the plane of the porphyrin. When O2 binds to deoxymyoglobin to form oxymyoglobin, the iron is converted from five-coordinate (high spin) to six-coordinate (low spin) (Fig. 2). Because low-spin Fe2+ and Fe3+ are smaller than high-spin Fe2+, the iron atom moves into the plane of the porphyrin ring to form an octahedral complex. The O2 pressure at which half of the molecules in a solution of myoglobin are bound to O2 (P1/2) is about 1 mm Hg (1.3 × 10−3 atm). Fig 1. The Structure of Deoxymyoglobin, showing the Heme Group. Fig 2. Oxygen Binding to Myoglobin CHEMISTRY PAPER No. : 11. Inorganic Chemistry –III (Metal-Complexes and Metal Clusters) 18: Transport and storage of oxygen in animals… (Note: A vacant coordination site at a metal center in a protein usually indicates that a small molecule will bind to the metal ion, whereas a coordinatively saturated metal center is usually involved in electron transfer). Hemoglobin consists of two subunits of 141 amino acids and two subunits of 146 amino acids, both similar to myoglobin; it is called a tetramer because of its four subunits but the framework of each subunit is same as that for myoglobin. Because hemoglobin has very different O2-binding properties, however, it is not simply a “super myoglobin” that can carry four O2 molecules simultaneously (one per heme group). The shape of the O2-binding curve of myoglobin (Fig. 3) can be described mathematically by the equation: MbO2 ⇌ Mb + O2 Kdiss = [Mb][O2][MbO2] …Equation 1 In contrast, the O2-binding curve of hemoglobin is S shaped (Fig. 3). The curve for myoglobin can be described by a simple equilibrium between deoxy- and oxymyoglobin, but the S-shaped curve for hemoglobin can be described only in terms of a cooperative interaction between the four hemes. When oxygen binds to the iron complex, it causes the iron atom to move back toward the center of the plane of the porphyrin ring At the same time, the imidazole side-chain of the histidine residue interacting at the other pole of the iron is pulled toward the porphyrin ring. This interaction induces a strain in the protein helix containing the histidine as it moves nearer to the iron atom. This strain is transmitted to the remaining three monomers in the tetramer, where it induces a similar conformational change in the other heme sites such that binding of oxygen to these sites becomes easier. In the tetrameric form of normal adult hemoglobin, the binding of oxygen is, thus, a cooperative process. The binding affinity of hemoglobin for oxygen is increased by the oxygen saturation of the molecule, with the first oxygens bound influencing the shape of the binding sites for the next oxygens, in a way favorable for binding. This positive cooperative binding is achieved through steric conformational changes of the hemoglobin protein complex as discussed above, that is, when one subunit protein in hemoglobin becomes oxygenated, a conformational or structural change in the CHEMISTRY PAPER No. : 11. Inorganic Chemistry –III (Metal-Complexes and Metal Clusters) 18: Transport and storage of oxygen in animals… whole complex is initiated, causing the other subunits to gain an increased affinity for oxygen. Fig. 3 The O2-Binding Curves of Myoglobin and Hemoglobin As shown in the curves, at low oxygen pressures, the affinity of deoxyhemoglobin for O2 is substantially lower than that of myoglobin, whereas at high O2 pressures the two proteins have comparable O2 affinities. The physiological consequences of the unusual S- shaped O2-binding curve of hemoglobin are enormous. In the lungs, where O2 pressure is highest, the high oxygen affinity of deoxyhemoglobin allows it to be completely loaded with O2, giving four O2 molecules per hemoglobin. In the tissues, however, where the oxygen pressure is much lower, the decreased oxygen affinity of hemoglobin allows it to release O2, resulting in a net transfer of oxygen to myoglobin. Function of protein part is to provide crevice which keeps heme from getting oxidized. In the absence of protein Fe2+ will get converted to Fe3+ and hemoglobin/myoglobin would become inactive. Heme buried in hydrophilic environment of protein so that O2 binding does not result in oxidation. Oxygen is not unique in its ability to bind to a ferrous heme complex; small molecules such as CO and NO bind to deoxymyoglobin/hemoglobin even more tightly CHEMISTRY PAPER No. : 11. Inorganic Chemistry –III (Metal-Complexes and Metal Clusters) 18: Transport and storage of oxygen in animals… than does O2. The interaction of the heme iron with oxygen and other diatomic molecules involves the transfer of electron density from the filled t2 gorbitals of the low- 6 2+ 2+ spin d Fe ion to the empty π* orbitals of the ligand. In the case of the Fe –O2 interaction, the transfer of electron density is so great that the Fe–O2 unit can be 3+ 5 − described as containing low-spin Fe (d ) and O2 . We can therefore represent the binding of O2 to deoxyhemoglobin and its release as a reversible redox reaction: 2+ 3+ − Fe + O2 ⇌ Fe –O2 ….Equation 2 As shown in Fig. 4 the Fe–O2 unit is bent, with an Fe–O–O angle of about 130°.
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