Oxygen Transport Proteins PDF

Oxygen transport proteins Oxygen •O2 Essential for cellular respiration. Structure and function of hemoglobin •O2 is poorly soluble in plasma. •Impossible to transport by simple diffusion. •Hemoglobin is found exclusively in red blood cells (RBCs), where its main function is to transport oxygen (O...

Oxygen transport proteins Oxygen •O2 Essential for cellular respiration. Structure and function of hemoglobin •O2 is poorly soluble in plasma. •Impossible to transport by simple diffusion. •Hemoglobin is found exclusively in red blood cells (RBCs), where its main function is to transport oxygen (O2) from the lungs to the capillaries of the tissues. •O2 is transported using two proteins (hemoglobin and myoglobin) That contains essential prosthetic group. (heme: contains Fe atom) •Hemoglobin A, the major hemoglobin in adults, is composed of four polypeptide chains (2α chains “141 residue each” and 2β chains “146 residue each”) held together by noncovalent interactions. • 3D structure of α& β is very similar MYOGLOBIN AND HEMOGLOBIN Structure and function of myoglobin •Myoglobin, a hemeprotein present in heart and skeletal muscle, functions both as a reservoir for oxygen, and as an oxygen carrier that increases the rate of transport of oxygen within the muscle cell. •Each subunit has stretches of α -helical structure, and a heme-binding pocket. •The tetrameric hemoglobin molecule is structurally and functionally more complex than myoglobin. For example, hemoglobin can transport H+ and CO2 from the tissues to the lungs, and can carry four molecules of O2 from the lungs to the cells of the body. Heme is composed of a ringlike organic compound known as a porphyrin, to which an iron atom is attached. Heme is a complex of protoporphyrin and ferrous iron (Fe2+). The iron is held in the center of the heme molecule by bonds to the four nitrogens of the porphyrin ring. The heme Fe2+ can form two additional bonds, one on each side of the planar porphyrin ring, one of these positions is coordinated to the side chain of a histidine residue of the globin molecule, whereas the other position is available to bind oxygen. •The ferrous(+2) state binds the oxygen • HbFe+3 Ferric state (3+)/ (methemoglobin) can not bind Oxygen, 7 •Myoglobin consists of a single polypeptide chain (153 residue). that is structurally similar to the individual subunit polypeptide chains of the hemoglobin molecule. • : f- •This homology makes myoglobin a useful model for interpreting some of the more complex properties of hemoglobin. •The interior of the myoglobin molecule is composed almost entirely of nonpolar amino acids. They are packed closely together, forming a structure stabilized by hydrophobic interactions. In contrast, charged amino acids are located almost exclusively on the surface of the molecule, where they can form hydrogen bonds, both with each other and with water. Structure of heme Oxygen dissociation curve: •A plot of saturation measured at different partial pressures of oxygen (pO2). • P50 is the partial pressure of O2 (PO2) at which the protein is half-saturated with O2. Hemoglobin consists of : globin and heme pigment •Globin Consists of two a and two b subunits •Each subunit binds to a heme group • Each heme group bears an atom of iron, which binds reversibly with one molecule of oxygen MYOGLOBIN AND HEMOGLOBIN BINDING TO OXYGEN •Myoglobin can bind only one molecule of oxygen, because it contains only one heme group. In contrast, hemoglobin can bind four oxygen molecules, one at each of its four heme groups. •P50 Expresses the Relative Affinities of Hemoglobin and myoglobin for Oxygen •The curves for myoglobin and hemoglobin show important differences. This figure illustrates that myoglobin has a higher oxygen affinity (more tightly binds) at all pO2 values than does hemoglobin. •The partial pressure of oxygen needed to achieve halfsaturation of the binding sites (P50) is approximately 3 mm Hg for myoglobin and 26 mm Hg for hemoglobin. The higher the oxygen affinity the lower the P50. - ALLOSTERIC EFFECTS ODC for Myoglobin The oxygen dissociation curve for myoglobin has a hyperbolic shape . This reflects the fact that myoglobin reversibly binds a single molecule of oxygen. ODC Hemoglobin (Hb): Deoxygenated haemoglobin •The deoxy form of hemoglobin is called the “T,” or taut form. In the T form, the two α β dimers interact through a network of ionic bonds and hydrogen bonds that constrain the movement of the polypeptide chains. • The T form is the low oxygen affinity form of hemoglobin. •The oxygen dissociation curve for hemoglobin is sigmoidal in shape, indicating that the subunits cooperate in binding oxygen. Oxygenated haemoglobin •Cooperative binding of oxygen by the four subunits of hemoglobin means that the binding of an oxygen molecule at one heme group increases the oxygen affinity of the remaining heme groups in the same hemoglobin molecule. •The binding of oxygen to hemoglobin causes the rupture of some of the ionic bonds and hydrogen bonds between the α β dimers. This leads to a structure called the “R,” or relaxed form, in which the polypeptide chains have more freedom of movement. •O2 bind to one subunit —-> affinity for other subunit inc • The R form is the high oxygen- affinity form of hemoglobin. •The affinity of hemoglobin for the last oxygen bound is approximately 300 times greater than its affinity for the first oxygen bound. •The cooperativity in oxygen binding in hemoglobin comes from conformational changes in tertiary structure that take place when O2 binds The ability of hemoglobin to reversibly bind oxygen is affected by the following parameters: • pO2 • 2, 3-bisphosphoglycerate. • pH • pCO2 • pCO • temperature •high altitudes pd " - _¥J÷ " 0 0 These are collectively called allosteric effectors (the binding of a ligand/substrate at one site in a multisubunit protein that influences the subsequent binding of other ligands/ substrates to other subunits called allosteric effect), because their interaction at one site on the hemoglobin molecule affect s the binding of oxygen to heme groups at other locations on the molecule. 2’3-bisphosphoglycerate effect: •The highly anionic 2,3-BPG is present in red blood cells. binds to haemoglobin (one molecule per tetramer) and decreases the affinity for O2, promoting release in the tissues. •The presence of 2,3-BPG significantly reduces the affinity of hemoglobin for oxygen by binding to deoxyhemoglobin but not to oxyhemoglobin (The oxygenated haemoglobin has a smaller central gap and excludes 2,3-BPG). •2,3-BPG binds in the central cavity of the tetramer, interacting with three positively charged groups (2 His, 1 Lys) on each β chain so a mutation of one of these amino acids can result in hemoglobin variants with abnormally high oxygen affinity. Significance of the sigmoidal oxygen dissociation curve: •The steep slope of the oxygen dissociation curve over the range of oxygen concentrations that occur between the lungs and the tissues permits hemoglobin to carry and deliver oxygen efficiently from sites of high to sites of low pO2. •This preferential binding stabilizes the taut conformation of deoxyhemoglobin, shifting the oxygen dissociation curve to the right. Why doesn't myoglobin exhibit cooperative binding? •A molecule with a hyperbolic oxygen dissociation curve, such as myoglobin, could not achieve the same degree of oxygen release within this range of partial pressures of oxygen. Instead, it would have maximum affinity for oxygen throughout this oxygen pressure range and, therefore, would deliver no oxygen to the tissues. •Because it only has one heme group, and thus only one oxygen binding site. • Myoglobin is fully saturated when bound to one oxygen; its affinity cannot increase when there are no additional oxygen binding sites. •This reduced affinity enables hemoglobin to release oxygen efficiently at the partial pressures found in the tissues. •The physiological adaptation to high altitude involves increased tissue concentrations of BPG, leading to more efficient O2 release to compensate for the reduced O2 tension. ← € Physiological significance: Bohr Effect (H+ & CO2 effect): The concentration of 2,3-BPG in the RBC increases in response to chronic hypoxia, such as: The release of oxygen from hemoglobin is enhanced when the pH is lowered or when the hemoglobin is in the presence of an increased pCO2. Both result in a decreased oxygen affinity of hemoglobin and, therefore, a shift to the right in the oxygen dissociation curve. 1. Chronic obstructive pulmonary disease (COPD). 2. At high altitudes. 3. Anemia. Elevated 2,3-BPG levels lower the oxygen affinity of hemoglobin, permitting greater unloading of oxygen in the capillaries of the tissues. Differential oxygen affinity of foetal and maternal red blood cells •fetal haemoglobin contains a variant of the β chain, called ɤ (gamma), which has a His→Ser substitution in the 2,3-BPG-binding site. The fetal haemoglobin thus has a reduced affinity for 2,3-BPG, resulting in an enhanced O2binding affinity that allows transfer of O2 from the maternal to the foetal red blood cells 2,3-bisphosphoglycerate (2,3-BPG) enhances hemoglobin's ability to release oxygen. 2,3-BPG interacts much more with hemoglobin A than hemoglobin F. This is because the adult β subunit has more positive charges than the fetal γ subunit, which attract the negative charges from 2,3-BPG •This regulation of O2-affinity by pH and CO2 is called the Bohr effect after its discoverer, Christian Bohr(1904). Physiological significance: •The differential pH gradient (lungs having a higher pH, tissues a lower pH) favors the unloading of oxygen in the peripheral tissues, and the loading of oxygen in the lung. •Thus, the oxygen affinity of the hemoglobin molecule responds to small shifts in pH between the lungs and oxygenconsuming tissues, making hemoglobin a more efficient transporter of oxygen. •Also the rapidly metabolising tissues, such as contracting muscle, have a high need for O2 and generate large amounts of H+ and CO2. The binding of CO2 stabilizes the T (taut) or deoxy form of hemoglobin, resulting in a decrease in its affinity for oxygen and a right shift in the oxygen dissociation. -Both of these species interact with haemoglobin to promote O2-release. 8,0 The effect of CO: Sickle cell anaemia: Carbon monoxide (CO) binds tightly (but reversibly) to the hemoglobin iron, forming carboxyhemoglobin. It shifts the oxygen dissociation curve to the left, and changes the normal sigmoidal shape toward a hyperbola. As a result, the affected hemoglobin is unable to release oxygen to the tissues. • The affinity of hemoglobin for CO is 220 times greater than for oxygen. Consequently, even minute concentrations of CO in the environment can produce toxic concentrations of carbon carboxyhemoglobin in the blood. •Treated by 100% oxygen therapy A haemoglobinopathy caused by a point mutation in the gene for β -globin, in which glutamate at position six has been replaced with valine, forming sickle shaped hb Hbs insoluble while deoxygenated and so aggregate into tubular fibers Which variables increase sickling? Any variable that increase HbS deoxystate • Low O2 as a result of high altitude or flying • High CO2 • Low pH • Dehydration • High BPG Oxygen Dissociation Curve in Anemia: In anemia, the curve is still sigmoidal, but the saturation is reduced. Hemi1n