2-myoglobin and hemoglobin-33_240306_190100.pdf

myoglobin hemoglobin 1 1 2-myoglobin and hemoglobin-33 - 4 Mart 2024 Biomedical Importance of myoglobin and hemoglobin The heme proteins myoglobin and hemoglobin maintain a supply of oxygen, essential for oxidative metabolism. Myoglobin, a monomeric protein of red muscle, stores oxygen as a reserve...

myoglobin hemoglobin 1 1 2-myoglobin and hemoglobin-33 - 4 Mart 2024 Biomedical Importance of myoglobin and hemoglobin The heme proteins myoglobin and hemoglobin maintain a supply of oxygen, essential for oxidative metabolism. Myoglobin, a monomeric protein of red muscle, stores oxygen as a reserve against oxygen deprivation. 2 2 2-myoglobin and hemoglobin-33 - 4 Mart 2024 3 3 2-myoglobin and hemoglobin-33 - 4 Mart 2024 4 4 2-myoglobin and hemoglobin-33 - 4 Mart 2024 5 5 2-myoglobin and hemoglobin-33 - 4 Mart 2024 Biomedical Importance of myoglobin and hemoglobin Hemoglobin, a tetrameric protein of erythrocytes, transports O2 to the tissues and returns CO2 and protons to the lungs. Cyanide and carbon monoxide kill because they disrupt the physiologic function of the heme proteins cytochrome oxidase and hemoglobin, respectively. The secondary-tertiary structure of the subunits of hemoglobin resembles myoglobin. 6 6 2-myoglobin and hemoglobin-33 - 4 Mart 2024 7 7 2-myoglobin and hemoglobin-33 - 4 Mart 2024 8 8 2-myoglobin and hemoglobin-33 - 4 Mart 2024 9 9 2-myoglobin and hemoglobin-33 - 4 Mart 2024 10 10 2-myoglobin and hemoglobin-33 - 4 Mart 2024 11 11 2-myoglobin and hemoglobin-33 - 4 Mart 2024 Biomedical Importance of myoglobin and hemoglobin The tetrameric structure of hemoglobin permits cooperative interactions that are central to its function. For example, 2,3bisphosphoglycerate (BPG) promotes the efficient release of O2 by stabilizing the quaternary structure of deoxyhemoglobin. Hemoglobin and myoglobin illustrate both protein structure-function relationships and the molecular basis of genetic diseases such as sickle cell disease and the thalassemias. 12 12 2-myoglobin and hemoglobin-33 - 4 Mart 2024 Heme & Ferrous Iron Confer the Ability to Store & to Transport Oxygen Myoglobin and hemoglobin contain heme, a cyclic tetrapyrrole consisting of four molecules of pyrrole linked by -methylene bridges. This planar network of conjugated double bonds absorbs visible light and colors heme deep red. The substituents at the -positions of heme are methyl (M), vinyl (V), and propionate (Pr) groups arranged in the order M, V, M, V, M, Pr, Pr, M 13 13 2-myoglobin and hemoglobin-33 - 4 Mart 2024 Pyrrole 14 14 2-myoglobin and hemoglobin-33 - 4 Mart 2024 Porphyrins- Porphin Nucleus-Tetrapyrroles 15 15 2-myoglobin and hemoglobin-33 - 4 Mart 2024 (Protoporphyrin III) 16 16 2-myoglobin and hemoglobin-33 - 4 Mart 2024 Heme & Ferrous Iron Confer the Ability to Store & to Transport Oxygen One atom of ferrous iron (Fe2+) resides at the center of the planar tetrapyrrole. Other proteins with metal-containing tetrapyrrole prosthetic groups include the cytochromes (Fe and Cu) and chlorophyll (Mg) Oxidation and reduction of the Fe and Cu atoms of cytochromes is essential to their biologic function as carriers of electrons. By contrast, oxidation of the Fe2+ of myoglobin or hemoglobin to Fe3+ destroys their biologic activity. 17 17 2-myoglobin and hemoglobin-33 - 4 Mart 2024 Myoglobin Is Rich in α Helix Oxygen stored in red muscle myoglobin is released during O2 deprivation (eg, severe exercise) for use in muscle mitochondria for aerobic synthesis of ATP. A 153-aminoacyl residue polypeptide (MW 17,000), myoglobin folds into a compact shape that measures 4.5 x 3.5 x 2.5 nm. Unusually high proportions, about 75%, of the residues are present in eight righthanded, 7–20 residue helices. Starting at the amino terminal, these are termed helices A–H. 18 18 2-myoglobin and hemoglobin-33 - 4 Mart 2024 Myoglobin Is Rich in α Helix Typical of globular proteins, the surface of myoglobin is polar, while—with only two exceptions—the interior contains only nonpolar residues such as Leu, Val, Phe, and Met. The exceptions are His E7 and His F8, the seventh and eighth residues in helices E and F, which lie close to the heme iron where they function in O2 binding. 19 19 2-myoglobin and hemoglobin-33 - 4 Mart 2024 20 20 2-myoglobin and hemoglobin-33 - 4 Mart 2024 Histidines F8 & E7 Perform Unique Roles in Oxygen Binding The heme of myoglobin lies in a crevice between helices E and F oriented with its polar propionate groups facing the surface of the globin. The remainder resides in the nonpolar interior. The fifth coordination position of the iron is linked to a ring nitrogen of the proximal histidine, His F8. The distal histidine, His E7, lies on the side of the heme ring opposite to His F8. 21 21 2-myoglobin and hemoglobin-33 - 4 Mart 2024 The Iron Moves Toward the Plane of the Heme When Oxygen Is Bound The iron of unoxygenated myoglobin lies 0.03 nm (0.3 Å) outside the plane of the heme ring, toward His F8. When O2 occupies the sixth coordination position, the iron moves to within 0.01 nm (0.1 Å) of the plane of the heme ring. Oxygenation of myoglobin thus is accompanied by motion of the iron, of His F8, and of residues linked to His F8. 22 22 2-myoglobin and hemoglobin-33 - 4 Mart 2024 The Oxygen Dissociation Curves for Myoglobin & Hemoglobin Suit Their Physiologic Roles Why is myoglobin unsuitable as an O2 transport protein but well suited for O2 storage? The relationship between the concentration, or partial pressure, of O2 (PO2) and the quantity of O2 bound is expressed as an O2 saturation isotherm The oxygen-binding curve for myoglobin is hyperbolic. Myoglobin therefore loads O2 readily at the PO2 of the lung capillary bed (100 mm Hg). However, since myoglobin releases only a small fraction of its bound O2 at the PO2 values typically encountered in active muscle (20 mm Hg) or other tissues (40 mm Hg), it represents an ineffective vehicle for delivery of O2. 23 23 2-myoglobin and hemoglobin-33 - 4 Mart 2024 The Oxygen Dissociation Curves for Myoglobin & Hemoglobin Suit Their Physiologic Roles Oxygen-binding curves of both hemoglobin and myoglobin. Arterial oxygen tension is about 100 mm Hg; mixed venous oxygen tension is about 40 mm Hg; capillary (active muscle) oxygen tension is about 20 mm Hg; and the minimum oxygen tension required for cytochrome oxidase is about 5 mm Hg. Association of chains into a tetrameric structure (hemoglobin) results in much greater oxygen delivery than would be possible with single chains. 24 24 2-myoglobin and hemoglobin-33 - 4 Mart 2024 The Allosteric Properties of Hemoglobins Result from Their Quaternary Structures The properties of individual hemoglobins are consequences of their quaternary as well as of their secondary and tertiary structures. The quaternary structure of hemoglobin confers striking additional properties, absent from monomeric myoglobin, which adapts it to its unique biologic roles. 25 25 2-myoglobin and hemoglobin-33 - 4 Mart 2024 Hemoglobin Is Tetrameric Hemoglobins are tetramers composed of pairs of two different polypeptide subunits. Greek letters are used to designate each subunit type. The subunit composition of the principal hemoglobins are α2 β2 (HbA; normal adult hemoglobin), α2 γ2 (HbF; fetal hemoglobin), Α2 S2 (HbS; sickle cell hemoglobin), and α2 δ2 (HbA2; a minor adult hemoglobin). The primary structures of the β, γ , and δ chains of human hemoglobin are highly conserved. 26 26 2-myoglobin and hemoglobin-33 - 4 Mart 2024 Myoglobin & the Subunits of Hemoglobin Share Almost Identical Secondary and Tertiary Structures Despite differences in the kind and number of amino acids present, myoglobin and the β polypeptide of hemoglobin A have almost identical secondary and tertiary structures. Similarities include the location of the heme and the eight helical regions and the presence of amino acids with similar properties at comparable locations. Although it possesses seven rather than eight helical regions, the polypeptide of hemoglobin also closely resembles myoglobin. 27 27 2-myoglobin and hemoglobin-33 - 4 Mart 2024 Oxygenation of Hemoglobin Triggers Conformational Changes in the Apoprotein Hemoglobins bind four molecules of O2 per tetramer, one per heme. A molecule of O2 binds to a hemoglobin tetramer more readily if other O2 molecules are already bound termed cooperative binding, this phenomenon permits hemoglobin to maximize both the quantity of O2 loaded at the PO2 of the lungs and the quantity of O2 released at the PO2 of the peripheral tissues. Cooperative interactions, an exclusive property of multimeric proteins, are critically important to aerobic life. 28 28 2-myoglobin and hemoglobin-33 - 4 Mart 2024 Oxygenation of Hemoglobin Is Accompanied by Large Conformational Changes The binding of the first O2 molecule to deoxyHb shifts the heme iron toward the plane of the heme ring from a position about 0.6 nm beyond it. This motion is transmitted to the proximal (F8) histidine and to the residues attached thereto, which in turn causes the rupture of salt bridges between the carboxyl terminal residues of all four subunits. 29 29 2-myoglobin and hemoglobin-33 - 4 Mart 2024 Oxygenation of Hemoglobin Is Accompanied by Large Conformational Changes As a consequence, one pair of α / β subunits rotates 15 degrees with respect to the other, compacting the tetramer. Profound changes in secondary, tertiary, and quaternary structure accompany the high-affinity O2-induced transition of hemoglobin from the low-affinity T (taut) state to the high-affinity R (relaxed) state. These changes significantly increase the affinity of the remaining unoxygenated hemes for O2, as subsequent binding events require the rupture of fewer salt bridges. The terms T and R also are used to refer to the low-affinity and high-affinity 30 conformations of allosteric enzymes, respectively. 30 2-myoglobin and hemoglobin-33 - 4 Mart 2024 Transition from the T structure to the R structure. In this model, salt bridges linking the subunits in the T structure break progressively as oxygen is added, and even those salt bridges that have not yet ruptured are progressively weakened. The transition from T to R does not take place after a fixed number of oxygen molecules have been bound but becomes more probable as each successive oxygen binds. The transition between the two structures is influenced by protons, carbon dioxide, chloride, and BPG; the higher their concentration, the more oxygen must be bound to trigger the transition. Fully oxygenated molecules in the T structure and fully deoxygenated molecules in the R structure are not shown because they are unstable. 31 31 2-myoglobin and hemoglobin-33 - 4 Mart 2024 The Bohr effect in hemoglobin, which refers to the dependence of the oxygen affinity on the pH, plays an important role in its cooperativity and physiological function. The dominant contribution to the Bohr effect arises from the difference in the pKa values of His residues of the unliganded (deoxy) and liganded (carbonmonoxy) structures 32 32 2-myoglobin and hemoglobin-33 - 4 Mart 2024 The Bohr Effect In the tissues, the pH is lower due to the presence of CO2.. The lowered pH causes Hb to lose O2. This is known as the BOHR EFFECT and increases the delivery of O2 to the tissues. The origin of the Bohr effect lies in the fact that deoxy Hb is a weaker acid than oxy Hb: A major contribution to the Bohr effect involves the C-terminal His of each β subunit. In the deoxy state, this His forms a salt bridge to Asp 94, if the His ring is protonated. The salt bridge stabilizes the protonated form of the His, increasing the pK a (weaker acid) in the deoxy state. The salt bridge does not form in the oxy state. Thus protonation of His-146 favors the deoxy conformation. 33 2-myoglobin and hemoglobin-33 - 4 Mart 2024 Protonation states and hydrogen bonding for the main-chain and side-chain groups of βHis146 in subunits β1 and β2. Nuclear density is contoured at the 1.5σ level. Hydrogen bonding is shown as magenta dotted lines, while the C—H O contact between β2His146 and the carboxylic moiety of β2Asp94 is depicted as an orange dashed line. Nuclear density is contoured at the 1.5σ level. D O distances and N—D O angles for the hydrogen bonds and H O distances for the C—H O contacts are given. 34 2-myoglobin and hemoglobin-33 - 4 Mart 2024 Important residues for His143 and His146 35 2-myoglobin and hemoglobin-33 - 4 Mart 2024 36 36 2-myoglobin and hemoglobin-33 - 4 Mart 2024 37 37 2-myoglobin and hemoglobin-33 - 4 Mart 2024 38 38 2-myoglobin and hemoglobin-33 - 4 Mart 2024