BIOCHEMISTRY TRANS - GLOBULAR PROTEIN.pdf

1A BIOCHEMISTRY GLOBULAR PROTEINS DR. JANDOC B. STR...

1A BIOCHEMISTRY GLOBULAR PROTEINS DR. JANDOC B. STRUCTURE AND FUNCTION OF MYOGLOBIN OUTLINE I. Globular Hemeproteins  Found in heart and skeletal muscle A. Structure of Heme  Reservoir and Carrier of Oxygen 1. Formation of Methemoglobin  α – helical content B. Structure and Function of Myoglobin  80% of chain folded into 8 α-helices (A-H), terminated C. Structure and Function of Hemoglobin by proline D. Binding of Oxygen to myoblobin and haemoglobin E. Allosteric effect F. Minor hemoglobins II. Organization of Globin genes A. α-gene family B. β-gene family C. Steps in globin chain synthesis III. Hemoglobinopathies A. Sickle Cell Anemia B. Hemoglobin C disease  Location of polar and nonpolar AA C. Hemoglobin SC disease  Nonpolar AA – interior, packed closely via D. Methemoglobinemia hydrophobic interactions E. Thalassemia  Polar AA – surface, form hydrogen bond with water  Binding of the heme group  Crevice of myoglobin, lined by nonpolar AA  Except 2 Histidine residues I. GLOBULAR HEMEPROTEINS  Proximal – bind to Fe of heme  Distal – stabilize binding of O2 to Fe  Proteins with heme as prosthetic group C. STRUCTURE AND FUNCTION OF HEMOGLOBIN  Cytochromes  Found exclusively on RBCs  Electron carrier, alternately oxidized or reduced  Main fxn: transport of O2 from lungs to tissues  Catalase  Hemoglobin A  Heme in active site, Breakdown of H2O2  4 polypeptide chains - 2α and 2β  Hemoglobin and Myoglobin  Transport H and CO2 from tissues  Most abundant hemeproteins, reversibly bind O2  Carry 4 molecules of O2, regulated by interaction with allosteric effectors A. STRUCTURE OF HEME  Quaternary structure of Hemoglobin  Ferrous iron + protoporphyrin IX  Two identical dimers: (αβ)1 and (αβ)2  Able to move via polar bonds  Held by: hydrophobic interactions, ionic and hydrogen bond  T form  Tense or Taut  Network of ionic and hydrogen bonds (protonation of AA)  Bury binding pocket  Low-oxygen affinity (DEOXYHEMOGLOBIN)  R form  Relaxed  Rupture of ionic and hydrogen bond (deprotonation of AA)  Expose binding pocket  Iron can form 6 bonds:  High-oxygen affinity (OXYHEMOGLOBIN)  4 with porphyrin nitrogens  2 additional bonds (above and below the planar porphyrin ring)  One position; side chain of histidine  Other: binding of oxygen Trans 1 | Raff 1 of 6 BIOCHEMISTRY GLOBULAR PROTEIN D. BINDING OF OXYGEN TO MYOGLOBIN AND HEMOGLOBIN  Myoglobin - one heme = one oxygen  Higher affinity to oxygen  Hemoglobin - 4 heme = 4 oxygen; 0-100% saturation  Oxygen-dissociation curve  X = % saturation with O2  Y = partial pressure of pO2 E. ALLOSTERIC EFFECT  Reversibility of O2 binding in hemoglobin (Allosteric effectors) – myoglobin-O2 binding not affected)  pO2  pH of environment  pCO2  2,3 – biphosphoglycerate availability  Heme-Heme interaction st  Last oxygen – 300x more affinity than 1 O2  Loading and unloading of oxygen  Changes in pO2 (alveoli vs tissue capillaries)  Significance of the sigmoidal oxygen dissociation curve  Permits Hb to carry and deliver oxygen from high to low sites of pO2  Hyperbolic curve (Mb) = not same degree of O2 release  P50 – partial pressure of oxygen needed to achieve half- saturation of binding site  1 mmHg = myoglobin  26 mmHg = haemoglobin  Higher oxygen affinity = lower P50  MYOGLOBIN  Hyperbolic shape; reversible binding to 02  Mb + O2 MbO2  Left or Right shift as to addition or release of O2  Bind O2 released by haemoglobin at low pO2  Release O2 within the muscle cell in case of demand  HEMOGLOBIN  Sigmoidal shape  Cooperative O2 binding (heme-heme interaction)  Binding of O2 to a heme increases the oxygen affinity st  more difficult for 1 O2 to bind  Bohr effect  Release of O2 = decreased pH or increased pCO2 RAFF 2 of 6 BIOCHEMISTRY GLOBULAR PROTEIN  Decrease O2 affinity of hemoglobin = SHIFT TO THE  2,3-BPG is expelled on oxygenation of Hb RIGHT  Shift of the oxygen dissociation curve  Increased pH or decreased pCO2  Presence of 2,3-BPG = reduce affinity of Hb for O2  Increased O2 affinity of hemoglobin = SHIFT TO THE LEFT  Sources of proton that lower pH  CO2 and H in the capillaries of metabolically active tissue  Organic acids: Lactic acid (anaerobic metabolism)  Mechanism of Bohr Effect  deoxyHb has greater affinity of protonation than oxyHb  caused by ionizable groups (histidine), higher pKa in deoxyHb than oxyHb  protonation = ionic bond (salt bridge) formation = stabilize deoxy form  HbO2 + H HbH + O2  Effects of 2,3-biphosphoglycerate on oxygen affinity  Important regulator of O2 binding OXYGEN DISSOCIATION CURVE SUMMARY  Most abundant organic PO4 in RBC Shift to the Left Shift to the Right  Glycolytic pathway intermediate O2 affinity Increased Decreased pH Increased Decreased pCO2 Decreased Increased 2,3-BPG Decreased Increased  Response of 2,3-BPG levels to chronic hypoxia or anemia  Increased 2,3-BPG = chronic hypoxia (emphysema, high altitude), chronic anemia  Lowers O2 affinity = greater unloading or release  Role of 2,3-BPG in transfused blood  Stored blood = low 2,3-BPG = high O2 affinity = fail to release O2 (oxygen trap)  Binding of CO2  Most C02  hydrated and transported as HCO3  Some CO2  carried as carbamate bound to the N- terminal amino group of Hb  Binding of CO  CO binds tightly (but reversibly) to the Hb forming  Binding of 2,3-BPG to deoxyHb carboxyhemoglobin  Decreases O2 affinity of deoxyHb  One or more of the 4 heme sites  HbO2 + 2,3-BPG Hb-2,3-BPG + O2  Hb affinity is 220x greater for CO than O2  Binding site of 2,3-BPG  Increased in tobacco smokers  TXT: 100% oxygen at high pressure (hyperbaric O2 therapy)  CO dissociation  NO – carried by Hb, potent vasodilator F. MINOR HEMOGLOBINS  Fetal Hb - 2α + 2ϒ  HbF synthesis during development th  5 week of gestation  Major Hb in fetus and newborn (60%) 