Hb and Mb

• Protein classification based on higher levels of structural organisation


  • Fibrous proteins
  • Globular proteins
• Fibrous proteins


  • Polypeptide chains arranged in long strands or sheets; usually consist largely of a single type of secondary structure
• Globular proteins


  • Polypeptide chains folded into a spherical or globular shape; often contain several types of secondary structures
• Fibrous proteins


  Provide support, shape, and external protection to vertebrates
• Globular proteins


  Most enzymes and regulatory proteins are globular proteins
• Hemeproteins


  A group of specialized proteins that contain heme as a tightly bound prosthetic group

  
• Roles of heme group in globular hemeproteins

  • Cytochromes - function as electron carriers, alternately oxidized and reduced
  • Catalase – heme is a part of the active site of the enzyme that catalyzes the breakdown of H2O2
  • Hemoglobin and myoglobin – bind oxygen

  
• Hemoglobin


  Transports O2 from the lungs to the tissues and CO2 and H+ from the tissues to the lungs; transports and releases NO in the blood vessels

  
• Myoglobin


  O2 reservoir and an oxygen carrier within the myocytes for ATP synthesis when energy is highly consumed

  
• Neuroglobin


  In brain, neuroprotective effect
• Alveolar-arterial gradient


  Hb → Nb → Mb, Affinity to oxygen

  
• Myoglobin and hemoglobin both bind O2, contain heme, which is involved in O2 binding, and have almost identical secondary and tertiary structure of the monomer Mb and the α subunits of the tetramer Hb

  
• Heme is active only if it contains Fe2+, and not Fe3+
• Heme


  Consists of a complex organic ring structure, protoporphyrin, to which an iron atom in its ferrous (Fe2+) state is bound

  
• Heme


  • Fe2+ atom in the center of the heme group has two bonding (coordination) positions perpendicular to the plane of the heme

  
• Differences between myoglobin and hemoglobin

  • Structural: Hb – quaternary structure, 4 polypeptide chains; Mb – 1 polypeptide chain, tertiary structure
  • Functional: Hb carries O2 from lungs to tissues, where it is released; Mb releases O2 for muscle cells at pO2 ~ 5mm Hg

  
• Myoglobin structure


  • 80% from all amino acids are involved in 8 alpha helixes (A-H)
  • Nonpolar residues are in the interior of the globule, the hydrophilic – on the surface
  • Heme is situated in a pocket formed by F and E helixes

  
• Histidines F8 & E7 in myoglobin


  • One of the Fe2+ bonds is to the R group of the F8 His residue (proximal His); the other is to a site at which O2 molecule binds (distal E7 His)
  • The iron moves toward the plane of the heme when O2 is bound

  
• Hemoglobin structure


  • Hb A1 (major hemoglobin in adults) – 4 polypeptide chains (2α and 2β)
  • Similar tertiary structure (similar conformations) between Hb α chain and the Mb despite the quite different primary structures

  
• Hemoglobin quaternary structure


  • Hb is built of 2 identical dimers - (αβ)1 and (αβ)2
  • The polypeptide chains within the dimers are held tightly together by numerous hydrophobic interactions, as well as ionic and hydrogen bonds
  • The two dimers are held together primarily by ionic and hydrogen bonds, allowing them to occupy different relative positions in deoxyhemoglobin (T, "taut") as compared with oxyhemoglobin (R, "relaxed")

  
• Oxygen dissociation curve


  A plot of degree of saturation (Y) measured at different partial pressures of oxygen (pO2) for the monomeric Mb (tertiary structure) and the tetrameric Hb (quaternary structure)

  
• Myoglobin oxygen dissociation curve has a hyperbolic shape, indicating that myoglobin reversibly binds a single molecule of oxygen

  
• At the low partial pressures of oxygen in muscles (20-40mm Hg) 90% of Mb is saturated with oxygen. MbO2 and Mb exist in a simple equilibrium

  
• Mb starts to release oxygen only when partial pressures of oxygen falls below 5 mm Hg

  
• Mb has a greater affinity for binding oxygen than Hb

  
• Hemoglobin oxygen dissociation curve

  • pO2 in the lungs is 90-100mm Hg and Hb is fully saturated with O2
  • pO2 in the tissues is 40mm Hg, and in capillaries ~ 20 mmHg – at those values of pO2 oxygen is easily released by Hb
  • The dissociation curve is sigmoidal in shape indicating the cooperative interaction between the subunits when binding and releasing O2

  
• Heme-heme interactions in hemoglobin

  Binding of one oxygen molecule to one heme group increases the oxygen affinity of the remaining heme groups in the same hemoglobin molecule, resulting in a 300 times greater affinity for oxygen

  
• Allosteric effectors of hemoglobin

  • CO2 (stabilizes the deoxy form - T → reduced affinity for O2)
  • H+ (stabilizes the T form)
  • 2,3-Bisphosphoglycerate (2,3-BPG) (stabilizes the T form)

  
• Bohr effect


  O2 release from Hb at lowered pH and increased CO2 concentration in the tissues

  
• The Bohr effect


  2CO2 → 2H2CO3 → 2HCO3- + 2H+ → 4O2 Hb.2H+ → Hb.4O2 (lungs)

  4O2 → Exhaled (lungs)

  2H+ + 2HCO3- → 2H2CO3 → 2CO2 + 2H2O (Carbonic anhydrase in erythrocytes) (peripheral tissues)
• Increase in pH (or CO2)

  Increased affinity for oxygen, a shift to the left in the oxygen dissociation curve

  
• 2,3-Bisphosphoglycerate (2,3-BPG)


  Synthesized from an intermediate of the glycolytic pathway, reduces the Hb affinity for oxygen, which is released from Hb

  
• Negatively charged molecule of 2,3 BPG binds to the positively charged cavity formed by the beta chains of deoxyHb
• The concentration of 2,3-BPG in erythrocytes increases in response to chronic hypoxia, chronic anemia, or at high altitudes, reducing Hb affinity to O2 and improving oxygen supply to the tissues

  
• CO2 transport


  15-20% of all CO2 molecules are transported bound to Hb, the rest – in the form of HCO3-

  CO2 does not bind to the heme

  Some CO2 is carried as carbamate bound to the uncharged α-amino groups of hemoglobin → carbamino-hemoglobin is formed (the T deoxyHb is formed, resulting in a affinity for oxygen)

  
• The affinity of Hb for CO is 220 times greater than that for O2
• Carbon monoxyhemoglobin (HbCO)


  CO binds tightly (but reversibly) to the heme Fe, forming carbon monoxyhemoglobin

  When CO binds to one or more of the four heme sites, Hb shifts to the R conformation, causing the remaining heme sites to bind O2 with high affinity

  The saturation curve turns from sigmoidal to hyperbolic

  60% Hb-CO from total Hb – is fatal!

  
• Oxygen therapy is used to replace CO!
• Hemoglobin F (HbF)


  HbA synthesis starts at the 8th month of pregnancy and HbA gradually replaces HbF

  BPG binds more weakly to HbF than to HbA. The γ chains of HbF contain Ser at position 21 instead of His. This results in a HbF having a lower affinity to 2,3-BPG and, respectively, higher affinity for O2 than HbA

  Thus O2 is transferred from HbA to HbF
• Hemoglobin A1C


  Under physiologic conditions, HbA is slowly and nonenzymically glycosylated, the extent of glycosylation being dependent on the plasma concentration of glucose

  Glucose binds covalently to the NH2-groups of the N-terminal valines of the β-globin chains

  A marker for continuously high blood glucose in diabetes mellitus