structures of myoglobin and hemoglobin myoglobin (mb) - monomeric protein that facilitates the...
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Structures of Myoglobin and Structures of Myoglobin and HemoglobinHemoglobin
• Myoglobin (Mb) - monomeric protein that facilitates the diffusion of oxygen in vertebrates
• Hemoglobin (Hb) - tetrameric protein that carries oxygen in the blood
• Heme consists of a tetrapyrrole ring system called protoporphyrin IX complexed with iron
• Heme of Mb and Hb binds oxygen for transport
Heme Fe(II)-protoporphyrin IXHeme Fe(II)-protoporphyrin IX
Protein component of Mb and Protein component of Mb and Hb is globinHb is globin
• Myoglobin is composed of 8 helices
• Heme prosthetic group binds oxygen
• His-93 is complexed to the iron atom, and His-64 forms a hydrogen bond with oxygen
• Interior of Mb almost all hydrophobic amino acids
• Heme occupies a hydrophobic cleft formed by three helices and two loops
Sperm whale oxymyoglobinSperm whale oxymyoglobin
Hemoglobin (Hb)Hemoglobin (Hb)
• Hb is an tetramer (2 globin subunits, 2 globin subunits)
• Each globin subunit is similar in structure to myoglobin
• Each subunit has a heme group
• The chain has 7 helices, chain has 8 helices
Hemoglobin tetramerHemoglobin tetramer
(a) Human oxyhemoglobin (b) Tetramer schematic
Oxygen Binding to Mb and HbOxygen Binding to Mb and Hb
• Oxymyoglobin - oxygen bearing myoglobin
• Deoxymyoglobin - oxygen-free myoglobin
• In oxymyoglobin, six ligands are coordinated to the ferrous ion in octahedral symmetry
• Oxygen is coordinated between the iron and the imidazole sidechain of His-64
Oxygen-binding site of Oxygen-binding site of whale oxymyoglobinwhale oxymyoglobin
• Octahedral geometry of coordination complex (six ligands around iron)
• His-93 (proximal histidine) liganded to Fe
• His-64 (distal histidine)
Oxygen-binding curvesOxygen-binding curves
(a) Comparison of O2-binding to Mb and Hb
Oxygen-Binding Curves of Oxygen-Binding Curves of Myoglobin and HemoglobinMyoglobin and Hemoglobin
• Curves show reversible binding of O2 to Mb and Hb
• Fractional saturation (Y) is plotted versus the partial pressure of oxygen, pO2 (oxygen concentration)
• The shape of the Hb curve shows a positive cooperativity in the binding of 4 O2 molecules (i.e. the O2 affinity of Hb increases as each O2 molecule is bound)
Oxygen-binding curvesOxygen-binding curves
(a) Comparison of O2-binding to Mb and Hb
OO22 binding curves (continued) binding curves (continued)
Mb-O2 binding curve is hyperbolic, indicating a single equilibrium constant for binding O2
Hb-O2 binding curve is sigmoidal, and reflects the binding of 4 molecules of O2, one per each heme group
Oxygen-binding curvesOxygen-binding curves
(a) Comparison of O2-binding to Mb and Hb
Oxygen-binding curvesOxygen-binding curves
Binding of the R (high-affinity) and T (low affinity) forms of Hb
Conformational changes in a Conformational changes in a hemoglobin chain induced by hemoglobin chain induced by
oxygenationoxygenation
• Oxygen binding to Fe pulls the His toward ring plane
• Helix with His shifts position, disrupting some ion pairs between subunits (blue to red position)
Oxygen-binding site of Oxygen-binding site of whale oxymyoglobinwhale oxymyoglobin
• Octahedral geometry of coordination complex (six ligands around iron)
• His-93 (proximal histidine) liganded to Fe
• His-64 (distal histidine)
Hemoglobin is an Allosteric Hemoglobin is an Allosteric ProteinProtein
• Oxygen binding and release from Hb are regulated by allosteric interactions
• Allosteric effectors (modulators) bind to a protein at a site separate from the functional binding site (may be activators or inhibitors)
• The activity of an allosteric protein is regulated by allosteric effectors
Two conformations of Two conformations of hemoglobin: T and Rhemoglobin: T and R
• Active (R state) and inactive (T state) forms are in rapid equilibrium in allosteric proteins
• Binding of substrates and allosteric activators stabilize the R state and shift the equilibrium in the R direction
• Allosteric inhibitors stabilize the T state and shift the equilibrium in the T direction
BisBisphospho-D-glycerate phospho-D-glycerate (2,3BPG)(2,3BPG)
• 2,3BPG is an allosteric effector of Hb
• 2,3BPG lowers the affinity of deoxyHb for oxygen (raises the P50 of Hb from ~12 to ~26 torr)
• Negatively charged 2,3BPG is bound to six (+) charged groups of deoxyhemoglobin
BisBisphospho-D-glycerate phospho-D-glycerate (2,3BPG)(2,3BPG)
Binding of 2,3BPG to Binding of 2,3BPG to deoxyhemoglobindeoxyhemoglobin
• (-) Charges on 2,3BPG pair with (+) charges lining the central cavity, stabilizing the DeoxyHb form
• -Subunits pink, -subunits blue, heme groups red
Oxygen-binding curvesOxygen-binding curves
(a) Comparison of O2-binding to Mb and Hb
Bohr effectBohr effect
• Lowering the pH decreases the affinity of Hb for oxygen
Carbamate adductCarbamate adduct
• Carbon dioxide is transported from the tissues to the lungs in two ways:(1) Dissolved bicarbonate ions(2) Carbamate adducts of hemoglobin (N-terminal globin residues react with CO2 to form carbamates)
Review of Relevant ParametersReview of Relevant Parameters
(1) Low P50 indicates high O2 affinity
(2) Low pH (through CO2 intake) stabilizes 2,3-BPG and lowers O2 affinity
(3) Raising P50 causes unloading of O2
Oxy-Hb Deoxy-Hb
(R) (T)
Case StudiesCase Studies
Shock victims are given intravenous HCO3-
Why?
HCO3- generates CO2 to the tissues
and lowers the O2 affinity of Hb,
thus releases O2 from HbO2 to the tissues
Case StudiesCase Studies
Fetal Hb (HB-F) contains ser in place of the cationic his at position 143 of the chains of adult Hb (HB-A). Residue 143 faces the central cavity between the chains
Outcomes:
his in Hb-A is protonated and thus binds more tightly to negatively charged 2,3-BPG;
ser in Hb-F is not protonated and does not bind to 2,3-BPG as strongly; thus Hb-F has a greater fraction of HbO2
Case StudiesCase Studies
Fetal Hb (HB-F) contains ser in place of the cationic his at position 143 of the chains of adult Hb (HB-A). Residue 143 faces the central cavity between the chains
Outcomes:
Hb-F has a greater fraction of HbO2 which means greater O2 affinity and lower P50 (18 torr)
Since average P50 for Hb-A is 26 torr, oxygen can efficiently be transferred from maternal blood to fetus
Antibodies Bind Specific Antibodies Bind Specific AntigensAntigens
• Vertebrate immune systems synthesize protein antibodies (immunoglobulins) to eliminate bacteria, viruses, other foreign substances
• Antibodies specifically recognize and bind antigens
• Antibodies are synthesized by lymphocytes (white blood cells)
Human antibody structureHuman antibody structure
• Heavy chains (blue) and light chains (red)
• Disulfide bonds (yellow)
• Variable domains colored darker
Stereo view of the Stereo view of the immunoglobin foldimmunoglobin fold
• Two antiparallel sheets linked by nonrepetitive segments
Binding of three different Binding of three different antibodies to an antigenantibodies to an antigen