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BICH 410: EXAM 2

What is myoglobin?
An intracellular protein found in muscle tissue.
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What is the function of myoglobin?
To facilitate oxygen transport in rapidly respiring muscle tissue.
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How does myoglobin affect the solubility of oxygen in muscle tissue?
- It increases the effective solubility of oxygen in muscle tissue. - Helps facilitates oxygen diffusion.
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What is myoglobin's composition?
Single polypeptide chain of 153 residues in 8 a-helices (a-helices are labeled A-H)
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How is oxygen bounded by myoglobin?
By a heme.
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What is a heme?
A porphyrin that consists of 4 pyrole rings linked by methylene bridges.
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How is metmyoglobin (MetMb)/ methemoglobin (MetHb) formed from regular myoglobin and hemoglobin?
When the Fe(II) ferrous state central iron in the protophyrin is oxidized into Fe(III) ferric state.
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What happens to oxygen when it is bounded to metmyoglobin?
The free heme in the solution will readily bind to oxygen, but the oxygen quickly oxides it into the ferric state.
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What is the binding arrangement of the iron atoms of the heme? Is the the same in both the ferrous or ferric state?
In BOTH the ferric or ferrous state, the iron atoms prefer to bind six ligands in an octahedral geometry. - 4 of the ligands are the nitrogen atoms of the 4 pyrrole rings of the heme - 5th ligand comes from histidine F8 - 6th ligand occurs when oxygen binds to the ferrous heme/ water binds for the ferric heme
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What is oxymyoglobin?
It is the oxygenated heme of the myoglobin.
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How is the oxygen bounded to the heme structurally?
60 degrees to the plane of the heme. Same side of the plane is His-E7 => makes the oxygen binding site a sterically hindered region.
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What is deoxymyoglobin?
It is the unoxygenated heme of myglobin. - in this form, the 6th coordination site is vacant
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What is the affinity of the free heme in solution for CO compared to O2?
It has 25,000 times more affinity for CO than O2.
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What is the affinity of the heme in myoglobin/hemoglobin for CO compared to O2?
CO only binds 250 times greater than O2 - because the His-E7 forces the CO molecule to tilt away from the preferred perpendicular alignment with the plane of the heme.
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How is oxygen bounded to the deoxymyoglobin structurally?
- Ferrous iron atom has five ligands and lays 0.55 A above the plane of the heme towards His-F8 (gives the iron porphyrin the complex dome shape) - When oxygen binds, the iron is pulled back into the plane of the heme such that it is only 0.26 A above the porphyrin
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What is the binding equilibria oxygen binding?
Mb + O2 <=> MbO2
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Dissociation constant K =
K = ([Mb][O2]) / [MbO2]
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What is fractional saturation (YO2)?
The fraction of O2-binding sites occupied by O2.
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YO2 (fractional saturation) =
YO2 = [MbO2]/ ([Mb] + [MbO2]) YO2 = [O2] / (K + [O2]) YO2 = pO2/ (K + pO2)
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What is p50?
The value of pO2 when YO2 = 0.5 The partial pressure when half of the myoglobin binding sites are occupied with oxygen.
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YO2 (in terms of p50) =
YO2 = p50/ (K + pO2) YO2 = pO2/ (p50 + pO2)
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What is the fraction of unbound site in myoglobin?
1 - YO2
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What is the ratio of fractional saturation to the free myoglobin?
YO2 / (1 - YO2) = pO2 / K
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What is the Hill plot?
A graph of log Y/(1-Y) versus log pO2
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What is the Hill Coefficient?
- the midpoint of binding - the slope of the hill plot at the point of log (Y/1-Y) = 0
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What is the Hill Coefficient for Myoglobin?
1
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Hill coefficient = 1 means what?
The oxygen atoms bind independently of each other.
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What is hemoglobin?
It is a compact globular protein.
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What is the components of hemoglobin?
- It is a tetramer - four polypeptide chains - each subunit contains a heme allowing hemoglobin to bind four oxygen atoms
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What type of tetramer is an adult hemoglobin?
a2B2 - type tetramer
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What are the two different types of subunits in hemoglobin?
alpha and beta
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How is the beta subunit of hemoglobin compared to the structure of myoglobin?
- 7 amino acid residues shorter than Mb 153 - its last helix H is shorter than myoglobins
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How is the alpha subunit of hemoglobin compared to the structure of myoglobin?
- 12 amino acid residues shorter - has a shorter H helix - lacks helix D
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Why is the tetrameric quarterinary structure of hemoglobin important to its function?
when a molecule of oxygen binds to heme in Hb, the heme iron is drawn into the plane of the the porphyrin ring => sets off chain of conformation events - dramatically enhancing the affinity of the heme for oxygen
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What happens to deoxyhemoglobin when exposed to O2?
It shatters due to the change in the structure that occurs upon oxygen binding.
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What is the structure of hemoglobin like?
- highly symmetrical and spherical - hemes are located in the clefts between the E and F helices and are exposed to the aqueous solvent - heme groups are far apart - the closed two hemes are between a1 and B2 or a2 and B1 are separated by 25 A
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Describe the subunit interactions in hemoglobin.
- occurs between a1 and B1(or a1 and B1) interface consisting of 35 residues ;contacts involve helices B, G and H and the GH corner; important for subunit packing - occurs between a1 and B2 (or a2 and B1) interface consists of 19 residues; these contacts are called sliding contact ; involve helices C and G and the FG corner
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What is the binding curve for hemoglobin?
It is sigmoidal shaped.
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What is the binding curve for myoglobin?
It is hyperbolic shaped.
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Myoglobin binds oxygen under the condition of what?
when Hemoglobin releases the oxygen.
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What is the Hill Equation?
- it describes the degree of saturation of a multisubunit protein as a function of the ligand concentration Ys = [S]^n / (K + [S]^n) Ys: fractional saturation S: ligand S n: number of subunits (related to the degree of cooperativity among interacting ligand binding sites)
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Hill constant (n)
- increases with degree of cooperativity of a reaction and provides a way of characterizing a ligand binding reaction
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Hill constant (n) =1
- myoglobin - hyperbolic binding curve - binding is NONCOOPERATIVE
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Hill constant (n) > 1
- positively cooperative - binding increases the affinity of the protein for further ligand binding
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Hill constant (n) < 1
- negatively cooperative - ligand binding decreases the affinity of the protein for subsequent ligand binding
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Ys / (1 - Ys) =
- ratio of the fractional saturation to the free protein - Ys / (1 - Ys) = [S]^n / K
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Why must the shift to the R-state must occur simultaneously at both the a1-B2 and a2-B1 interfaces?
Due to the inflexibility of the a1B1 and the a2B2 interface. - no one subunit can independently exist in the R-state - no one aB dimer exist in the R state
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What interfaces can change upon oxygenation?
a1-B2 and a2-B1
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Describe the transition from the T to the R-state?
It requires a quaternary shift of the a1C-B2FG contacts one turn up the a1C-helix; the sliding contact shift one turn up the C-helix during the quaternary shift from the T to the R state. - Result in a lot more sliding contacts
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How are the two states stabilized?
- R-state is stabilized by oxygen binding - when oxygen is not present, T-state is more stable => salt bridges electrostatically stabilize the T-state
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Where does the energy that cause the T-> R transition come from?
The formation of the Fe-O2 bond.
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What is the equation for the bicarbonate buffer system of blood plasma?
H2CO3 <=> H+ + HCO3-
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What is the pKa of carbonic acid at 37 degrees Celsius?
3.57
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What is the physiological pH?
7.4
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At the pH of 7.4, what is the concentration of H2CO3?
It is miniscule
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Why does the bicarbonate system works well?
- The critical level of the carbonic acid is maintained by equilibrium with dissolved CO2 gas produced in the tissues and is available as gaseous CO2 in the lung. - The gaseous carbon dioxide from the lungs and tissues is dissolved in the blood plasma designated by CO2 and is hydrated to form H2CO3.
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What is the three equilibriums of the bicarbonate buffer system?
1. H2CO3 <=> H+ + HCO3- (in blood of capillaries) 2. CO2(d) + H2O <=> H2CO3 (in blood of capillaries) 3. CO2 (g) <=> CO2 (d) (in alveoli of lungs) ---------------------------- Net: CO2(g) + H2O <=> H+ + HCO3-
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What happens to the bicarbonate buffer system when you exercise?
you generate H+ => drives equilibrium towards carbonic acid formation - increase [CO2d] - exhale more [CO2g]
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What happens to the bicarbonate buffer system when you hyperventilate?
- increase [O2] - decrease [CO2g] - decrease [CO2d] - decrease carbonic acid - decrease H+ concentration - decrease bicarbonate concentration - increase in blood pH
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What happens to most of the Co2 produced int he muscles and tissues?
They diffuse through the tissues to the plasma in the CO2(d) because of the slow rate of bicarbonate formation.
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What is carbonic anhydrase?
- An enzyme found in erythrocytes. - It catalyze the net reaction of the first two equilibria of the bicarbonate buffer system - the enzyme catalyzes at the diffusion limit = the rate determining step is the diffusion of CO2 (d) to the enzyme - this enzyme prevent high [CO2g], which would resulted in bubbles of CO2(g) in our blood.
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What is the Bohr Effect I?
- On binding oxygen, the conformational change makes the hemoglobin a slightly stronger acid. It release protons upon binding oxygen. - Conversely increasing the pH stimulates Hb to bind oxygen. Hb(O2)nHx + O2 <=> Hb(O2)n+1 + xH+ n=1,2,3 x=0.6 under physiological conditions
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What is the Bohr Effect II?
In the capillaries, pO2 is low, the H+ generated by bicarbonate formation is taken up by the Hb - Binding of protons decreases the affinity for O2 => hemoglobin unloads its oxygen => more bicarbonate formation - Conversely in the lungs, pO2 is high, oxygen binding by Hb releases the Bohr protons, which then drive off the CO2
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What is the major contributor to the Bohr effect?
His-B146
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What is BPG (formerly DPG)?
D-2,3-Bisphosphoglycerate - it binds to hemoglobin and promotes oxygen release by stabilizing the deoxyHb (T) form - 1 BPG binds per hemoglobin tetramer - has a net charge of -5 at physiological pH - binding site is located within the central cavity of Hb between the four subunits
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What does BPG do tho the saturation curve?
- shifts the saturation curve to the right - facilitates oxygen delivery
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Why do marathon runners train at high altitude?
oxygen concentration is low => elevated BPG levels - obtain high BPG blood concentration so you can use oxygen more efficiently and have a distinct competitive advantage at sea level
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How is fetal hemoglobin different from adult hemoglobin?
- two gamma chain instead of two Beta chains (a2v2) - gamma chain (histidine is substituted with a serine) - cannot electrostatically interact with BPG => BPG binds much weaker to hemoglobin - greater affinity for O2
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What is the T-state of hemoglobin?
the quarterinary conformation of deoxyHb - the ligand used to induce the state does not matter - (i.e. H+, BPG, CO2)
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What is the R-state of hemoglobin?
- the quarterinary conformation of oxyHb - the ligand used to induce the state does not matter - (i.e. O2, CO, CN-, NO)
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What are hemoglobinopathies?
Genetic disorders caused by the synthesis of abnormal globin chains.
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Sickle Cell Anemia
- most common and severe hemoglobinopathy - caused by a single point mutation of the sixth amino acid of the B-subunit from a a glutamate to a valine; this residues is the third amino acid in the A helix - HbS 6(A3)BGlu->Val - sickle like shaped erythrocytes occurring at low oxygen concentrations - treatment: dilute the concentration of HbS in the red blood cel by inducing the expression of fetal hemoglobin
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Mutations that decrease the stability of hemoglobin
decrease in stability result in increased rates of the degradation of hemoglobin => cause RBC to lyse open HEMOLYTIC ANEMIA 1/ Hb Savannah 24(B6)B Gly->Val 2/ Hb Bibba 136(H19)a Leu->Pro
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Mutations that effect heme binding
1/ Hb Bristol 67(E11)B Val->Asp 2/ Hb Sydney 67(E11)B Val->Ala 3/ Hb Hammersmith 42(CD1)B Phe-Ser [heme loss->hemoglobin become unstable->severe anemia]
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Mutations that effect the oxidation state of the iron
Only heterozygous individuals are found. Homozygous individuals never observed (fatal); ferric iron makes blood choclate brown in color and makes the skin bluish 1/ Hb (Iwate) 87(F8)a His-> Tyr 2/ Hb Hyde Park 92(F8)B HIs-> Tyr 3/ Hb Boston 58(E7)a His-> Tyr 4/ Hb Saskatoon 63(E7)B His-> Tyr 5/ Hb Milwaukee 67(E11)B Val-> Glu
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Mutations that favor the R state
These mutations increase the affinity of hemoglobin for oxygen by eliminating a hydrogen bond or salt bridge or other stabilizing interaction of the T-state 1/ Hb Chesapeake 92(FG4)a Arg-> Leu 2/ Hb Philly 35(C1)B Tyr-> Phe 3/ Hb Yakima 99(G1)B Asp -> His
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Mutations that stabilize the T state
T-state causes lower oxygen affinity Hb Kansas 102(G4)B Asn-> Thr
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Mutations that affect BPG binding
1/ Hb Syracuse 143(H21)B His->Pro [eliminate one of the electrostatic interactions and decreases the affinity of hemoglobin for BPG -> shifts equilibria to favor the R state-> high oxygen affinity) 2/ Hb Shepherd's Bush 74(E18)B Glu->Asp [add negatively charged aspartate -> repulses BPG -> low affinity of hemoglobin]
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Mutations that affects the Bohr effect
1/ Hb Cowtown 146(HC3)B His->Leu [T->R-state -> diminish Bohr effect -> increase oxygen affinity] 2/ Hb McKees Rocks 145(HC2)B Tyr-Stop [prematurely stops B-subunit before His146 -> diminish Bohr effect -> increase oxygen affinity]
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