View
- Term
- Definition
- Both Sides
Study
- All (79)
Shortcut Show
Next
Prev
Flip
BICH 410: EXAM 2
What is myoglobin? |
An intracellular protein found in muscle tissue. |
What is the function of myoglobin? |
To facilitate oxygen transport in rapidly respiring muscle tissue. |
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. |
What is myoglobin's composition? |
Single polypeptide chain of 153 residues in 8 a-helices (a-helices are labeled A-H)
|
How is oxygen bounded by myoglobin? |
By a heme. |
What is a heme? |
A porphyrin that consists of 4 pyrole rings linked by methylene bridges.
|
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. |
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. |
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 |
What is oxymyoglobin? |
It is the oxygenated heme of the myoglobin. |
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. |
What is deoxymyoglobin? |
It is the unoxygenated heme of myglobin.
- in this form, the 6th coordination site is vacant
|
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. |
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. |
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 |
What is the binding equilibria oxygen binding? |
Mb + O2 <=> MbO2 |
Dissociation constant K = |
K = ([Mb][O2]) / [MbO2] |
What is fractional saturation (YO2)? |
The fraction of O2-binding sites occupied by O2. |
YO2 (fractional saturation) = |
YO2 = [MbO2]/ ([Mb] + [MbO2])
YO2 = [O2] / (K + [O2])
YO2 = pO2/ (K + pO2) |
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. |
YO2 (in terms of p50) = |
YO2 = p50/ (K + pO2)
YO2 = pO2/ (p50 + pO2) |
What is the fraction of unbound site in myoglobin? |
1 - YO2 |
What is the ratio of fractional saturation to the free myoglobin? |
YO2 / (1 - YO2) = pO2 / K |
What is the Hill plot? |
A graph of log Y/(1-Y) versus log pO2
|
What is the Hill Coefficient? |
- the midpoint of binding
- the slope of the hill plot at the point of log (Y/1-Y) = 0 |
What is the Hill Coefficient for Myoglobin? |
1 |
Hill coefficient = 1 means what? |
The oxygen atoms bind independently of each other. |
What is hemoglobin? |
It is a compact globular protein. |
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
|
What type of tetramer is an adult hemoglobin? |
a2B2 - type tetramer |
What are the two different types of subunits in hemoglobin? |
alpha and beta |
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 |
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 |
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 |
What happens to deoxyhemoglobin when exposed to O2? |
It shatters due to the change in the structure that occurs upon oxygen binding.
|
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 |
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
|
What is the binding curve for hemoglobin? |
It is sigmoidal shaped. |
What is the binding curve for myoglobin? |
It is hyperbolic shaped. |
Myoglobin binds oxygen under the condition of what? |
when Hemoglobin releases the oxygen.
|
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) |
Hill constant (n) |
- increases with degree of cooperativity of a reaction and provides a way of characterizing a ligand binding reaction
|
Hill constant (n) =1 |
- myoglobin - hyperbolic binding curve
- binding is NONCOOPERATIVE |
Hill constant (n) > 1 |
- positively cooperative
- binding increases the affinity of the protein for further ligand binding |
Hill constant (n) < 1 |
- negatively cooperative
- ligand binding decreases the affinity of the protein for subsequent ligand binding |
Ys / (1 - Ys) = |
- ratio of the fractional saturation to the free protein
- Ys / (1 - Ys) = [S]^n / K
|
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
|
What interfaces can change upon oxygenation? |
a1-B2 and a2-B1 |
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
|
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 |
Where does the energy that cause the T-> R transition come from? |
The formation of the Fe-O2 bond. |
What is the equation for the bicarbonate buffer system of blood plasma? |
H2CO3 <=> H+ + HCO3- |
What is the pKa of carbonic acid at 37 degrees Celsius? |
3.57 |
What is the physiological pH? |
7.4 |
At the pH of 7.4, what is the concentration of H2CO3? |
It is miniscule |
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.
|
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-
|
What happens to the bicarbonate buffer system when you exercise? |
you generate H+ => drives equilibrium towards carbonic acid formation
- increase [CO2d]
- exhale more [CO2g] |
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 |
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.
|
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. |
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 |
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 |
What is the major contributor to the Bohr effect? |
His-B146 |
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 |
What does BPG do tho the saturation curve? |
- shifts the saturation curve to the right
- facilitates oxygen delivery |
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 |
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
|
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) |
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) |
What are hemoglobinopathies? |
Genetic disorders caused by the synthesis of abnormal globin chains. |
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
|
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 |
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]
|
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 |
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 |
Mutations that stabilize the T state |
T-state causes lower oxygen affinity
Hb Kansas 102(G4)B Asn-> Thr
|
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] |
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] |