Lesson 6: Protein Structure and Function - Myoglobin and Hemoglobin Flashcards
where is myoglobin found
- skeletal muscle –> job is to “store”
myoglobin
- tertiary structure
- 8 helical segments
- porphyrin ring with a central Fe+
- proximal and distal His residues are important in O2 binding
what is hemoglobin’s job
transport O2 form lungs and deliver to tissues
hemoglobin
- composed of 4 polypeptide subunits; therefore quaternary structure
- composed of 2 alpha and 2 bet subunits
- each subunit is structurally similar to an individual Mb ( each 4 chains on Hb has a folded subunity structure similar to that of Mb and each carries a heme)
- porphyrin ring
F helix of Hb contains what
proximal His residue
E helix of Hb contains what
distal residue
the distal His of Hb binds to what
(also give the number of residue)
distal His 64 forms hydrogen bond with the bound O2
where is the oxygenation of myoglobin and hemoglobin bound through ()
- what are critical for this binding
- 6th coordination bond
- 2 histidines (proximal and distal) are critical to this binding
what is the shape of myoglobin curve
hyperbolic
what does the curve shape and saturation of Mb graph indicate
indicats that there is a single binding site for O2 and binding can be described by a single binding constant
P50:
pressure where 50% of molecules are saturated
what relationship is there between P50 and oxygen binding affinity
INVERSE RELATIONSHIP with binding affinity
ex:
if Mb had a low p50 value—-
- high affinity for O2
- means that it takes a small amount of O2 to reach 50% mark
— indication of binding affinity
O2 path (step 1)
hemoglobin transports O2 from the lungs to the respiring tissues where it is used for aerobic metabolism in the mitochondria
O2 path (step 2)
inside cells, dissolved O2 diffuses freely or is bound to myoglobin, which aids transport of O2 to mitochondria. Myoglobin can also store O2 for later use (as in deep-diving mammals)
O2 path (step 3)
CO2 produced by oxidative processes in the tissues is carried back to the lungs by hemoglobin, or in the plasma as HCO3-, and released
hyperbolic =
single binding site - no cooperativity
sigmoidal =
- cooperative bindng (cooperativity) –> subunits are working together
- =must be >1 ligand binding site
- binding of ligand to 1 binding site influences binding of ligand to other sites
why does fetal HbF have a lower P50 value than HbA
as O2 is exchanged in the placenta, the fetal Hb needs to “compete” with maternal tissues for available O2. Hence it needs to have a higher affinity and a lower p50 value
HbF
Fetal Hb –> (a2) (y2)
HbA
adult Hb —> (a2) (b2)
synthesis of normal Hb from globin genes: yolk sac, liver, spleen
in utero, gene expression unregulates alpha and gamma. Yolk sac, liver, and spleen synthesize fetal Hb`
synthesis of normal Hb from globin genes: bone marrow
gene expression shifts postnatally. Bone marrow synthesizes adult Hb
T-state
- low O2 affinity state
- deoxy state (oxygen is unbound)
- puckered porphyrin ring plane, large and basic central cavity
- tense state
R - state
- high O2 affinity state
- oxy state (oxygen is bound)
- flat porphyrin ring upon O2 binding
- stimulates a series of large scale conformational changes
- relaxed state
when no ligands are bound,
T state is more stable-
- less stable upon oxygenation –> higher in energy
when 4 ligands are bound
R state is more stable
- more stable upon oxygenation –> lower in energy
without oxygen…
reduced oxygen is no longer planar because the ion is to large and the heme is puckered
- this is called deoxyhemoglobin
puckered heme
no O2 binding
planar heme
there is O2 binding
where does the H -bond form in the deoxyhemoglobin (T state)
between Tyr 42’s OH and Asp 99’’s O-
where does the H-bond form in the oxyhemoglobin (R state)
asp 94’s O and asn 102’s nH
concerted and sequential models of O2 binding (step 1)
1st O2 (ligand) binds
concerted and sequential models of O2 binding (step 2)
2nd subunit undergoes T–> and binds 2nd O2
concerted and sequential models of O2 binding (step 3)
3rd subunit undergoes T—-> R and bonds 3rd O2
concerted and sequential models of O2 binding (step 4)
4th subunit undergoes T ——->R and binds 4th O2
what does the binding of O2 to Hb stimulate
the release of H+ from Hb (bohr effect)
N-terminus:
bohr position
His 146
bohr position
the bohr effect
1.) protonation of “bohr positions” (amino termini of alpha; His 146 of beta) stabilizes T state in absence of O2
2.) Binding of O2 releases H+ from “bohr positions”, thereby shifting equilibrium to R state
pH 7.6 vs 6.8
- at pH7.6, Hb is about 99 percent saturated in lungs and 75 percent saturated in the tissue –> inefficient O2 transfer
- at pH 6.8, Hb is 99 percent saturated in the lungs and around 35 percent saturated in the tissue
^^^^^ much more efficient O2 transfer at this pH
the efficiency of oxygen unloading () significantly as the pH drops
increases
in oxygenated Hb,
- r state favored
- favors deprotonation of Bohr positions; therefore His more acidic
(lower pKr)
in dexogyenated Hb,
- t state favored
- favors protonation of bohr positions; therefore His is more basic (higher pKr)
2,3-biphosphohlycerate (BPG) modulates the conformational changes within Hb
A.) highly purified (stripped) Hb has a higher affinity for O2 than Hb in whole blood (similar to Mb!!!)
B.) blood must contain some other compound that affects O2 binding to Hb
C.) the metabolite BPG is that compound
TAKE AWAY ——–»»» BPG stabilizes T-state
stripped Hb
dangerously R state favored
blood Hb
bpg stabilizes t state so O2 can be released
2,3 - BPG Binding: high O2 favors Oxy.
shift T-> R.
“squeeze out” BPG
2,3 - BPG Binding: Heterotrophic negative allosteric modifier
BPG binds.
Shift R —> T
O2 is a positive allosteric effector of Hb
O2 binding helps shift T–> R
example of homotropic allostery:
binding of O2 promotes binding of the same ligand (O2)
positive allostery b/c binding of O2 () binding of the ligand O2 to other sites on the molecule
increases
—– add more O2, increase binding capautor
what are the Bohr positions and how are they different from proximal His, Distal His, and His residues at central cavity
Bohr Positions:
- 2 His 146 residues of B chains
2 N-termini of alpha chains
His 2 and His 143 - central cavity residues
4 proximal and 4 distal His residues
