Biochem - Globins Flashcards
What do hemoglobin and myoglobin do to O2’s reactivity?
reduce it
2 reasons for O2 transport proteins:
- reduces O2 activity
- gets more solubility in the blood
Oxidation vs reduction
O: loss of e- (increase in oxidation #)
R: gain of e- (decrease in oxidation #)
Oxidant vs reductant
O: accepts e-‘s (oxidation state decreases)
R: donates e-‘s (oxidation state increases)
Example of powerful oxidant
oxygen
How do hemoglobin and myoglobin affect oxygens reactivity?
reduce it by forming a protective environment around it so it cant react
Hemoglobin picks up _____, so it acts as a biological _____
protons
buffer
What are heme iron’s two oxidation states?
Fe2+ (ferrous)
Fe3+ (ferric)
How many bonds can ferrous iron form?
6 bonds
- 4 bonds with the nitrogens of the pyrrole rings
- 1 bond with the histidine of the globin chain
- 1 bond with oxygen
Carbon monoxide binds
_____ more tightly than oxygen
200-fold
T/F: Ferric iron doesnt bind to oxygen?
True
O2 doesn’t bind to protein components, it binds to _____ groups
prosthetic
What is an example of prosthetic groups?
vitamins
What is the heme prosthetic group derived from?
precursors in the body (NOT vitamins)
Heme structure
- forms 4 pyrroles (because of the 4 nitrogens)
- 1 central iron
- flat
- bulk of molecule is hydrophobic/nonpolar
- one edge that is polar (because of the proprionic acid groups)
Myoglobin structure:
- 153 amino acids (80% in α helix form)
- overall nonpolar molecule
- has 1 polar surface (between histidines)
Heme and myoglobin:
- Heme slides into a pocket between E and F helices
- Histidine in helix F binds heme iron (proximal His)
- Histidine in helix E binds O2 (distal His)
- the surface between the 2 histidine portions is POLAR (prevents H2O from entering and oxidizing the iron)
Which residue does histidine bind to the heme iron?
Residue position 93
Histidine in helix F:
- proximal His (more anterior when looking at a 3D structure)
- binds heme iron
Histidine in helix E:
- distal His (more posterior when looking at a 3D structure)
- binds O2
- actually makes contact with O2, unlike prox. histidine
Hemoglobin structure:
- heterotetramer
- 2α and 2β subunits (so hemoglobin can bind up to 4 O2)
How many oxygen can hemoglobin vs myoglobin bind?
H: 4 oxygens
M: 1 oxygen
What is the sequence identity btwn Hb and Mb?
17%
What is the sequence identity btwn α and b globins?
40%
Structural conservation between globin chains is greatest at which structural level?
tertiary
Forms of Hemoglobin in the Human Circulatory System:
- 92.5% HbA1 α2b2 (alpha 2 beta 2)
- 2.5% HbA2 α2δ2 (alpha 2 delta 2)
- 5% HbA1c α2 (b-NH-Glc)2
overall: 95% adult hemoglobin is in blood, 5% is HbA1
Forms of Hemoglobin in the Fetal/Embryonic Circulatory System:
Fetus:
- HbF α2γ2 (alpha 2 gamma 2)
Embryo:
- Hb Gower ζ2ε2 (zeta 2 epsilon 2)
- Hb Portland ζ2γ2 (zeta 2 gamma 2)
Fetal development and clinical manifestations
- when beta chains replace gamma chains at birth, visible manifestations become observed (not affected during embryonic development, because no beta chains in the embryo)
- disorder to alpha chains is expressed during fetal development
Quaternary Structure of Hemoglobin:
- non covalent assembly of polypeptide chains
- has a distinctive hole in middle of structure
- when treated with urea, becomes 2 alpha-beta dimer “halfs”
Myoglobin-O2 binding equation
Y = fraction of Mb with O2 bound
pO2 = partial pressure of O2
Kd = equilibrium dissociation constant
note: if pO2 > Kd : Y=1 (this is why shape of binding curve plateau’s at 100% binding)
note: if pO2 < Kd : Y = pO2/Kd
Myoglobin-O2 binding continued
- Kd and affinity are inversely related
- Low Kd = high binding strength (affinity)
- Low K-1 (slow and small) = high affinity (results in more bound species)
Myoglobin-O2 binding shifts
if curve shifts to right:
- Kd increases (bc bigger pO2 value)
- affinity for O2 decreases
if curve shifts to left:
- Kd decreases
- affinity for O2 increases
Why is Mb binding simpler than Hb binding?
- Mb is a single O2 binding to a single polypeptide chain
- Hb binding equation has exponents
- Hb shape is sigmoidal
Hemoglobin-O2 binding equation
N = cooperativity (hill coefficient)
Does Hb display positive or negative cooperativity?
positive cooperativity (when the binding on one ligand stimulates the binding of another)
In Hb, the last O2 bind with _____ greater affinity than the first O2
100-fold
What are the 2 reasons positive cooperativity is ideal?
- it fully loads partial pressures in the lung
- it significantly unloads partial pressure in tissues
O2 binding properties - Hb/Mb
- Hb releases O2 to tissues
- Mb within the cells of the tissues picks it up
- at the level where Hb releases O2, Mb binds (bc it has high affinity at those partial pressures)
When O2 binds to DeoxyHb, how many degrees is the orientation shifted to become OxyHb?
15 degrees
Hb movement at the atomic level
Deoxy state: iron is “puckered out”
Oxy state: iron is flush (gets pulled into the ring)
Another name for DeoxyHb
T state
Another name for OxyHb
R state
Histidine in DeoxyHb vs OxyHb
Deoxy:
+ charged His
pKa 8 (high pKa = weak acid = stronger binding to proton)
Oxy:
neutral His
pKa 6 (normal His pKa = releases the proton)
Relationship between O2 and protons in Hb
- inversely related
- O2 binds to Hb, protons come off
- protons bind to Hb, O2 comes off
overall: protons influence the affinity of Hb for O2
pH effect: protons and O2
- High proton concentrations decrease pH values
- proton binds -> decreased pH
- decreased pH = right shift
- right shift = increased Kd
- increased Kd = lower affinity for O2
What happens when CO2 binds to Hb?
- right shift
- lower affinity O2
- is used as a way to get more O2 to tissues
Where does BPG bind?
What is net charge of BPG?
What does it do?
How does BPG affect affinity for O2?
- central cavity of Hb
- -5
- stabilizes deoxyHb
- lowers affinity for O2
Fetal Hb has a _____ affinity for BPG and a _____ affinity for O2 when compared to maternal Hb
- lower
- higher
T/F: Fetal Hb and maternal Hb have a similar relationship to Hb and Mb
TRUE
The globin gene family is thought to have evolved through a series of…
- gene duplications
- random mutational drift
Sick cell hemoglobin (HbS) is a balanced polymorphism with good and bad characteristics
bad: sickle cell disease
good: resistance to malaria
What is the malaria parasite known as?
Plasmodium falciparum
In HbS, what amino acid can substitute Glutamic acid at position 6 of beta globin chains?
- Valine (non-polar substituting polar)
- Val can fit between Leu and Phe hydrophobic pockets
- Causes Hb to go through polymerization (distorts RBC, causes the “sickle” shape)
Tendency to polymerize is favored by which state?
- T state
What factors favor polymerization (“sickling”)?
- High proton concentration
- Low O2
- Low pH
