Block2 10.hemoglobin And Myoglobin

Front 1

Hemoglobin vs. Myoglobin: The Basics

Back 1

Hemoglobin: Circulating form
• Binds O2 at high pH
• Releases O2 at low
pH
• Carries CO2 and H+
back to lungs
• Adapts to conditions
of higher or lower O2
in atmosphere
Myoglobin: Facilitates O2
diffusion (and
storage) in tissues
• Binds to O2 more
tightly than
hemoglobin
• pH change does not
change its affinity for
O2 (tons of myglobin in marine mammals)

Front 2

Heme Structure

Back 2

Heme = protoporphyrin plus Fe2+. Iron has 6 coordinate groups, 
4 are bound to the heme. One other 
binds to a histidine, and last one reacts
 with O2 molecule. Note: If you take heme molecule out of protein and put into solution, the Iron is easily oxidized from 2+ to 3+

Front 3

Myoglobin vs. Hemoglobin Structure

Back 3

They are both comprised entirely of alpha helices and turns. The major difference is that hemoglobin has a patch of hydrophobic residues on its exterior. This helps in tetramer formation due to hydrophobic interactions (tetramer also due to ionic bonds)

Front 4

Equilibrium dissociation constant (Kd)

Back 4

The concentration of ligand
(units=molar) at which ½ of the available ligand binding sites (on average) are occupied. The more tightly a protein binds a
ligand, the lower the value of Kd.

Front 5

Myoglobin vs. Hemoglobin binding curves

Back 5

Myoglobin - one site binding curve, very left-shifted because it has higher affinity than hemo.
Hemoglobin sigmoidal because of cooperative binding that takes place.

Front 6

Tense and Relaxed States for Hgb

Back 6

The iron sitting in the heme is like a dinner plate. In the tense state, the plate is slightly angled; this decreases the affinity for oxygen by the iron. In the relaxed state, the plate is nicely planar, and the oxygen binds more easily. IMPORTANT- the tense state is a result of increased ionic binding (salt bridges) in the hemoglobin tetramer.

Front 7

Cooperative binding of hmeoglobin

Back 7

When the first oxygen binds, it causes the breaking of an ionic bond. This makes it easier for the next oxygen to bind, and so on and so forth.

Front 8

Effects of pH on binding of O2 to Hgb

Back 8

Concentration of H+ (pH) - higher pH, more left-shifted curve because affinity is higher. Low pH causes more salt bridge formation. High pH in lungs allows lots of O2 to bind up there. pH is lower at tissues (curve right shifted), so the hemoglobin can more easily dump off its oxygen there.

Front 9

Effect of CO2

Back 9

1. 10% CO2 remains dissolved in plasma. The rest diffuses into the red blood cells.
2. 23% CO2 forms reversible, non-covalent adduct with Hgb,
carbaminohemoglobin.
3. ~70% converted to HCO3- and H+, catalyzed by carbonic anhydrase.
CO2 + HOH <===> H2CO3 <===> H+ + HCO3-
Reaction is reversed in the lungs. Increased H+ can ↓ O2 binding affinity.

Front 10

Carbon Monoxide Poisoning

Back 10

CO binds to Hb ~250 times more tightly than does O2.
CO stablizes the R state of Hb, which shifts the binding curve to the left, and prevents O2 from being delivered to the tissues.
Earliest symptoms of CO poisoning are nonspecific
and readily confused with flu-like
syndromes (headache, nausea, vertigo).
High concentrations of HbCO (50-60%) lead to seizures, coma and death.

Front 11

Methemoglobinemia

Back 11

Oxidation of Fe from 2+ to 3+. This ruins oxygen binding to hemoglobin. Exposure to exogenous oxidizing drugs (eg.,
benzocaine, dapsone).
• Ingestion of compounds containing nitrates (bismuth
nitrate) or well water contaminated by nitrates. Infants are particularly susceptible to nitrates ingested in drinking water (leads to blue-baby
syndrome)

Front 12

2,3 Bisphosphoglycerate (BPG)

Back 12

Molecule we use to adjust our hemoglobin's binding affinity for oxygen. It sits on a patch of positively charged aa's, (2,3 bisphosphoglycerate is highly negative). This makes more salt bridges, and more salt bridges reduce hemoglobin's binding to oxygen, making it able to deliver better to tissues. At sea level (5mM BPG), Hb is ~95% saturated with O2 in the lungs and ~57% saturated in the tissues. It can therefore release ~38% of its bound O2 to the tissues. The delivery is reduced to ~30% at high altitudes.
After adaptation to high altitudes (8mM BPG) Hb is 90% saturated with O2 in the lungs and 53% saturated in the tissues. Again, it can therefore release 37% of its bound O2 to the tissues.


Front 13

Fetal Hemoglobin and BPG

Back 13

Adult hemo has two alpha and two beta subunits, but fetal has two alpha and two gamma! In each γ subunit, two serines replace the two histidines
present in the BPG binding pocket of the β subunit. This greatly reduces fetal hemoglobin's binding with BPG, giving it higher binding affinity for oxygen. If a pregnant mother experiences a rapid decrease in
oxygen tension, the fetus will be spared.