Biochemistry of RBCs Flashcards
The two states of hemoglobin
There are two states in the hemoglobin, the T state (the tensestate) and the R state (the relaxed state). The T state has a less of an affinity for oxygen than the R state. In the concerted mode of cooperativity, the hemoglobin must either be in its T state or R state.
Several factors affect the equilibrium between the R and T state; these have evolved to facilitate efficient gas transport
Fetal Hemoglobin
- Fetal hemoglobin (HbF) has a different composition than adult hemoglobin (HbA). It substitutes γ chains for the β chains in HbA, so its composition is α2/γ2.
- γ chains supply less (+) charge to central cavity than do β chains. This difference makes HbF have a much lower affinity for the negatively charged 2,3-bisphosphoglycerate than HbA.
- Purified HbF and HbA have very similar O2 affinities but in red blood cells HbF has a higher O2 affinity.
- The affinity difference allows O2 to be transferred from a mother’s red blood cells to the fetal red blood cells in utero
2,3-bisphosphoglycerate and hemoglobin stabilization
- stabilizes the T state of hemoglobin
- 2,3-bisphosphoglycerate is present in red blood cells in mM concentrations.
- 2,3-bisphosphoglycerate binds to the central pocket that is exposed in the T state of hemoglobin.
- This stabilizes the T state relative to the R state, which promotes O2 release; pocket in the T state has lots of positive charges that can compliment the negatives on 2,3BPG
Myoglobin
- Monomeric O2 binding protein
- Similar 3D fold; also binds to a heme group
- Hemoglobin can release O2 in the tissues at 20-30 torr
- Myoglobin releases O2 only under very low pO2 conditions (it is an O2 storage protein in muscles to provide oxygen under extreme conditions
- Hemoglobin can bind multiple oxygen; cooperative nature in binding of more oxygen
Effect of H+ Binding to Hemoglobin
- Bohr effect
- Binding of H+ to hemoglobin favors the T state.
- Some weak acid groups are weaker acids in the T-state (two his residues (C-terminal residues on the β chains) and the α- amino termini of the α globin chains)
- Hemoglobin releases protons when bound to oxygen
- Lower pH (higher H+) thus favors oxygen release (the Bohr effect); stabilizes the T state
- Interaction of histidine in hemoglobin; tight in R and separated in T
Gas Transport betweeb RBC and capillary at tissue and at lung
How does oxygen transport work?
O2 transport requires three major steps:
(1) O2-transport proteins must bind oxygen in the lungs.
(2) O2 must be carried to sites where it is needed.
(3) O2 must be released in the correct tissues.
How does O2 binding affect hemoglobin?
(1) O2 binding alters the structure of heme, which moves the histidine residue that helps to ligand the iron in heme
(2) These alterations lead to local changes in the structure of the hemoglobin subunit that destabilize contacts formed in the T state
Factors that affect gas transport by hemoglobin
- (1) Small molecules that alter O2 binding to modulate gas transport
- (a) 2,3-bisphosphoglycerate
- (b) H+
- (c) CO2
- (d) CO
- (2) Fetal hemoglobin
What does hemoglobin look like?
- Adult hemoglobin is comprised of 4 protein chains – 2 are called “alpha” chains and 2 are called “beta” chains.
- Each alpha and beta chain forms a “globin” fold with a heme prosthetic group bound.
- There are two major arrangements of these chains, one is more “open” and the other is more “closed”. These are referred to as the T and R states, respectively.
- The R state (closed state) binds O2 with high affinity whereas the T state (open state) binds O2 with low affinity
Bohr Effect
The Bohr Effect refers to the observation that increases in the carbon dioxide partial pressure of blood or decreases in blood pH result in a lower affinity of hemoglobin for oxygen. This manifests as a right-ward shift in the Oxygen-Hemoglobin Dissociation Curve described in Oxygen Transport and yields enhanced unloading of oxygen by hemoglobin.
CO2 effects on hemoglobin O2 binding
CO2 favors the T-state in two ways
(1) CO2 preferentially binds to the T-state of hemoglobin, favoring the T-state and weakening oxygen binding. This is referred to as the Haldane effect – helps to target oxygen delivery to sites in the body with high concentrations of carbon dioxide
(2) CO2 reacts with water in the red blood cell to form carbonic acid (H2CO3), which dissociates to form H+ and bicarbonate (HCO3 - ). The increase in H+ concentration lowers the pH in the red blood cell, which contributes to the Bohr effect
Effects of CO on gas transport
- Carbon monoxide (CO) is found in abundance in burned fuel exhaust and tobacco smoke.
- CO binds to the heme in hemoglobin, meaning that it competes with O2 for binding.
- Hemoglobin binds CO ~200-times tighter than it does O2 and it dissociates from hemoglobin very slowly. CO binding thus reduces the fraction of hemoglobin that is available for gas transport.
- This make CO a potentially fatal poison that disrupts proper gas transport
How does 2,3-bisphosphoglycerate alter the hemoglobin O2 binding curve?
- Right shift means that hemoglobin is able to release oxygen at higher partial pressures of oxygen
- High elevation adaptation – allows greater dumping of oxygen from hemoglobin to target tissues
