Hemoglobin: structure and function Flashcards
Describe the overall structure of hemoglobin, indicating the site of oxygen binding.
68kD tetramer, with 2 pairs of globin polypeptide chains (1 pair of alpha-globin and 1 pair of non-alpha-globin). A heme prosthetic groups, consisting of a protoporphyrin ring bound to iron, is associated with each globin chain of the hemoglobin tetramer. It is the heme group that binds oxygen.
Explain the concept of allostery as it relates to hemoglobin function
when oxygen (substate) binds to hemoglobin at one site, hemoglobin has a change in configuration, which alters its binding affinity of additional oxygen molecules at another site. This enables hemoglobin to easily pick up oxygen in the lungs (high O2 levels) and unload it in the tissues (low O2 levels). If the allosteric changes lead to increased affinity for substrate at the other binding sites, it is termed positive cooperativity.
Explain the concept of positive cooperativity as it relates to hemoglobin function
the phenomenon of binding to substrate (oxygen) leading to increased affinity for additional substrate (more oxygen!) is called positive cooperativity. When 1 oxygen binds, the configuration of hemoglobin changes so that the other 3 sites have a higher binding affinity. As the number of occupied sites increases, the affinity for the remaining sites continues to increase.
explain what is meant by taut (T) and relaxed (R) configurations.
a. Taut (T): Deoxygenated, under conditions where the oxygen concentration is low enough that none of the 4 binding sites are occupied, the binding affinity to oxygen is relatively low, T-configuration is present due to inter-and intra-salt bonds, hydrogen bonding, and hydrophobic interactions within the molecule.
b. Relaxed (R): Oxygenated, as oxygen becomes more available, 1 oxygen binds, the configuration changes and the other sites have higher binding affinity for oxygen. The sequential breaking of salt bonds leads to the R-configuration.
- Draw a typical oxygen dissociation curve. Explain why it is sigmoidal in shape.
The oxygen dissociation curve is sigmoidal in shape because of cooperativity: binding of substrate leads to increased affinity for additional substate. Allows oxygen to easily bind at higher pO2 level in lungs and unload at lower pO2 level in tissues
Define the p50.
the partial pressure of oxygen at which the oxygen carrying protein (hemoglobin or myoglobin, usually) is 50% saturated. Normal conditions: p50 for hemoglobin is 27 mmHg and p50 for myoglobin is 2.75 mmHg
Explain the effects of pH on the oxygen dissociation curve
low pH (more acidic)= decreased oxygen affinity, oxygen is unloaded, curve shifts to the right. Higher pH (more alkaline)= increased oxygen affinity, oxygen is held more tightly, curve shifts to the left. Known as the BOHR EFFECT
Explain the effects of CO2 concentration on the oxygen dissociation curve
when CO2 is released into the bloodstream, the carbonic anhydrase converst CO2+H20 –> carbonic acid –> bicarbonate and H+ (drops the pH). This continues the Bohr Effect. Tissues with higher metabolic rate will release more CO2 and lactic acid, leading to a drop in pH, which then shifts the curve to the right, allowing greater release of oxygen to the tissues. Efflux of CO2: pH of blood rises, increased oxygen affinity, easier loading of oxygen to the hemoglobin
Explain the effects of temperature on the oxygen dissociation curve
Higher temperatures=more oxygen is unloaded to tissues and less is bound by hemoglobin (think: when you exercise your temperature rises because metabolic rates are higher, therefore increased need for oxygen). Curve shifts to the right
Explain the effects of [2,3-BPG] on the oxygen dissociation curve
: by product of the anaerobic glycolytic pathway (normal level=5 mmol/L). When level is much higher (in states of increased O2 use, chronic HPX, chronic anemia), oxygen affinity of hemoglobin decreases and shifts the curve to the right, increasing delivery of O2 to the tissues. 2,3-BPG alters O2 affinity by binding to deoxyhemoglobin and stabilizing it in the T-configuration, leading to decreased affinity of the hemoglobin for oxygen.
Compare oxygen dissociation curves for myoglobin and hemoglobin and explain the reason for the differences.
Hemoglobin is a tetramer, myoglobin is a monomer and therefore cannot undergo allosteric regulation or cooperativity. Myoglobin curve is shaped like a hyperbola, very high O2 affinity at very low O2 concentrations. Myoglobin would be a poor protein to use for O2 transport from lungs to tissues because it holds very tightly to O2 and does not release until O2 levels are extremely low. But, myoglobin is good to use for oxygen storage intracellularly where O2 levels are very low and where high oxygen affinity is needed to transfer the O2 from hemoglobin to myoglobin
List and describe the typical hemoglobin variants seen during fetal development and in adulthood and explain how amounts of these different hemoglobins change during development.
- Embryos (at 4-14 weeks) have 3 distinct hemoglobins with higher affinity for O2 than hemoglobin A (allows for O2 transfer from mom to baby) (Hemoglobin Gower I, Gower II and Portland).
- At 8 weeks, fetal hemoglobin predominates (HbF). HbF binds 2,3-BPG poorly, thereby stabilizing the Relaxed hemoglobin state and shifting the oxygen dissociation curve to the left. The Bohr effect also increases by 20% in HbF so that as fetal blood passes through the intravillous spaces of placenta, H+ ions are transferred to the maternal circulation and the pH rises, leading to increased O2 affinity and a further shift of the curve to the left.
- At birth: 65-95% HbF and 20% HbA
- Adults (actually around age 5, although HbF remains high in premature babies and infants of mothers with diabetes): 96-97% HbA, 2% HbA2, ˂1% HbF
- HbA2 (α2/δ2): functions much like HbA, has same Bohr effect, cooperatitivity and response of 2,3-BPG but is more heat stable and has slightly higher O2 affinity. Can be used diagnostically for Beta-thalassesmias, sickle cell trait, hyperthyroidism and megaloblastic anemias.
Describe how structural differences in hemoglobin affect oxygen affinity and explain the physiologic effects of altered oxygen affinity.
After week 8 of gestation, fetal hemoglobin or hemoglobin F (α2γ2) predominates. The γ- chain differs from the β-globin chain by 39 amino acids. Fetal red cells have a higher oxygen affinity than adult red cells, primarily because hemoglobin F binds 2,3-BPG poorly, stabilizing the hemoglobin in the R state and shifting the oxygen dissociation curve to the left. The Bohr effect is also increased by 20% in fetal hemoglobin, so that as fetal blood passes through the intravillous spaces of the placenta, H+ ions are transferred to the maternal circulation and the pH rises, leading to increased oxygen affinity and a further shift of the curve to the left. These changes favor transfer of oxygen from the maternal circulation to the fetal circulation
