Hemoglobin structure and function Flashcards
Describe the overall structure of hemoglobin, indicating the site of oxygen binding. Explain the
concepts of allostery and positive cooperativity as they relate to hemoglobin function and explain
what is meant by taut (T) and relaxed (R) configurations.
Hb is a ~68 kD tetramer, comprised of 2 pairs of globin polypeptide chains: one pair of alpha chains and one pair of non-alpha globin chains. A heme prosthetic group, consisting of a protoporphyrin ring bound to iron, is associated with each glob chain. The heme group binds oxygen.
Hb molecule must be able to easily pick up available oxygen in an environment where oxygen concentration is relatively high and unload it easily under condition of lower oxygen availability—> accomplished via allosteric regulatiion of Hb.
When O2 conditions are very low and 0/4 heme sites are bound to O2, binding affinity to O2 is relatively low—> THIS IS THE TAUT (T) configuration .
As O2 becomes more available and one of the binding sites become oxxupied, the configuration of the molecule changes such that the other 3 sites have high binding affinity and can more easily bind to additional O2 molecules—> this is the relaxed conformation.
Draw a typical oxygen dissociation curve. Explain why it is sigmoidal in shape. Define the p50. Explain the effects of pH, [CO2], temperature, and [2,3-BPG].
Curve is sigmoidal become of positive cooperativity in binding O2 to hemoglobin.
P50 is the partial pressure of Oxygen at which the oxygen carrying protein is 50% saturated. P50 for hemoglobin is 27 mm Hg.
pH: O2 affinity of Hb increases over a pH range of 6-8.5, O2 is mroe tightly held by Hb in alkaline situations, and more easily released in more acidic environments.
Temerpature: Decreased temperature, increased Oxygen affinity. Increased temperature, decreased affinity for O2.
2,3-BPG- a byproduct of anaerobic glycolysis, when increased, Hb affinity for oxygen is decreased. 2,3-BPG alters oxygen affinity by binding to deoxyhemoglobin, stabilizing it in the T conformation.
Compare oxygen dissociation curves for myoglobin and hemoglobin and explain the reason for the differences.
Oxygen dissociation curve for hemoglobin is sigmoidal because of allosteric binding (its a tetramer), while its not for myoglobin because its a monomer and is not able to undergo allosteric regulation.
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 have 3 distinct hemoglobins that are present only between 4 and 14 weeks gestation: Gower 1 , Gower 2, and Portland. Each has a higher affinity for o2 than does HbA
Fetal hemoglobin (Hb F) predominates in the fetus (alpha 2, gamma 2).
At birth there is 65%-95% HbF and about 20% Hb A. The norma adult level of fetal Hb is approached by 1 yerar and achieved by 5 yrs.
Under normal conditions, adults have 96-97% HbA and HbF makes up less than 1% of total Hb and is unevenly distributed in red cells.
Describe how structural differences in hemoglobin affect oxygen affinity and explain the physiologic effects of altered oxygen affinity.
Variants altering Hb-oxygen affinity can lead to either higher or lower than normal O2 affinity. Variants producing a physically unstable molecules can lead to meolytic anemia, aka Heinz body anemia (Heinz bodies are denatured Hb. Hemoglobin S causes sick cell disease.
High affinity Hb: erythrocytosis (high RBC)—> reduced O2 delivery to tissues, leads to erythropoeitin release.
Low affinity Hb: Cyanosis, more oxygen delivered to tissues.
Unstable Hb may or not bind O2 differently than HbA. e.g. HB Zurich has a single point mutation that increases binding to carbon monoxide.
Describe what methemoglobinemia is, what causes it, how to diagnose it, and how to treat it.
Methemoglobin is Hb in the ferric (+3) form, (normally it needs to be in the +2 form.
Ferric iron is reduced to ferrous iron via the NADPH methemoglobin reducatase pathway.
Methemoglobin occurs b/c of too much methemoglobin production or because of decreased methemoglobin production. Can be genetic or acquired.
Diagnosis is made when a person looks cyanotic but the arterial partial pressure of oxygen is normal on arterial blood gas. Blood looks dark red, chocolate, or brown blue, and does not change with oxygen exposure.
Treatment depends on the cause. Congenital patients are treated with methylene blue or ascorbic acid. Acquired patients are treated by removing the inciting drug/chemical. Methylene blue is given intravenously and provides an artificial electron acceptor for the reduction of methemoglobin via NADPH pathway.
Explain the pathophysiology of carbon monoxide poisoning and its treatment.
When one heme binds CO, an allosteric change occurs so the other 3 hemes download Oxygen less well, increasing the Hb’s O2 affinity, and decreases delivery of oxygen to tissues.
Treatment is will 100% oxygen, which will compete with CO for the binding sites of the heme moiety. Hyperbaric oxygen can also be considered.
Explain in basic terms how a pulse oximeter works. Describe situations where a pulse oximeter reading may inaccurately reflect a patient’s true oxygenation status.
Pulse oximeter probe has two light emitting diodes, emitting light at the frequency for deoxyhemoglobin absorbs maximally (660 nm) and oxyhemoglobin absorbs maximally (940nm).
Pulse oximetry may be inaccurate if the probe isn’t placed correctly, if only one diode is working, if there is too much motion, nail polish is present, deeply pigmented skin, anemia, shock, or abnormal Hb.
CarboxyHb will given an abnormally high reading. Methemoglobin will give a false reading,.
