Which of the following has highest affinity for Hb?
Ans. a (CO). (Ref. Ganong, Physiology, 21st/pg.537,694: Harrison's Medicine 18th/Ch. 104).The two most important acquired hemoglobinopathies are carbon monoxide poisoning and methemoglobinemia.CO has a higher affinity for hemoglobin than does oxygen; it can replace oxygen and diminish O2 delivery. Chronic elevation of carboxyhemoglobin levels to 10 or 15%, as occurs in smokers, can lead to secondary polycythemia.Carboxyhemoglobin is cherry red in color and masks the development of cyanosis usually associated with poor O2 delivery to tissues.HEMOGLOBIN# Hemoglobin is composed of four polypeptide subunits (2 alpha, 2 beta) and exists in two forms--T (taut) form has low affinity for oxygen and R (released) form has high affinity for oxygen.# Hemoglobin exhibits positive cooperativity and negative allostery (accounts for the sigmoid-shaped 02 dissociation curve for hemoglobin), unlike myoglobin.# Carbon monoxide has a 257 times greater affinity for hemoglobin than oxygen.# Oxygen dissociation curve of Hb is sigmoid in shape indicating that subunits co-operate in binding oxygen, which results from the fact that binding of oxygen molecule at one heme increases the oxygen affinity or remaining heme groups in the same Hb molecule. The net effects that the affinity of Hb for last oxygen bound is approximately 300-fold greater than affinity of first oxygen bound.# 2-3 BPG is essential for normal transport function of Hb, e.g., storing blood in acid citrate dextrose (formerly used medium) leads to decreased 2, 3 BPG in RBCs and such blood displays an abnormally high oxygen affinity, acting virtually as an oxygen "trap" rather than oxygen transport system, failing to unload properly its bound oxygen in tissues. However, decrease in 2-3 BPG can be prevented by adding Inosine to storage medium as it can enter RBC where it is converted to 2, 3 BPG.HbAalpha and beta chains (synthesis starts at about eighth month of pregnancy)HbA2alpha and delta chains (first appear about 12 weeks after birth)HbFalpha and gamma chains.
CO
O2
CO2
NO2
Respiratory System