Question

1 a. Mutations can change hemoglobins O2 binding properties and cause disease. Explain why mutations can increase or decrese the oxygen affinity and cooperativity of hemoglobin. How can the body compensate for these changes?

b. Explain why O2 binding behavior of myoglobin and hemoglobin can be summed up by a single number, (the p50)?

Answer 1

a) Hemoglobin is formed by genes that are in charge of the expression of the hemoglobin protein. Failings in these genes can form irregular hemoglobin and anemia, which are conditions termed "hemoglobin disorder". Irregular hemoglobin appears in these three conditions.

1. Structural failure in the hemoglobin molecule. Changes in the gene for one of the two hemoglobin subunit chains, alpha (α) or beta (β), are called mutations. Often, mutations change a single amino acid building block in the subunit. Most commonly the change is innocuous, perturbing neither the structure nor function of the hemoglobin molecule. Occasionally, alteration of a single amino acid dramatically disturbs the behavior of the hemoglobin molecule and produces a disease state. Sickle hemoglobin represents this phenomenon.

2. Reduced production of one of the two sub-units of the hemoglobin molecule. Mutations that form this condition are termed "thalassemias." Equal numbers of hemoglobin alpha and beta chains are essential for normal function. Hemoglobin chain inequity damages and destroys red cells thereby producing anemia. Although there is a death of the affected hemoglobin subunit, with most thalassemias the few subunits created are structurally normal.

3. Irregular relations of otherwise normal sub-units. A single sub-unit of the alpha chain and a single subunit from the β-globin locus combine to create a normal hemoglobin dimer. With severe α-thalassemia, the β-globin subunits start to associate into groups of tetramers due to the scarcity of potential α-chain partners. These tetramers of β-globin subunits are functionally inactive and do not carry oxygen. No similar tetramers of alpha globin subunits form with severe beta-thalassemia. Alpha subunits are quickly destroyed in the absence of a partner from the beta-globin gene cluster.

b) The differences between hemoglobin and myoglobin are most important at the level of quaternary structure. Hemoglobin is a tetramer composed of two each of two types of closely related subunits, alpha and beta. Myoglobin is a monomer (so it doesn't have a quaternary structure at all). Myoglobin binds oxygen more tightly than does hemoglobin. This difference in binding energy reflects the movement of oxygen from the bloodstream to the cells, from hemoglobin to myoglobin.

Each myoglobin molecule is capable of binding one oxygen, becausemyoglobin contains one heme per molecule. Even though the reaction of myoglobin and oxygen takes place in solution, it is convenient to measure the concentration of oxygen in terms of its partial pressure, the amount of gas in the atmosphere that is in equilibrium with the oxygen in solution.

The titration curve of myoglobin with oxygen is a hyperbola, as shown in Figure of the form:

where Y is the fraction of oxygenated myoglobin, pO ₂ is the partial pressure of O ₂, expressed in torr (mm Hg; 760 torr = 1 atmosphere) and P ₀ is the partial pressure of O ₂ required to bind 50% of the myoglobin molecules. The derivation of this equation from the equilibrium constant for binding is not reproduced here but may be found in many standard chemistry and biochemistry textbooks. In the above equation, if Y is set at 0.5, P ₅₀ = pO ₂.

The P ₅₀ of hemoglobin is greater than the P ₅₀ for myoglobin, hemoglobin in the blood gives up O ₂ to myoglobin in the tissues. This follows from the principle discussed above, that a greater P ₅₀ means lower affinity, and weaker binding means greater release.