BIOCHEMISTRY - L. Stryer - 1984

VOLUME 1

PART I. CONFORMATION AND DYNAMICS

CHAPTER 3. OXYGEN CARRIERS—MYOGLOBIN AND HEMOGLOBIN

QUESTIONS AND PROBLEMS

1. The average volume of a red Blood Cell is 87 µm³. The concentration of Hemoglobin in red Blood Cells averages 34 g/100 mL.

a) What is the weight of hemoglobin contained in a single red blood cell?

b) How many hemoglobin molecules are contained in a single red blood cell?

c) Can the concentration of hemoglobin in red blood cells increase significantly? (Hint: Assume that hemoglobin inside THE RED BLOOD cell is packed as cubic crystals with an edge length of 65 Å.)

2. How much iron is contained in the hemoglobin of a 70-kg human? Assume a blood volume of 70 mL/kg and a blood hemoglobin concentration of 16 g/100 mL.

3. The Myoglobin content in certain human Muscles is 8 g/kg of Muscle. In the sperm whale, this value reaches 80 g/kg of muscle.

a) How much oxygen is bound by myoglobin in human muscle versus sperm whale muscle? Assume that myoglobin is saturated with O₂.

b) Tissue Water (in equilibrium with venous blood at t = 37 °C) dissolves approximately 3.5 • 10^-5 M O₂. What is The ratio of bound oxygen (to myoglobin) to freely dissolved oxygen in sperm whale muscle?

4. Sperm whale myoglobin has the following Amino Acid Composition:

a) Determine the net charge of ferromyoglobin at pH 2, 7, and 9.

b) Determine the isoelectric point of myoglobin. The isoelectric point is the pH value at which the molecule carries no net charge.

5. A myoglobin solution is treated with Cyanogen bromide.

a) What are the Cleavage products? Use the provided Amino Acid Sequence of the myoglobin molecule.

b) In aqueous solution, the cleavage products contain very few α-helical segments. What does this indicate about the stability of α-helices in an aqueous environment?

6. The binding of oxygen to myoglobin is described by a simple equilibrium expression:

Mb + O₂ ⇄ MbO₂.

Let [Mb] be the concentration of deoxymyoglobin; [MbO₂] be the concentration of oxymyoglobin; pO₂ be the concentration of O₂ (expressed as the partial pressure of O₂); and P₅₀ be the pO₂ at which [Mb] = [MbO₂]. Derive an equation analogous to the Henderson-Hasselbalch equation, expressing the ratio of oxymyoglobin to deoxymyoglobin as a function of pO₂ and P₅₀.

7. The Equilibrium Constant K for oxygen binding by myoglobin is 10^-6 M, where K = [Mb][O₂]/[MbO₂]. The rate constant for the binding of O₂ to myoglobin is 2 • 10⁷ M^-1 • s^-1.

a) What is the rate constant for the dissociation of O₂ from oxymyoglobin?

b) What is the average lifetime of the oxymyoglobin complex?

8. Light absorption A of a solution is defined as

A = lg(/₀//),

where /₀ is the incident light intensity (at the entrance), and /-the intensity of light passing through the solution (at the exit). The light absorption of a solution depends on the molar absorption coefficient (extinction coefficient) e (dimension cm^-1 • M^-1), concentration c (in M), and light path length I (in cm):

А = elc.

The extinction coefficient of myoglobin at 580 nm is 15000 cm^-1 M^-1. What is the light absorption of a 1 mg/ml myoglobin solution at a light path length of 1 cm? What percentage of the incident light passes through the solution?