Hemoglobin structure and function concepts

1. If you simply copy and paste text from the book, power points or from internet sources, you get ZERO points.
2. you can work with your classmates, however, if your answers are 50% similar to any of your classmates, you will get 50% of the total points.

Below are 2 cases (8 and 9) from your book.

Use only the space provided, write short and concise that contain key words to demonstrate your knowledge.

Case 8

Hemoglobin, the Oxygen Carrier

Focus concept

A mutation in the gene for hemoglobin results in an altered protein responsible for the disease sickle cell anemia. An understanding of the biochemistry of the disease may suggest possible treatments.

Prerequisite

Hemoglobin structure and function concepts.

Background

Normal adult hemoglobin is called Hemoglobin A (Hb A). Ninety-eight percent of adult hemoglobin is Hb A and 2% is Hb A2. There are other forms of hemoglobin. For example, the developing fetus has a different kind of hemoglobin than most normal adults. Fetal hemoglobin (or Hemoglobin F) consists of two α chains and two γ chains, whereas adult hemoglobin (Hemoglobin A) consists of two α chains and two β chains. Fetal hemoglobin is synthesized beginning at the third month of gestation and continues up through birth. After the neonate is born, hemoglobin F synthesis declines (because synthesis of the γ chain declines) and hemoglobin A is synthesized (because synthesis of β chains begins). By the time the baby is six months old, 98% of its hemoglobin is Hemoglobin A.

There is also a mutant form of hemoglobin called Hemoglobin S which is found in persons with the disease sickle cell anemia. The disease sickle cell anemia is one of the major health problems facing the African-American community. The World Health Organization estimates that 250,000 babies worldwide are born with sickle cell anemia. Currently there is no cure. A person afflicted with sickle cell anemia has inherited a defective gene from each parent. (Parents who are carriers of the sickle cell gene are heterozygous AS, whereas the person afflicted with sickle cell anemia is SS; non-carriers are designated AA.) The defective gene is the one coding for the β-chain. The amino acid at position 6 on each β chain has been mutated from a glutamate to a valine. Normal α chains have a decreased affinity for the mutated β chains; thus assembly of the HbS tetramer is more difficult. Red blood cells containing HbS form a sickle shape because the Hb S molecules polymerize. Hb S molecules are more likely to polymerize when in the deoxygenated T form than in the oxygenated R form. The polymerized Hb deforms the normal discoid shape of the red blood cells, producing a sickle-shaped cell. The sickle shaped red blood cells become trapped in capillaries and organs, depriving the victim of adequate oxygen supply and causing chronic pain and organ damage.

In this case we will consider our patient, a 10-year-old black male child named Michael B., who was admitted to the hospital because he was experiencing severe chest pain. He had been hospitalized on several previous occasions for vaso-occlusive episodes that caused him to experience severe pain that could not be managed with non-prescription drugs such as ibuprofen. He was slightly jaundiced, short of breath and easily tired, and feverish. A chest x-ray was taken and was abnormal. An arterial blood sample showed a pO2 value of 6 kPa (normal is 10-13 kPa).

Questions (use only the space provided)

2. You suspect that Michael has sickle cell anemia and you have ordered an isoelectric focusing analysis of the child‟s lysed red blood cells. (Lysing the red blood cells releases the hemoglobin.) Draw a diagram of the predicted results. Why will this test allow you to diagnose this child‟s disease?
3. Why do you think that Hb S molecules would be likely to clump together whereas Hb A molecules do not?

3. In the emergency room, oxygen (100%) was administered to the patient. (Inspired air normally is about 20% oxygen.) Why was this an effective treatment?

Case 9

Allosteric Interactions in Crocodile Hemoglobin

Focus concept

The effect of allosteric modulators on oxygen affinity for crocodile hemoglobin is unique when compared with other species.

Prerequisite

Hemoglobin structure and function concepts.

Background

While most human beings are able to hold their breath for only a minute or two, other species are able to stay under water for much longer periods of time. In this case study we will examine the physiological adaptations that allow some organisms to deliver oxygen to tissues while submerged under water.

Deep sea-diving mammals, such as whales and seals, are able to stay under water for long periods of time. These mammals are able to stay submerged because their muscles contain many-fold higher concentrations of myoglobin (Mb) than humans.

Crocodiles are also able to stay submerged under water for periods of time exceeding one hour. This adaptation allows the crocodile to kill small mammals by drowning them. However, the crocodile doesn’t have large amounts of myoglobin in its muscle as the deep sea-diving mammals do, so their physiological adaptation must be different. In 1995, Nagai and colleagues described in the British journal Nature a possible mechanism that allowed the crocodile hemoglobin to deliver a large fraction of bound oxygen to the tissues. They suggested that bicarbonate, HCO3 – , binds to hemoglobin to promote the dissociation of oxygen in a manner similar to 2,3-bisphosphoglycerate (BPG) in humans.

This case is important because information gathered from experiments like those described here will allow scientists to design effective blood replacements.

Questions

1. In humans, oxygen is effectively delivered to the tissues because of the presence of several allosteric modulators. Name three of these modulators and explain how their presence allows oxygen to be delivered to the tissues.

2. Explain why having higher concentrations of Mb would allow whales and seals to stay submerged under water for a long period of time.

3. Let us consider the hypothesis that bicarbonate serves as an allosteric modulator of hemoglobin binding in crocodiles. What is the source of HCO3 – in tissues? Describe the changes occurring in the deeper tissue.

4. Draw oxygen-binding curves for crocodile hemoglobin in the presence and absence of bicarbonate. Which conditions give rise to a greater p50 value for crocodile hemoglobin? What does this tell you about the oxygen binding affinity for hemoglobin under those conditions?

5. Komiyama et al. investigated the bicarbonate binding site on the crocodile hemoglobin by constructing human-crocodile chimeric hemoglobins in which amino acids in the human hemoglobin were replaced with amino acids found in the crocodile hemoglobin at the same location. (The investigators wanted to see if they could make a synthetic human hemoglobin that resembled the crocodile hemoglobin in terms of its ability to bind bicarbonate anions.) They found the bicarbonate binding site to be located at the a1b2-subunit interface, where the two subunits slide with respect to one another during R ¾ T transitions. Based on their results the authors modeled a stereochemically plausible binding site that included the phenolate anion of Tyr 41b, the e-amino group of Lys 38β, and the phenolate anion of Tyr 42b.

What kinds of interactions do you think the aforementioned amino acid side chains will have with the bicarbonate anion? (It might be helpful to draw the Lewis electron dot structure of bicarbonate).