Passage:Hemoglobin

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Hemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of vertebrates. In mammals, the protein makes up about 97% of the red blood cell’s dry content. Hemoglobin transports oxygen from the lungs or gills to the rest of the body, such as to the muscles, where it releases the oxygen for cell use.


Hemoglobin also has a variety of other roles in gas transport and effect-modulation which vary from species to species, and are quite diverse in some invertebrates. Hemoglobin can bind protons and carbon dioxide which causes a conformational change in the protein and facilitates the release of oxygen. Protons bind at various places along the protein, and carbon dioxide binds at the alpha-amino group forming carbamate.



When oxygen binds to the iron center, it causes contraction of the iron atom and movement back into the center of the porphyrin ring plane. This causes a tug on the peptide strand which tends to open up heme units in the remainder of the molecule, so that there is more room for oxygen molecules to bind at their heme sites. In the tetrameric form of normal adult hemoglobin, the binding of oxygen is thus a cooperative process. As a consequence, the oxygen binding curve of hemoglobin is sigmoidal, or S-shaped, as opposed to the normal hyperbolic curve associated with noncooperative binding. Hemoglobin binds with carbon monoxide 240 times more readily than with oxygen. The presence of carbon monoxide on one of the four heme sites causes the oxygen on the other heme sites to bind with greater affinity. This makes it difficult for hemoglobin to release oxygen to the tissues and has the effect of sequestering hemoglobin away from proper use. With an increased level of carbon monoxide, a person can suffer from severe hypoxemia while still maintaining a normal PO2.


In people acclimated to high altitudes, the concentration of 2,3-Bisphosphoglycerate (2,3-BPG) in the blood is increased, which allows these individuals to deliver a larger amount of oxygen to tissues under conditions of lower oxygen tension. This phenomenon, where molecule Y affects the binding of molecule X to a transport molecule Z, is called a heterotropic allosteric effect.


A decrease of hemoglobin, with or without an absolute decrease of red blood cells, leads to symptoms of anemia. Anemia has many different causes, although iron deficiency and its resultant iron deficiency anemia are the most common causes in the Western world. As absence of iron decreases heme synthesis, red blood cells in iron deficiency anemia are hypochromic (lacking the red hemoglobin pigment) and microcytic (smaller than normal). Other anemias are rarer. Some mutations in the globin chain are associated with the hemoglobinopathies, such as sickle-cell disease and thalassemia. Other benign mutations are referred to merely as hemoglobin variants. One group of genetic disorders, known as the porphyrias, are characterized not by mutations in the hemoglobin proteins but by errors in metabolic pathways of heme synthesis. King George III was probably the most famous sufferer of a porphyria condition, specifically, an autosomal dominant condition known as varietage prophyria.


1. Blood temperature in the circulatory system _______

remains constant in both the peripheral regions and internal regions of the body.
remains constant in the peripheral regions of the body but varies in the internal regions.
varies in the peripheral regions of the body but remains constant in the internal regions.
varies in both the peripheral regions and internal regions of the body.
→ Homeostasis insures that the body keeps a regular temperature regardless of the outside temperature, however blood that travels to and from the peripheral regions (hands, feet, etc) will fluctuate due to cooling/heating from the external environment.

2. Which of the following will shift the oxygen dissociation curve to the right?
          i) A decrease in blood pH
          ii) High levels of 2,3-BPG
          iii) High levels of carbon monoxide

i and ii only
ii and iii only
i and iii only
i, ii, and iii
→ A decrease in pH shifts the standard curve to the right, while an increase shifts it to the left. This is known as the Bohr effect and is a result of hemoglobin's structure to improve release of oxygen in acidic regions which are more likely to need it (muscles). High levels of 2,3-BPG imply a high altitude adaptation which allows an individual to better release oxygen in their bodies during chronic oxygen deprived environments. Thus, 2,3-BPG results in a shift to the right as it allows oxygen to be released more easily (just as acidic conditions do). Carbon monoxide makes it difficult for hemoglobin to release oxygen thus it has the effect of shifting the curve to the left.

3. Which of the following circulatory regions contains the least oxygenated hemoglobin?

aorta
→ The aorta is the first artery that leaves the heart with oxygenated blood. Blood then branches through all the arteries and arterioles to the tissues to provide oxygen rich blood.
pulmonary vein
→ The pulmonary vein is the only vein in the body which contains oxygenated blood. In general veins, which always flow back to the heart, have deoxygenated blood, however this vein comes from the lungs, thus bringing oxygenated blood from the lungs to the heart.
pulmonary artery
→ The pulmonary artery brings deoxygenated blood from the heart to the lungs so that they can become reoxygenated. This is the only artery in the body which pumps deoxygenated blood.
inferior vena cava
→ The inferior vena cava returns blood back to the heart from the lower regions of the body. Along with the superior vena cava, which returns blood from the upper regions of the body, both veins retain very little oxygen, however once in the heart their contents are pumped to the pulmonary artery and immediately to the lungs.

4. By which transport mechanism does oxygen in the alveoli enter red blood cells for hemoglobin transport?

active transport
secondary transport
osmosis
diffusion
→ The aveoli of the lungs are designed to be extremely thin to allow oxygen to diffuse more readily into the adjacent red blood cells in the capillaries. Of the processes listed, one can also recognize that active transport or secondary transport would require far too much energy for the body to ever maintain and osmosis is the movement of water, not oxygen. Diffuse is the natural flow of a molecule from high concentration to low concentration, in this case, oxygen, rich in the lungs, diffuses into the capillaries, which contain red blood cells depleted in oxygen.

5. Biliverdin, a component of bile, is produced from the natural breakdown of old erythrocytes. Which organ is primarily responsible for this breakdown?

The liver
The spleen
The pancreas
The kidneys
→ The spleen is the primary location of old erythrocyte breakdown. The product of heme destruction, biliverdin, is then expelled in bile but is in no way produced by either the liver or pancreas (the other major players in bile production). The kidneys are used for filtering blood, however in healthy individuals no blood is destroyed or broken down in the filtration process.

6. Carbon monoxide can best be considered a/an _____ of hemoglobin.

uncompetitive inhibitor
→ Uncompetitive inhibitors bind only to the enzyme-substrate complex (in this case the hamoglobin-oxygen complex) and thus do not compete with oxygen for binding. They simply reduce the enzymes effectiveness.
noncompetitive inhibitor
→ Noncompetitive inhibitors bind to a different location on the enzyme to decrease enzyme effectiveness.
competitive inhibitor
→ Competitive inhibitors directly compete with the ligand/substrate (in this case oxygen) for the enzyme (hemoglobin).
irreversible inhibitor
→ Irreversible inhibitors generally covalently bind to the enzyme thus rendering it inactive. Carbon monoxide is not an irreversible inhibitor as it simply positions itself in the same location the oxygen molecule would. People who suffer from carbon monoxide poisoning are generally given pure oxygen for a short period of time to allow the oxygen the chance to out compete the carbon monoxide.

7. If a heterozygous varietage prophyria male sufferer has a son with a nonsymptomatic female, what is the probability that the son does not have the condition?

0.125
0.25
0.375
0.5
→ From the passage, varietage prophyria is said to be autosomal and dominant. Thus it is not passed in either the X or Y chromosome but having one copy of the defective gene is all it takes to show signs. The fact that the person has a son is meaningless since it is an autosomal condition and sex is not involved. Now let us consider what the outcome of the infected father, who is heterzygous, and thus Zz crosses with a nonsymptomatic mother, and thus zz (because it is dominant she can not have the gene at all). This cross would lead to four different possible outcomes, Zz, Zz, zz, and zz, so the probability that a child gets the disease is 50% (and 50% for NOT getting it).

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