PROTEIN AS A TRANSPORT TOOL IN LIFE

Transport Proteins

Carrier Molecules

The next broad category of proteins we will consider are the carrier molecules or transport proteins. These transport proteins are often globular proteins. They are generally tightly packed with polar side groups on the outside to enhance their solubility in water. They typically have nonpolar side groups folded to the inside to keep water from getting in and unfolding them.
Serum albumin is one example. It transports water-insoluble lipids in the bloodstream.

Hemoglobin

Hemoglobin is another example. It carries oxygen from the lungs to the tissue. Myoglobin performs a similar function in muscle tissue, taking oxygen from the hemoglobin in the blood and storing it or carrying it around until needed by the muscle cells.
Hemoglobin and myoglobin also have similar structures. Myoglobin contains 151 amino acid residues plus a heme group to bond to oxygen. Hemoglobin has four similar chains, two with 141 residues and a heme group and two with 146 residues and a heme group. The molecular weight of hemoglobin is about 64,500 and can carry four oxygen molecules.
It is important that hemoglobin can bond to oxygen under certain conditions. But it is equally important that hemoglobin can release oxygen under other conditions. The ability of hemoglobin to bind oxygen is sensitive to several factors. They include pH, temperature, concentrations of O₂ and CO₂, and even the number of oxygen molecules already bound. It seems that when oxygen binds to hemoglobin, the structure of the hemoglobin changes slightly in a way that makes it better at binding to more oxygen, thus enhancing its ability to carry more oxygen.

Oxygen Binding Curve

This graph relates the ability of hemoglobin to bind oxygen to the concentration or the partial pressure of oxygen. (It is also found in Example 9 in your workbook where you will be able to read the details of the labeling on the axes.) The vertical axis shows the fraction or percentage of hemoglobin molecules that are saturated with oxygen. The horizontal axis shows the partial pressure of oxygen gas, a measure of how much oxygen is available in the air.
The partial pressure as used here is not a direct measure of the concentration of the oxygen in the blood. Instead, it refers to the aqueous concentration of oxygen that would be in equilibrium with gaseous oxygen having the stated partial pressure. If that makes no sense to you, don't worry about it. But if you wondered how a solution can have a partial pressure, it doesn't, and that is the explanation.
When the partial pressure of oxygen is high, virtually all of the hemoglobin molecules have oxygen molecules bound to them. The partial pressure of oxygen in the lungs is about 100 mm Hg (shown by the pencil), which is in the region just to the right of the steep portion of the curve.
When the partial pressure of oxygen is very low, virtually none of the hemoglobin molecules have oxygen molecules attached.
In the region near the steep part of the curve (at about 40 mm Hg) a very small change in the partial pressure of oxygen will cause a very large change in the fraction of hemoglobin molecules that bind oxygen. Compare the values on one side of the pencil to the values on the other.

The partial pressure of the oxygen in the body tissues is about 40 mm Hg or less, which is partway down the steep part of the graph. Thus, in the lungs, virtually all of the hemoglobin bonds to oxygen and the blood becomes rich in oxygen, turning a characteristic red color. When the blood reaches active body tissue, the hemoglobin releases a fair amount of its oxygen because of the low partial pressure of oxygen in the tissue and it turns the blue color characteristic of venous blood. Hemoglobin generally retains about half to three-quarters of its oxygen in venous blood, rather than giving it all to the cells under normal conditions. The value of this is that some reserve oxygen is available from the hemoglobin when strenuous exercise depletes the cellular oxygen to even lower partial pressures.
Hemoglobin can be used as an example to point out how important the conformation of a protein is. The shape and the functional groups must be such that they will attract oxygen molecules, but not nitrogen molecules (which are four times more abundant in air than are oxygen molecules), and not water molecules, and not sugar molecules, and so on. Ironically, another molecule, carbon monoxide, will bind to hemoglobin 200 times more readily than oxygen. That makes carbon monoxide very dangerous. Not only does the hemoglobin that has bonded to carbon monoxide not have oxygen to give to the cells, it cannot easily get rid of the carbon monoxide to be able to get some oxygen.

Sickle Cell Hemoglobin

The disease sickle cell anemia points out another important aspect of protein structure. As you know, the primary structure of a protein determines the secondary structure which determines the tertiary structure and the quaternary structure, which in turn determines the function of the protein.
In sickle cell hemoglobin the sixth amino acid residue is valine instead of glutamic acid.
That's it. That's the difference.
The consequence is that when the hemoglobin releases its oxygen, it reacts with other such proteins in a way that causes the shape of red blood cells to change. The red blood cells change to a sickled shape which does not readily pass through capillaries and thus causes a number of problems.

Linus Pauling, who helped uncover the alpha-helix primary structure of proteins, refers to diseases such as this as "molecular diseases." The change of a single amino acid in a protein, by the way, does not always have such a drastic effect on the function of the protein.

Cytochromes

Another quite different group of carrier molecules is the group known as the cytochromes. These are the electron carrier proteins that operate in the electron transport chain which is part of the respiration process. They carry electrons from the hydrogen atoms freed in the citric acid cycle to waiting oxygen molecules. At the end of that process, the hydrogen and oxygen combine to form H₂O. The energy released in this series of reactions is stored by using it to convert ADP to ATP.

"Inhibition" of Carrier Proteins

Carrier proteins can be affected by what can be called competitive inhibition.
For example, hemoglobin is a carrier protein that transports oxygen from the lungs to muscle tissue and other cells. However, carbon monoxide molecules compete with oxygen for the binding sites on the hemoglobin molecule. If they are present in high enough concentration, they prevent sufficient oxygen from getting to the tissues and the organism dies.
Cyanide is another poison that affects respiration. It acts by inhibiting the cytochrome proteins that are an integral part of the electron transport system in respiration.