Protein Function in Humans

Proteins are vital components of our bodies. They form part of the body's structure and perform many essential functions, such as allowing us to move, distributing oxygen around the body, clotting blood when we are wounded, fighting infections, transporting substances into cells, carrying out chemical reactions and carrying messages from one part of the body to another.

Red blood cells contain a protein called hemoglobin, which gives the cells their color. Hemoglobin picks up oxygen from the lungs. As the red blood cells travel around the body, the hemoglobin releases the oxygen to the tissue cells. Cells need this oxygen to make energy from the digested food and to produce substances that they need.

The liquid part of blood, called blood plasma, contains a protein called fibrinogen, as well as other dissolved proteins, which are all involved in the blood clotting process. When a blood vessel is broken a series of chemical reactions occur which convert fibrinogen into a solid protein called fibrin. The fibrin fibers form a mesh over the wounded area which traps escaping blood. This is the blood clot.

Another dissolved protein in blood, called albumin, helps to keep water in the blood and maintain the correct blood volume. Albumin also transports bilirubin to the liver. Bilirubin is a waste substance made from the breakdown of hemoglobin in old red blood cells. The liver converts the bilirubin into a form that can be excreted.

Blood also contains antibodies, which are proteins made by a type of white blood cell called a B lymphocyte, or a B cell. The antibodies help fight invaders such as bacteria and viruses. In addition, certain blood proteins, along with certain proteins attached to the cell membrane, form a system known as the complement system. The complement system has a number of functions in the immune system. For example, activated complement molecules cause the attraction of phagocytes – white blood cells that engulf and destroy invaders – when an infection is present. Complement molecules also cause lysis (bursting) of bacteria.

Actin and myosin are proteins which exist as filaments in muscles. When calcium ions are present, the filaments slide over each other, causing the muscle to contract. Myoglobin is a red pigment in muscles that binds to oxygen. It releases the oxygen to the muscle cells when they need to produce energy. Ferritin is a protein in cells that stores iron and releases it when it’s needed. Ferritin is found in the skeletal muscles and also in the liver, spleen, bone marrow and other areas of the body. A small amount of ferritin is present in blood.

The outer layer of cells, called the cell membrane, contains embedded proteins. Some of these proteins form channels that allow substances to move through the membrane; others carry substances through the cell membrane. Some membrane proteins act as enzymes and cause chemical reactions to take place. Other proteins are receptors, which join to specific substances at the surface of the cell. An example of the importance of this process is the joining of insulin, a protein hormone made by the pancreas, to a receptor protein. This union causes the membrane to become more permeable to glucose, so that sufficient glucose can enter the cell. Another example is the binding of neurotransmitters released by the end of a stimulated nerve cell to protein receptors on the next nerve cell. This binding causing a nerve impulse to be produced in the second nerve cell, and is the method by which nerve impulses travel from nerve to nerve.

Glycoproteins - proteins attached to carbohydrates - are found on the surface of cell membranes and have many functions, including acting as cell adhesion molecules to “stick” cells together.

Cytokines are small proteins released by cells to communicate with other cells. They are often made by cells in the immune system when an infection is present. The cytokines stimulate the immune system to produce T cells, also called T lymphocytes, which fight the infection.

Some hormones are protein molecules. For example, erythropoietin is a protein hormone made by the kidneys to stimulate red blood cell production in the bone marrow. HCG (Human Chorionic Gonadotropin) is a protein hormone that is produced by the embryo and by the placenta during early pregnancy. Its function is to maintain the correct levels of estrogen and progesterone in a woman’s body to support the continued pregnancy. Pregnancy tests check for HCG in a woman’s urine or blood.

A cell contains a network of protein filaments and tubules called the cytoskeleton. The cytoskeleton maintains the cell’s shape and allows its parts to move. Some cells have short hair-like extensions on their surface, called cilia. Other cells have one or more long extensions called flagella. Cilia and flagella are made of protein microtubules. They are used to move the cell or to move fluids surrounding the cell.

Keratin is a structural protein found in our skin, hair and nails. Collagen protein fibers are located in many parts of the body, including the muscles, tendons, ligaments and bones. Collagen and another protein, called elastin, are often found together. Collagen fibers provide strength and elastin fibers provide flexibility. Collagen and elastin are found in the lungs, in the walls of blood vessels and in the skin.

Enzymes are chemicals that catalyze (speed up) the chemical reactions in the body, Without enzymes, the reactions would happen too slowly or would not happen at all. Since a huge number of chemical reactions are happening all the time in our bodies, life would be impossible without enzymes.

Proteins are made of chains of amino acids. The amino acid chains are twisted, coiled and folded into a variety of complex shapes. These shapes must be maintained or the proteins cannot function. Our bodies can make some of the amino acids needed to make our body proteins, but we must obtain the others from our diet. The amino acids which we can make are called “nonessential” amino acids, while those that we can’t make are “essential” amino acids. The distinction between a nonessential and an essential amino acid is not always clear, however, since adults can make certain amino acids while children can’t.

A protein in our diet that contains all the essential amino acids in adequate quantities is called a complete protein. Proteins from animal sources - meat, fish, eggs and dairy foods – are complete proteins. Plant proteins are generally incomplete, although there are some exceptions, such as soybean protein. Since different plants lack different essential amino acids, by eating a variety of plant foods a person can obtain all the amino acids that he or she needs.