Protein Functions in the Human Body and in Cells
Proteins are vital components of our bodies. They form part of the body's structure and perform many essential functions. They enable us to move, distribute oxygen around the body, clot blood when we're wounded, fight infections, transport substances into and out of cells, control chemical reactions, and transmit messages from one part of the body to another.
Protein molecules are made of chains of amino acids. Our bodies digest the proteins that we eat, converting them into individual amino acids that are absorbed into the bloodstream. Our cells then use these amino acids and ones that we make to produce the specific proteins that we need.
Good sources of protein in the diet include meat, poultry, fish, dairy products, eggs, legumes or pulses (beans, lentils and peas), and nuts. Many nutritionists recommend that we eat lean meats and low fat dairy if these foods are part of our diet.
Proteins are found throughout our body. This article focuses on major ones in blood, muscle, and cell membranes. It also discusses signaling and structural proteins as well as enzymes. Many other proteins exist in the human body. They are important molecules.
Hemoglobin, Fibrinogen, and Albumin in Blood
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 oxygen to the tissue cells. These need oxygen to make energy from digested food and to produce substances that they need.
The liquid part of blood is called plasma. It contains a protein called fibrinogen, which is involved in the blood clotting process. When a blood vessel is broken, a series of chemical reactions convert fibrinogen into a solid protein called fibrin. The fibrin fibers form a mesh over the wounded area that traps escaping blood. The mesh and the trapped blood form the blood clot.
Albumin is another protein in blood plasma. It helps to keep water in the blood and to maintain the correct volume of liquid in the vessels. Albumin also transports bilirubin to the liver. Bilirubin is a waste substance made from the breakdown of hemoglobin in old and damaged red blood cells. The liver converts the bilirubin into a form that can be excreted.
Antibodies and the Complement System
Proteins are important in our immune system, which fights infections. For example, blood contains antibodies, which are proteins made by a type of white blood cell called a B lymphocyte or a B cell. The antibodies fight invaders such as bacteria and viruses.
Certain proteins in blood and specific ones attached to the cell membrane form the complement system. This system has a number of functions in the immune system. 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, Myosin, Myoglobin, and Ferritin in Muscle
Actin and myosin are proteins that exist as filaments in muscle fibers (or muscle cells). When calcium ions are present the filaments slide over each other, causing the muscle to contract. The proteins are also found in other types of cells and are responsible for various movements of and within the cells.
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. Myosin has some similarities to hemoglobin but also some differences.
A polypeptide is a single chain of amino acids. Some proteins contain only one polypeptide, but others have multiple ones joined together. A myoglobin molecule consists of only one polypeptide chain while a hemoglobin molecule contains four. The heme group in myoglobin and hemoglobin binds to oxygen. Myoglobin has one heme group and hemoglobin has four.
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 is called the cell membrane or the plasma membrane. It's made chiefly of a double layer of phospholipids (the "phospholipid bilayer"), cholesterol molecules, and protein molecules.
Membrane proteins are classified into three major categories.
- Peripheral proteins are present at the outer and/or inner surface of a membrane. The bond between a peripheral protein and the cell membrane is weak and often temporary. Peripheral proteins frequently sit on the surface of the membrane but sometimes extend a small distance into it.
- Integral proteins are not only present on the membrane surface but also penetrate the membrane. Most extend all the way through the membrane and are known as transmembrane proteins. Some integral proteins span the membrane multiple times.
- Lipid-bound or Lipid-linked proteins are located entirely within the phospholipid bilayer and don't extend to either membrane surface. They are rarer than the other types of membrane proteins.
Functions of Membrane Proteins
The protein molecules in membranes have a variety of functions. Some 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. Others are receptors, which join to specific substances at the surface of the cell.
An example of a receptor in action is the joining of insulin to a receptor protein. Insulin is a protein hormone made by the pancreas. The union of the insulin and the receptor causes the membrane to become more permeable to glucose. This enables sufficient glucose to enter the cell, where it's used as a nutrient.
Receptors are also involved in the transmission of nerve impulses. A chemical called an excitatory neurotransmitter is released from the end of a stimulated neuron, or nerve cell. The neurotransmitter binds to a receptor on the next neuron. This binding causing a nerve impulse to be produced in the second neuron and is the method by which nerve impulses travel from one nerve cell to another.
Glycoproteins—proteins attached to carbohydrates—are found on the outer surface of cell membranes and have many functions, including acting as cell adhesion molecules to “stick” cells together. The outer surface of the membrane also has glycolipids, which are phospholipids attached to carbohydrates.
Signaling Proteins and Hormones
Cytokines are small proteins released by cells to communicate with other cells. They are often made 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 continuation of the pregnancy.
Pregnancy tests check for HCG in a woman’s urine or blood. If HCG is present, the woman may be pregnant because the hormone is made by an embryo and a placenta. It's important that a doctor confirms that the woman is pregnant if a testing kit suggests that she is, though. Several factors can cause a false result in the test, including the use of certain medications, certain conditions in the woman's body, and the condition of the test kit.
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 and 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 wouldn't occur at all. Since a huge number of chemical reactions are happening all the time in our bodies, life would be impossible without enzymes.
Digestive enzymes break down the food that we eat, producing small particles that are absorbed through the lining of the small intestine. The particles enter the bloodstream, which transports them around the body to our cells. The cells use the digested food particles as nutrients.
The amino acid chains of a protein molecule are twisted, coiled, and folded into a variety of complex shapes. These shapes must be maintained or proteins can't function.
How Enzymes Work
Enzymes work by joining with the chemical or chemicals that are reacting (the substrate or substrates). A substrate molecule joins to a place on the enzyme molecule known as the active site. The two fit together like a key fits into a lock, so the description of enzyme action is commonly referred to as the lock and key theory. It's believed that in some reactions (or perhaps in most of them) the active site changes its shape slightly to fit the substrate, which is known as the induced fit model of enzyme activity.
Essential Amino Acids and Complete Proteins
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” ones. The distinction between the two types is not always clear, however, since adults can make certain amino acids while children can’t.
A protein in our diet that contains all of 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. Protein in some form is a vital part of our diet, since it enables our bodies to make essential chemicals for life.
Protein facts from the National Institute of General Medical Sciences (Chapter 1 in the Structures of Life booklet)
Information about proteins from the U.S. National Library of Medicine
Structure of the plasma membrane from the Khan Academy
Introduction to cell signaling from the Khan Academy
Structure and function of enzymes from the Royal Society of Chemistry
Questions & Answers
Which part of our body is totally made up of proteins?
That’s an interesting question. Hair is mainly protein, but it also contains some lipid. The lens of the eye is mainly protein, but it also contains some carbohydrate molecules. Muscles are rich in protein as well. The actin and myosin filaments in a muscle are protein, but the muscle as a whole also contains carbohydrates and fatty acids.
Our fingernails and toenails are made of dead cells containing a protein called keratin. The production of a large quantity of keratin in living cells is known as keratinization. Keratinization happens in some other parts of the body besides the nails. The keratin replaces the contents of the cells. I don’t know how many of the chemicals from the living cells remain in nail cells that have been keratinized, however.Helpful 10
© 2010 Linda Crampton