A protein (from the Greek "protos" meaning "of primary importance") is a complex, high-molecular-weight organic compound that consists of amino acids joined by peptide bonds. Proteins are essential to the structure and function of all living cells and viruses. Many proteins are enzymes or subunits of enzymes, catalyzing chemical reactions. Other proteins play structural or mechanical roles, such as those that form the struts and joints of the cytoskeleton, serving as biological scaffolds for the mechanical integrity and tissue signalling functions. Still more functions filled by proteins include immune response and the storage and transport of various ligands. In nutrition, proteins are broken down through digestion to provide amino acids for the organism, including those the organism may not be able to synthesise itself.
Proteins make up about 15% of the mass of the average person. Protein molecules are essential to us in an enormous variety of different ways. Much of the fabric of our body is constructed from protein molecules. Muscle, cartilage, ligaments, skin and hair - these are all mainly protein materials.
Every function in the living cell depends on proteins.
- Motion and locomotion of cells and organisms depends on contractile proteins. [Examples: Muscles]
- The catalysis of all biochemical reactions is done by enzymes, which contain protein.
- The structure of cells, and the extracellular matrix in which they are embedded, is largely made of protein. [Examples: Collagens] (Plants and many microbes depend more on carbohydrates, e.g., cellulose, for support, but these are synthesized by enzymes.)
- The transport of materials in body fluids depends of proteins. [See Blood]
- The receptors for hormones and other signaling molecules are proteins.
- Proteins are an essential nutrient for heterotrophs.
- The transcription factors that turn genes on and off to guide the differentiation of the cell and its later responsiveness to signals reaching it are proteins.
- and many more — proteins are truly the physical basis of life.
It is vital that we eat enough protein. Proteins are the building blocks of the body. The different amino acids that make up proteins are important for growth, tissue repair and replacement. Bones, muscles, the walls of hollow organs - in fact almost every part of the body - needs a supply of amino acids in order to keep healthy.
Many foods contain protein (say: pro-teen), but the best sources are beef, poultry, fish, eggs, dairy products, nuts, seeds, and legumes like black beans. Protein builds up, maintains, and replaces the tissues in your body.
Proteins are generally classified as soluble, filamentous or membrane-associated (see integral membrane protein). Nearly all the biological catalysts known as enzymes are soluble proteins. Antibodies, the basis of the adaptive immune system, are another example of soluble proteins. Membrane-associated proteins include exchangers and ion channels, which move their substrates from place to place but do not change them; receptors, which do not modify their substrates but may simply shift shape upon binding them. Filamentous proteins make up the cytoskeleton of cells and much of the structure of animals: examples include tubulin, actin, collagen and keratin, all of which are important components of skin, hair, and cartilage. Another special class of proteins consists of motor proteins such as myosin, kinesin, and dynein. These proteins are "molecular motors," generating physical force which can move organelles, cells, and entire muscles.
Proteins are generally large molecules, having molecular masses of up to 3,000,000 (the muscle protein titin has a single amino acid chain 27,000 subunits long) however protein masses are generally measured in kiloDaltons (kDa). Such long chains of amino acids are almost universally referred to as proteins, but shorter strings of amino acids are referred to as "polypeptides," "peptides" or rarely, "oligopeptides". The dividing line is undefined, though "polypeptide" usually refers to an amino acid chain lacking tertiary structure which may be more likely to act as a hormone (like insulin), rather than as an enzyme (which depends on its defined tertiary structure for functionality).
FUNCTIONS OF PROTEINS
Your body uses the protein you eat to make lots of specialized protein molecules that have specific jobs. For instance, your body uses protein to make hemoglobin (say: hee-muh-glo-bin), the part of red blood cells that carries oxygen to every part of your body. Other proteins are used to build cardiac muscle. What's that? Your heart! In fact, whether you're running or just hanging out, protein is doing important work like moving your legs, moving your lungs, and protecting you from disease.
Proteins are essential for growth and repair. They play a crucial role in virtually all biological processes in the body. All enzymes are proteins and are vital for the body's metabolism. Muscle contraction, immune protection, and the transmission of nerve impulses are all dependent on proteins. Proteins in skin and bone provide structural support. Many hormones are proteins. Protein can also provide a source of energy. Generally the body uses carbohydrate and fat for energy but when there is excess dietary protein or inadequate dietary fat and carbohydrate, protein is used. Excess protein may also be converted to fat and stored.
Blood proteins such as albumin and globulin function to maintain fluid balance in the body. When the concentrations of proteins in the bloodstream are low, the fluid in the blood (serum) begins to seep into surrounding tissue. Proteins in the blood can counteract this effect by increasing the osmotic potential and forcing fluid back into the bloodstream. Therefore, low amounts of protein in the blood cause edema, a condition that is characterized by an abnormal amount of fluid in the tissue and extracellular space. Edema is seen in starvation, low calorie diets, and diseases like AIDS that decrease the amount of circulating antibodies and albumin.
Protein should account for 10% to 20% of the calories consumed each day. Protein is essential to the structure of red blood cells, for the proper functioning of antibodies resisting infection, for the regulation of enzymes and hormones, for growth, and for the repair of body tissue.
The proteins that allow cell and tissue movement, such as actin and myosin in muscle, or tubulin in the microtubules of the mitotic spindle or the eukaryotic cilium
Hormonal proteins coordinate the bodily activities. Insulin, ahormone secreted by the pancreas, helps regulate the concentration of sugar in the blood.
Receptor proteins are built into the membrane of a nerve cell and they detect chemical signals released by other nerve cells. They are involved in the cell's response to chemical stimuli. Proteins are also important in the immune response. There are two types of important proteins the immune system uses to scan the vast network of molecules in our body and determine self from nonself.
Proteins help to maintain the body's fluid and electrolyte balance. This means that proteins ensure that the proper types and amounts of fluid and minerals are present in each of the body's three fluid compartments. These fluid compartments are intracellular (contained within cells), extracellular (existing outside the cell), and intravascular (in the blood). Without this balance, the body cannot function properly.
SYMPTOMS OF HIGH INTAKE
Too much protein, however, should also be avoided as this can cause problems in people who have liver and kidney problems. These waste-removing organs become overloaded and have difficulty in coping. Furthermore if the main sources of protein are meat and dairy products - which can also contain a lot of saturated fat - there is a danger that blood cholesterol levels might increase.
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