Proteins are compounds made up of carbon, hydrogen, oxygen, and nitrogen that are arranged as strands of amino acid. They play an essential role in the maintenance of cells, growth, and functioning of the human body. Serving as the basic structural molecule of all the tissues in the body, protein makes up nearly 17% of total body weight.
Much of the body's dry weight is protein; even bones are about one-fourth protein. The animals humans eat and the microbes that attack an individual's body are likewise mostly protein. Leather, wool, and silk clothing are nearly pure protein. The insulin that keeps a patient with diabetes alive and the clot-busting enzymes that might save heart attack patients are also proteins. Proteins can even be found working at industrial sites: Protein enzymes produce the high-fructose corn syrup that sweetens most soft drinks and fuelgrade ethanol (alcohol) and other gasoline additives.
Within human bodies and those of other living organisms, proteins serve many functions. They digest foods and turn them into energy; they move the body and move molecules about within cells; they let some substances pass through cell membranes while keeping others out; they turn light into chemical energy, making both vision and photosynthesis possible; they allow cells to detect and react to hormones and toxins in their surroundings; and, as antibodies, they protect the body against foreign invaders. There are numerous types of proteins; more than 18,000 have been identified. As of 2016, scientists had begun mapping the human proteome, which describes all proteins in the body. With this and other new genetic information, scientists can begin to understand the role of proteins in health and disease, for example, a protein that binds to LDL cholesterol; with further study, the finding could lead to new ways to treat high cholesterol.
Amino acids are the fundamental building blocks of protein. Long chains of amino acids, called polypeptides, make up the large complexes of protein. The arrangement of amino acids along the chain determines the structure and chemical properties of the protein. Amino acids consist of the following elements: carbon, hydrogen, oxygen, nitrogen, and sometimes, sulfur. The general structure of amino acids consists of a carbon center with four substituents in an amino group (NH2), an organic acid (carboxyl) group (COOH), a hydrogen atom (H), and a fourth group known as the R-group. These four groups determine the structural identity and chemical properties of the amino acid. The first three groups are common to all amino acids. The basic amino acid structure is R-CH(NH2)-COOH.
The human body uses 20 different forms of amino acids. These forms are distinguished by the R-group, which can be a chain of different lengths or a carbonring structure. For example, if hydrogen represents the R-group, the amino acid is known as glycine, a polar but uncharged amino acid, whereas the methyl (CH3) group is known as alanine, a nonpolar amino acid. The chemical components of the R-group essentially determine the identity, structure, and function of the amino acid.
The structural and chemical relatedness of the Rgroup allow classification of the 20 amino acids into chemical groups. Amino acids can be classified according to optical activity (the ability to polarize light), acidity and basicity, polarity and nonpolarity, or hydrophilicity (water-loving) and hydrophobicity (water-fearing). These categories offer clues to the function and reactivity of the amino acids in proteins. The biochemical properties of amino acids determine the role and function of protein in the human body.
Meat, milk, eggs, poultry, seafood, and soya (soybean) are considered high-quality, complete proteins because they have all the essential amino acids (protein's building blocks) in adequate amounts. Those sources are considered more complete than vegetable protein, such as beans, peas, and grains, also considered a good—even if not complete—source of amino acids. Except for soy, plant sources—nuts, beans, seeds, and grains—are deficient in or contain low levels of one or more of the essential amino acids. But plant foods contain other vital nutrients (such as phytochemicals and fiber) not found in animal foods. Dietitians recommend a diet consisting of foods from a variety of sources and including 10%–20% of daily calories from protein (poultry, fish, dairy, soy protein, nuts, legumes, eggs, peanut butter, and vegetable sources). Incomplete proteins can be combined to provide all the essential amino acids, though combinations of incomplete proteins must be consumed at the same time, or within a short period of time (within four hours), for maximum nutritive value from the amino acids. Such combination diets generally yield a highquality protein meal, providing sufficient amounts and proper balance of the essential amino acids needed by the body to function.
Protein digestion begins when the food reaches the stomach and triggers the release of hydrochloric acid (HCl) by the parietal cells located in the lining of the gastrointestinal (GI) tract. Hydrochloric acid makes a highly acidic environment that helps the protein digestion process in two ways: first, by reacting to break peptide bonds (the chemical process of breaking peptide bonds is referred to as a hydrolysis reaction because water is used to break the bonds); and, second, by converting the gastric enzyme pepsinogen to pepsin (the active form of pepsinogen). Pepsinogen is stored and released by the chief cells that line the stomach wall. Once converted into the active form, pepsin attacks the peptide bonds that link amino acids together, breaking the long polypeptide chain into shorter segments of amino acids known as dipeptides and tripeptides. These protein fragments are then further broken down in the duodenum of the small intestines. The brush border enzymes on the surface of the small intestines hydrolyze the protein fragments into amino acids.
The cells of the small intestine actively absorb the amino acids through a process that requires energy. The amino acids travel through the hepatic portal vein to the liver, where the nutrients are processed into glucose or fat (or released into the bloodstream). The tissues in the body take up the amino acids rapidly for glucose production, growth and maintenance, and other vital cellular functioning. For the most part, the body does not store protein, as the metabolism of amino acids occurs within a few hours.
Amino acids are metabolized in the liver into useful forms that are used as building blocks of protein in tissues. The body might use the amino acids for either anabolic or catabolic reactions. Anabolic refers to the chemical process through which the body uses digested and absorbed products to effectively build or repair body tissues or to restore vital substances broken down through metabolism. Catabolic, by contrast, refers to the process that results in the release of energy through the breakdown of nutrients, stored materials, and cellular substances. Anabolic and catabolic reactions work together; the energy produced in catabolic processes is used to fuel essential anabolic processes. The vital biochemical reaction of glycolysis (oxidizing of glucose to produce carbon dioxide, water, and cellular energy) in the form of adenosine triphosphate (ATP) is a prime example of a catabolic reaction. The energy released, as ATP, from such a reaction fuels important anabolic processes, such as protein synthesis.
The metabolism of amino acids can be understood from the dynamic catabolic and anabolic processes. In the process referred to as deamination, the nitrogencontaining amino group (NH2) is cleaved from the amino acid unit. In this reaction, which requires vitamin B6 as a cofactor, the amino group is transferred to an acceptor keto-acid that can form a new amino acid. Through this process, the body is able to make the nonessential amino acids not provided by a patient's diet. The keto-acid intermediate can also synthesize glucose to ultimately yield energy for the body, and the cleaved nitrogen-containing group is transformed into urea, a waste product excreted as urine.
Proteins are vital to basic cellular and body functions, including cellular regeneration and repair, tissue maintenance and regulation, hormone and enzyme production, fluid balance, and the provision of energy.
CELLULAR AND TISSUE PROVISIONING. Protein is an essential component for every type of cell in the body, including muscles, bones, organs, tendons, and ligaments. Protein is also needed to help form enzymes, antibodies, hormones, blood-clotting factors, and blood-transport proteins. The body is constantly undergoing renewal and repair of tissues. The amount of protein needed to build new tissue or maintain structure and function depends on the rate of renewal or the stage of growth and development. For example, the intestinal tract is renewed every couple of days, whereas blood cells have a life span of 60–120 days. Furthermore, an infant uses as much as onethird of the dietary protein for building new connective and muscle tissues.
HORMONE AND ENZYME PRODUCTION. Amino acids are the basic components of hormones, which are essential chemical signaling messengers of the body. Hormones are secreted into the bloodstream by endocrine glands, such as the thyroid gland, adrenal glands, pancreas, and others and regulate body functions and processes. For example, the pancreas releases the hormone insulin, which helps to lower the blood glucose level after meals. Insulin is made up of 48 amino acids.
Enzymes play an essential role in biological reactions and are made up of a large protein molecule. Enzymes control the rate of reactions by acting as catalysts and lowering the activation energy barrier between the reactants and the products of the reactions. All chemical reactions that occur during the digestion of food and the metabolic processes in tissues require enzymes. Enzymes are vital to the overall function of the body and indicate the fundamental and significant role of proteins.
FLUID BALANCE. The presence of blood protein molecules, such as albumins and globulins, are critical factors in maintaining the proper fluid balance between cells and extracellular space. Proteins are present in the capillary beds—one-cell-thick vessels that connect the arterial and venous beds. Proteins cannot flow outside the capillary beds into the tissue because of their large size. Blood fluid is pulled into the capillary beds from the tissue through the mechanics of oncotic pressure, in which the pressure exerted by the protein molecules counteracts the blood pressure. Therefore, blood proteins are essential in maintaining and regulating fluid balance between the blood and tissue. The lack of blood proteins results in clinical edema, or tissue swelling, because there is not enough pressure to pull fluid back into the blood from the tissues. Edema is a serious condition that can lead to many medical problems.
ENERGY PROVISION. Protein is not a significant source of energy for the body when there are sufficient amounts of carbohydrate and fats available, nor is protein a storable energy, as in the case of fats and carbohydrates. However, if insufficient amounts of carbohydrates and fats are ingested, protein is used for energy needs of the body. The use of protein for energy is not necessarily economical for the body because tissue maintenance, growth, and repair are compromised to meet energy needs. If taken in excess, protein can be converted into body fat. Protein yields slightly more usable energy (4 kcal/g) than carbohydrates (3.75 kcal/g). Although not the main source of usable energy, protein provides the essential amino acids needed for adenine, the nitrogenous base of ATP, as well as other nitrogenous substances, such as creatine phosphate (nitrogen is an essential element for important compounds in the body).
Most people in the United States include enough protein in their diet. The recommended values of protein are generally considered enough protein for individuals who engage in recreational sports and fitness activities. Professional athletes, such as runners, triathletes, dancers, and individuals who play sports professionally may have increased protein needs. These needs might be increased during times of intense physical training when new muscle is being built. However, regularly eating more protein than recommended does not add to muscle tone.
Athletes should evaluate carefully their protein source. For example, specialty protein bars and shakes may contain incomplete proteins and may contain high levels of fat and calories. It is recommended that proteins come from natural animal and plant sources and be only minimally processed. Athletes who are vegetarian or vegan should mix different proteins in each meal (e.g., eggs, beans, soy products, nuts, and seeds) to ensure that their bodies can produce all of the amino acids needed for good health. The timing of eating protein can help athletes perform better; eating a high-quality protein within a few hours of exercise can help the muscles repair and grow.
Certain athletes such as dancers, gymnasts, and wrestlers who are especially weight conscious might be at an increased risk for protein deficiency if they do not eat enough food to provide the protein necessary each day. The nitrogen balance index (NBI) is used to evaluate the amount of protein used by the body compared with the amount of protein supplied from daily food intake. The body is in the state of nitrogen (or protein) equilibrium when the intake and usage of protein is equal. The body has a positive nitrogen balance when the intake of protein is greater than that expended by the body. In this case, the body can build and develop new tissue. Since the body does not store protein, the overconsumption of protein can result in the excess amount being converted into fat and stored as adipose tissue. The body has a negative nitrogen balance when the intake of protein is less than that used by the body. In this case, protein intake is less than required, and the body cannot maintain or build new tissues.
Most people in industrialized countries eat more protein than they need. In the United States, true protein deficiency is rare except when excess protein is lost and protein requirements are increased, as in the following cases:
High-protein diets for athletes and others are designed to provide about 1.5–2 g of protein for each kilogram of body weight. Complex proteins, such as milk and meats, should make up one-half to two-thirds of the daily protein requirement. High-protein diets are recommended for people who have the following characteristics:
A negative nitrogen balance represents a state of protein deficiency, in which the body is breaking down tissues faster than they are being replaced. The ingestion of insufficient amounts of protein or food with poor protein quality can result in serious medical conditions that compromise an individual's overall health. The immune system is severely affected; the amount of blood plasma decreases, leading to medical conditions such as anemia or edema; and the body becomes vulnerable to infectious diseases and other serious conditions. Treatment or prevention of this condition lies in adequate consumption of protein-rich foods.
See also Calories ; Metabolism and energy ; Nutrient timing .
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Revised by Tish Davidson, AM
Revised by Teresa Odle