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Protein is a macromolecule made up of amino acids. It plays a crucial role in the body and can be found in various food sources. Learn about protein structure, functions, denaturation, and common sources of protein.
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Protein is a macromolecule... A protein molecule is very large and complex, and contains many atoms. For that reason it is referred to as a ‘macromolecule’. Nearly 50% of the dry part of many body cells consists of protein. The protein molecule is made up of hydrogen, carbon, oxygen, and nitrogen, and sometimes other elements such as sulfur. Nitrogen is the critical difference between protein and carbohydrates and fats which do not contain nitrogen. Protein molecules are found in all the foods of the meats group on the food pyramid.
The building blocks of protein... Protein is made up of chains of substances called amino acids. There are 22 amino acids that combine to form the protein molecules. C Each amino acid has the same basic structure… a central atom of carbon with 4 bonds around it.
Amino acids... Shown here, bonded on one side of the central carbon atom, is the carboxyl group: COOH Bonded on the other side of the central carbon atom is the amine group: NH2 Bonded on a third side of the central carbon atom is a single hydrogen atom: H The only difference between one amino acid and another is the “R”, or side group. The amino acid ‘glycine’, shown at the left, has a side group consisting of a single Hydrogen atom. Another amino acid named ‘alanine’ has the side group CH3.
Amino acids form chains... Amino acids link together in chains to form molecules of protein. These attachments occur through ‘peptide bonds’. They always connect where one (H) hydrogen atom in the amine group meets the –OH of the carboxyl group. The result a condensation reaction and the release of water: H2O Some protein chains contain only a few amino acids. When amino acids link together in long chains, perhaps 100-500 amino acids, the protein molecule is referred to as a polypeptide.
Amino acid chains determine protein structure... Some amino acids are attracted to, or repelled by other amino acids and water. This is called ‘polarity’, and causes the amino acid chains to fold over on each other forming a ‘globular’ structure. Hemoglobin in the blood is a globular protein, well-suited to be a carrier of oxygen. Some amino acids chain together to form rope-like spirals called helixes. This type of protein is a ‘fibrous’ structure and is very strong. It forms connective tissue in the body. Collagen and elastin in muscle, tendons, and ligaments are fibrous proteins.
Denaturation and coagulation of protein... Denaturation (dee-nay-chuh-RAY-shun) is a process that causes protein to become a looser, less compact structure. It can be caused by heat, freezing, sound waves, mechanical treatment like beating, the addition of ingredients that raise or lower pH levels, or the presence of minerals such as sodium, copper, potassium, or iron. Denaturation is sometimes reversible. Coagulation (ko-ag-yuh-LAY-shun) changes a liquid protein into a soft, semisolid clot or solid mass. It occurs when polypeptides unfold during denaturation and then collide and clump together during cooking processes. Coagulation is not reversible. The normal size and folded shape of the protein molecules helps prevent them from dissolving in water.
Denaturation and coagulation in eggs... The processes of denaturation and coagulation are easily observable in our use of eggs. By applying heat during the cooking process, the protein in the eggs coagulates into soft clumps. The result is what we commonly call ‘scrambled eggs’. Adding salt or beating the eggs can cause the protein to denature, becoming ‘looser’. The yolks and the whites of an egg contain protein. The protein is ‘viscous’ (sticky, resists flow).
Denaturation of meat protein... The lean part of meat is 15-20% protein. Using a mechanical device to pound and cut the fibrous protein, cooking the meat with an acid such as tomato juice (to lower the pH), or cooking the meat slowly in liquid to soften the fibrous collagen proteins are all methods that will tenderize the meat. The elastin proteins are heat-resistant and must be broken down mechanically.
Denaturation of fish protein... Fish protein is very different than meat protein. The shorter muscle fibers are layered between thin sheets of connective tissue that is easily liquified, allowing the flesh to ‘flake’ apart when cooked. This protein structure is also the reason why fish cooks much more quickly than meat does.
Sources of protein... Tofu is made from soybeans. Soybeans are 40% protein and high in fiber, and their protein is equal in quality to meat protein. Eggs, meats, fish, poultry, nuts (generally too high in fat to be a main source of protein in the diet), legumes (dried beans and beans), milk, dairy products such as cheese, and soybeans are excellent sources of protein.
Protein is an emulsifier... Bonds with water The ‘polarity’ of some amino acids make protein foods good emulsifiers. One end of the amino acid chain is attracted to water where it makes hydrogen bonds, and the other end avoids water and bonds with oil. In this way, the protein can stabilize an oil and water mixture Bonds with oil Mayonnaise is an emulsion that is made by combining lemon juice or vinegar with egg yolks. Eggs bind the ingredients together and prevent separation. Then, oil is added drop by drop as the mixture is rapidly whisked. Adding oil too quickly (or insufficient, rapid whisking) will keep the two liquids from combining (emulsifying). But, as the sauce begins to thicken, oil can be added more rapidly. Seasonings are whisked in after all of the oil has been added. Blenders, mixers and food processors make it easier to make homemade mayonnaise.
Proteins help create foams... A foam is created when air bubbles incorporated into a foodstuff are trapped in a protein film during whipping. As egg whites or cream are whipped, the whipping action denatures the protein in the food and introduces air into the mixture. Then as the protein begins to coagulate, it forms a fine film around the pockets of air. This increases the mixture’s volume and lightness.
Factors that promote or inhibit the formation of foams... Some fluids make better foams than others. Milk does not have enough protein compared to its water content, so foams made with milk do not last long. Egg whites foam best at a pH of 4.6-4.8. The pH rises as eggs age. Cream of tartar, an acid, lowers pH and stabilizes the protein to make a higher quality foam. Fats, such as those found in egg yolks, inhibit the formation of foams. Sugars interfere with the coagulation of proteins, so must be added slowly. Overbeating, until the whites appear dry, denatures the protein to the point where it loses its elasticity.
Gelatin is protein... Gelatin is derived from connective tissues of animals, such as the collagen on the inner layer of hides and bones. Its structure allows it to dissolve as it heats in liquid. Upon cooling, the protein locks up the liquid in a semisolid mass. As meat drippings cool, it is the gelatin that causes them to congeal. Unflavored gelatin stabilizes liquids in food preparation. The sweetened gelatins, such as Jello brand gelatins, are mostly sugar, coloring, and flavoring. Gelatin is used to thicken, stabilize, or texturize. It may be used as an ingredient in gelatin desserts, marshmallows, gummy bears, margarines, jelly and jam, and ice cream.
Gluten is the protein in wheat... Gluten is the elastic substance formed by mixing water with the insoluble fibrous and globular proteins found in wheat (flour). Gluten is developed as the protein ‘casein’ denatures… in the process of kneading the dough. As the dough stretches, air is trapped within it; then it forms an elastic mesh to trap that air. Baking the dough coagulates the protein and sets the structure of the bread.
Building body protein from food protein... During digestion, proteins in food are denatured by the hydrochloric acid in the stomach. The stomach enzymes then break the peptide bonds, and the amino acid chains become shorter and shorter. By the time the protein is absorbed through the intestinal villi into the bloodstream, it is in the form of amino acids again. In the blood, the amino acids travel to the cells and genetic material re-assembles the amino acids into the types and forms of protein that are needed the most by the body. This includes muscles, tissues, virtually all cells, skin, nails, hair, blood, organs, enzymes, and hormones. Ex: during periods of growth, protein is used for bones; after an injury, protein is used for blood; during weightlifting, protein is used to build muscles; etc.
Functions of body proteins... Protein is needed for body structure, such as bones, muscle, tendons, and ligaments. Protein repairs and replaces cells in skin, tissue, hair, nails, and blood. Protein stabilizes the pH balance of acids and bases in the body. The enzymes that break down other proteins and the hormones that aid growth, balance body fluids, and regulate metabolism are made from protein. Some proteins pick up, deliver, and store nutrients. Antibodies are very large protein molecules that weaken or destroy foreign substances in the body.
Does protein supply energy? Proteins CAN supply energy, but it is a risky way to fuel the body. When the body is starved of carbohydrates, it will break down its own protein in search of glucose. That is one reason why people who participate in low-carb diets often ‘crave’ high-carb foods. When the body has to break down its own protein for energy, it will select the source of protein most easily broken down. Muscle and organ proteins would be more easily broken down than bone protein. This is dangerous, as the heart and lungs are body organs necessary for survival.
Essential amino acids... COMPLETE protein sources contain all the essential amino acids, such as eggs and meat. 13 of the 22 amino acids can be manufactured by the body itself. The other 9 must be included in the diet, and are called ‘essential’. If you don’t get them in the diet, the body will break down its own protein to find them. Essential amino acids: Isoleucine (eye-suh-LOO-seen) Leucine (LOO-seen) Lysine (LY-seen) Methionine (muh-THIGH-uh-neen) Phenylalanine (fen-ul-AL-uh-neen) Threonin (THREE-uh-neen) Tryptophan (TRIP-tuh-fan) Valine (VAY-leen) Histodine (HISS-tuh-deen) this amino acid is only essential in children INCOMPLETE protein sources lack one or more of the essential amino acids. Soybeans are the only plant source providing complete protein.
Vegetarian and vegan diets... In vegetarian and vegan diets, the dieter needs to eat foods from a variety of plant sources in order to get enough protein. Obviously, the vegetarian can include dairy products and eggs in the diet, which help greatly. The vegan diet, however, needs close monitoring for adequate protein supplies. Vegetarian and Vegan diets need sources of protein other than meat.
Recommended daily allowance... The body needs relatively small amounts of protein, considering how important it is. The recommended daily allowance of protein is .8 grams per kilogram of body weight. To figure your daily protein needs, divide your weight in pounds by 2.2 to get your weight in kilograms. Then take your weight in kg x .8 g = g of protein needed each day If your weight is 150 pounds: divide 150 by 2.2 = 68.2 kg. Take 68.2 kg x .8 g protein = 55 g 1 egg McMuffin contains 32 g protein Most Americans eat more protein than they need. A single 4 oz. hamburger contains 32 grams of protein. 1 pork chop contains 23 grams. 1 cup of tuna contains 40 grams. 1 cup of milk contains 8-9 grams. 1 cup of black beans contains 8 grams.
The disease called kwashiorkor is linked to protein deficiencies. Protein deficiency... Kwashiorkor is characterized by a swollen, pot belly, weight loss, skin problems and alternating bands of gray and dark hair. It is primarily a problem in developing countries, or for those who are crash dieting. Because brain neurons and synapses are cells and connections created with protein, a protein deficiency can lead to reduced intelligence or mental retardation.
Excess protein intake... Protein is NOT stored in the body. Excess amounts of protein are converted to sugars and fatty acids and stored as fat. The liver also removes the nitrogen atoms from the amino acids, so they can be burned as fuel. Many athletes believe that consuming large amounts of protein will increase muscle mass. However, muscle mass and strength can only increase as a result of physical activity and NOT excess protein consumption. Protein consumption does play an important role in the post exercise meal where it promotes the repair and restoration of muscle that was broken down or ‘denatured’ during exercise. The recommended protein intake for athletes ranges from 1.0-1.8 g/kg/day depending on the energy expenditure and demand of the exercise. This is more than the .8g/kg/day needed for average lifestyles.
PROTEIN The End
