Protein Function and Composition

Protein Function

• The primary function of protein is to build and repair tissues.

Protein Composition

• Proteins are built from simpler organic compounds called amino acids.
• Nitrogen is found only in protein.

Amino Acids

• The chemical structure that forms the base component of an amino acid is the amino group.
• Glycine is the simplest amino acid.
• There are 20 different amino acids that make up human proteins.
• 9 of the amino acids are considered indispensable or essential.
• Two amino acids are chemically joined by a peptide bond.
• Amino acids that can be synthesized by the body are dispensable.
• Amino acids can act as buffers.
• Proteins are absorbed primarily in the form of amino acids.
• Ammonia and urea are waste products from the metabolism of amino acids.
• The nutrient that has a protein-sparing effect is carbohydrate.
• Methionine and tryptophan play a role in forming neurotransmitters.

Protein Examples and Sources

• Hemoglobin is an example of a protein.
• Antibodies are an example of a tissue protein.
• A complete protein contains all the essential amino acids in the proper proportion.
• Gelatin is an animal protein that has negligible value as a dietary protein source when eaten alone.
• Proteins from plant sources such as grains, nuts, and legumes are classified as incomplete.
• Milk is an example of a complete protein.
• Meats provide the most frequent source of dietary protein for Americans.
• Eggs provide the greatest proportion of dispensable amino acids.

Factors Affecting Protein Quality

• The most significant measure of the protein quality of a diet depends on the variety of foods consumed.
• A 20-year-old male who wants to build muscle is vulnerable to health claims for amino acid supplements.
• The chemical score of a protein is the ratio of indispensable amino acids in a food versus a high-quality protein reference food.
• When analyzing the amino acid composition of foods, the amino acid occurring in the smallest amount is classified as limiting.

Dietary Considerations

• One of the potential consequences of eating an excess amount of dietary protein is to impair kidney function.
• A good reason that people choose to follow a vegetarian diet is health factors.
• A vegetarian diet in which dairy foods and eggs are included is known as a lactoovovegetarian diet.

Protein Digestion

• The gastric enzymes present in infants that coagulate milk is Rennin.
• HCL Hydrochloric acid, Pepsinogen and Rennin start the breakdown of protein in the stomach.
• HCL denatures the protein and unfolds the structure so enzymes can work on the protein in the stomach.
• Rennin digests and coagulates milk protein for infants.
• The mucous membrane of the stomach protects the stomach from the effects of HCL.
• Pepsin must be activated by pepsinogen because pepsin will destroy all linings once the stomach is empty, so it is activated only when needed for digestion.
• Pepsinogen secreted by the gastric cells is converted into pepsin by Hydrochloric Acid.
• There are no enzymes for protein digestion in the mouth.
• Protein digestion in the small intestine uses the following pancreatic enzymes: 1. Trypsin 2. Chymotrypsin 3. Carboxypepsidase 4. Aminopeptidase 5. Dipeptidase.
• Once all the pancreatic enzymes have broken down the protein, free amino acids are ready to be absorbed into the blood stream via portal circulation
• The enzyme trypsin is activated by Enterokinase.
• Chymotrypsin is found in pancreatic secretions.
• The pancreas secretes trypsin and chymotrypsin.
• The small intestines secrete aminopeptidase and dipeptidase.

Protein Metabolism

• Catabolism is when tissue proteins are constantly being broken down into amino acids
• Anabolism is the process of broken-down tissue is resynthesized into tissue proteins as needed.
• The constant turnover of protein occurs between tissue and plasma.
• The tissue that has the highest rate of protein turnover is the intestinal mucosa.
• The phase of metabolism that makes growth and repair possible is anabolism.
• Effects of an increased protein diet are kidney problems, dehydration, weight loss and ketosis.
• To maintain nitrogen balance, the part of the amino acid that contains nitrogen may be removed by Deamination.
• Nitrogen travels to the Liver to make Urea, the Kidneys to make Urine and then to the bladder (simplified).
• Vegetarians lack Vitamin B12 on main nutrient.
• A negative nitrogen balance occurs in an older adult with anorexia.
• Negative nitrogen balance is when the excretion of nitrogen exceeds the nitrogen intake.
• Childhood is when positive nitrogen balance is most likely to occur.
• Burn injury increases protein catabolism.
• The recommended dietary intake of protein for adults is 0.8 g/kg.

Digestion and Absorption Definitions

• Digestion is the process in which food is broken down in the GI tract, releasing many nutrients in forms the body can use.
• Absorption is the process in which nutrients are taken into the cells lining the GI tract.
• Transport is the movement of nutrients through the circulatory system from one area of the body to another.
• Metabolism is the sum of the vast number of chemical changes in the cell, the functional unit of life, which finally produces the essential materials necessary for energy, tissue building, and metabolic controls.
• Motility is the ability to move spontaneously.
• Muscles of the gastrointestinal wall interact to provide general types of movement.
• Muscle tone ensures continuous passage of the food mass and valve control.
• Periodic muscle contraction and relaxation are rhythmical waves that mix the food mass and move it forward and these alternating muscular contractions and relaxations that force the contents forward are known as peristalsis.
• Specific nerves regulate muscle action along the GI tract.
• A complex network of nerves in the GI wall extends from the esophagus to the anus called inramural nerve plexus which controls muscle tone in the wall, regulate the rate and intensity of the alternating muscle contractions, and coordinate all the various movements.
• Hydrochloric acid is needed to produce the correct pH necessary for enzyme activity.
• Digestive enzymes are proteins, specific in kind and quantity for breaking down nutrients.
• Secretions of mucus lubricate and protect the mucosal tissues lining the GI tract as well as help mix the food mass.
• The products of digestion are carried and circulated through the GI tract and into the tissues by water and electrolytes.
• Bile is made in the liver and stored in the gallbladder, bile divides fat into smaller pieces to expose more surface area for the actions of fat enzymes.
• Seretory cells produce each of the preceding substances for specific jobs in chemical digestion.
• The secretory action of these cells or glands is stimulated by the presence of food, nerve impulse, or hormones specific for certain nutrients
• Mastication (biting and chewing) occurs in the mouth and begins to break down food into smaller particles.
• Salivary amylase/ptyalin is a starch-splitting enzyme in the mouth, secreted by the salivary glands.
• Ebner glands secrete a lingual lipase and during infancy, lingual lipase is the more relevant enzyme for milk fat
• Salivary glands secrete mucous material that lubricate and bind food particles to facilitate the swallowing of each food bolus, or lump of food material
• Mucous glands line the esophagus and their secretions help move the food mass toward the stomach
• Pancreatic amylase converts starch to the disaccharides maltose and sucrose.
• Trypsin and chymotrypsin split large protein molecules into smaller and smaller peptide fragments and finally into single amino acids, carboxypeptidase removes end amino acids from peptide chains
• Pancreatic lipase converts fat to glycerides and fatty acids.
• Chyme is a semifluid food mass in the GI tract present after gastric digestion.
• Pepsin is the main gastric enzyme specific for proteins, pepsin begins breaking large protein molecules into shorter chain polypeptides and gastric hydrochloric acid is necessary for activation
• Pancreatic amylase is a major starch-splitting enzyme secreted by the pancrease that acts in the small intestine
• Trypsin is a protein-splitting enzyme formed in the small intestine where the inactive precursor trypsinogen is activated by enterokinase.
• Chymotrypsin is a protein splitting and milk curdling pancreatic enzymes activated in the small intestine from the precursor chymotrypsinogen and breaks specific amino acid peptide links of protein
• Carboxypeptidase is a protein enzyme that splits off the chemical group carboxyl (-COOH) at the end of peptide chains
• Pancreatic lipase is a major fat splitting enzyme produced by the pancreas and secreted into the small intestine to digest fat
• Secretin is produced by the mucosal glands in the first part of the intestine and controls the acidity and secretion of enzymes from the pancreas
• Cholecystokinin is secreted by intestinal mucosal glands when fat is present, triggers the release of bile from the gallbladder to emulsify fat.
• A nutrients bioavailability depends on the amount of nutrient present in the GI tract, competition between nutrients for common absorptive sites, and the form in which the nutrient is present
• Bioavailability influences dietary intake standards for all macronutrients and micronutrients.
• Functions of the liver include bile production for fat digestion, synthesis of proteins and blood clotting factors, metabolism of hormones and medications, regulations of blood glucose levels and urea production to remove waste products of normal metabolism

Specific Metabolism

• Lipolysis is the breaking down lipids to fatty acids and glycerol.
• Lipogenesis is the building up of lipids from fatty acids and glycerol.
• Glycolysis is breaking down glucose to pyruvate to enter the Krebs cycle
• Gluconeogenesis is converting noncarbohydrate substances into glucose.
• Glycogenolysis is breaking down glycogen to individual glucose units.
• Glycogenesis is combining units of glucose to store as glycogen.
• Protein degradation is breaking down proteins to single amino acids.
• Protein synthesis is building complete proteins from individual amino acids.
• Mucosal folds are large visible folds of the mucous lining of the small intestine that increase the absorbing surface area
• Villi are small protrusions from the surface of a membranep; fingerlike projections covering mucosal surfaces of the small intestine that further increases the absorbing surface area and are visible through a regular microscope.
• Microvilli are extremely small, hairlike projections covering all villi on the surface of the small intestine that greatly extend the total absorbing surface area; visible through an electron microscope.
• Simple diffusion is the force by which particles move outward in all directions from an area of greater concentration to an area of lesser concentration
• Small materials that do not need the help of a specific protein channel to move across the mucosal cell wall use simple diffusion
• Facilitated diffusion is similar to simple diffusion but uses a protein channel for carrier-assisted movement of larger items across the mucosal cell membrane
• Active transport is the force by which particles move against their concentration gradient and active transport mechanisms usually require some sort of carrier partner to help ferry the particles across the membrane.
• Pinocytosis is the penetration of larger materials by attaching to the thicker cell membrane and being engulfed by the cell
• Catabolism is the metabolic process of breaking down large substances to yield smaller building blocks
• Anabolism is the metabolic process of building large subsntaces from smaller parts and is the opposite of catabolism
• Glycogenesis is the formation of stored glycogen from glucose
• Stomach gas results from trapped air bubbles and occurs when a person eats too fast, drinks through a straw, or otherwise takes in extra air while eating.
• Stomach gas can be avoided by avoiding carbonated beverages, not gulping, chewing with your mouth closed, not drinking from a can or through a straw and not eating when you are nervous
• Intestinal gas forms in the colon, where bacteria attack undigested items, causing them to decompose and produce gas. Carbohydrates release hydrogen, carbon dioxide, and in some people methane
• In some people, protein produces hydrogen sulfide and volatile compounds which causes a distinctive aroma to the expelled air in intestinal gas
• To control intestinal gas, cut down on simple carbohydrates (sugars) where lactose intolerance may be the culprit. substitute cultured forms and use a prior leaching process before cooking dry beans to remove indigestible saccharides.
• It also helps to eliminate known food offenders such as beans, onions, cabbage, and high fiber wheat products
• Lipogenesis is the formation of fat
• Adipose tissue is the storage site for excess fat
• Gluconeogenesis is the formation of glucose from noncarbohydrate substances such as amino acids
• PKU (Phenylketonuria) is an autosomal recessive genetic disorder that results when phenylalanine hydroxylase, the enzyme responsible for breaking down the essential amino acids phenylalanine to trypsin, is not produced by the body
• Galactosemia is an autosomal recessive disorder that affects carbohydrate metabolism as the missing enzyme, GALT converts galactose to glucose
• Lactose intolerance is insufficient lactase to break down the milk sugar lactose which accumulates in the intestine, causing abdominal cramping and diarrhea
• Deficiency of any one of the disaccharides in the small intestine may produce a wide range of GI problems and abdominal pain.
• The digestive products of a large meal are not difficult to absorb because the area of the absorbent surface of the intestines is relatively small
• some enzymes must be activated by hydrochloric acid or other enzymes before they can work
• Bile is not an enzyme specifically used for the chemical break-down of fat
• The GI circulation provides a constant supply of water and electrolytes to carry digestive secretions and substances being produced
• Secretions from the GI accessory organs, the gall-bladder and the pancreas, do not mix with gastric se-cretions in the stomach to aid digestion
• One enzyme may not work on both car-bohydrate and fat breakdown
• Bile is released from the gall-bladder in re-sponse to a hor-monal stimulus
• During digestion, the major mus-cle action that moves the food mass forward in regular rhythmic waves is called peristalsis
• Mucus lubricates and protects the GI lining and is important GI secretion because it
• Pepsin is a gastric enzyme that acts of protein
• Bile is a fat-emulsifying agent and important secretion
• The route of fat absorption is the lymphatic system by way of villi lacteals.