Protein Structure and Metabolism Menu Verywell Fit Nutrition Weight Management Nutrition Facts Nutrition Basics Diets Meal Plans Meal Delivery Services View All News Fitness and Nutrition What to Buy How We Test Products Fitness Gear Nutrition Products Tools Recipe Nutrition Calculator Weight Loss Calorie Goal BMI Calculator Body Fat Percentage Calculator Calories Burned by Activity Daily Calories Burned Pace Calculator About Us Editorial Process Meet Our Review Board Search Nutrition Facts Proteins Protein Structure and Metabolism By Shereen Lehman, MS Shereen Lehman, MS Shereen Lehman, MS, is a healthcare journalist and fact checker. She has co-authored two books for the popular Dummies Series (as Shereen Jegtvig). Learn about our editorial process Updated on December 26, 2020 Medically reviewed Verywell Fit articles are reviewed by board-certified physicians and nutrition and exercise healthcare professionals. Medical Reviewers confirm the content is thorough and accurate, reflecting the latest evidence-based research. Content is reviewed before publication and upon substantial updates. Learn more. by Jonathan Valdez, RDN, CDCES, CPT Medically reviewed by Jonathan Valdez, RDN, CDCES, CPT Jonathan Valdez, RDN, CDCES, CPT is a New York City-based telehealth registered dietitian nutritionist and nutrition communications expert. Learn about our Medical Review Board Print Verywell / Alexandra Shytsman Table of Contents View All Table of Contents Structure Function Digestion Protein Sources Proteins are necessary for building the structural components of the human body, such as muscles and organs. You also need proteins to keep your immune system healthy, synthesize neurotransmitters, create and signal hormones, and much more. Protein Structure Proteins tend to be large molecules made from building blocks called amino acids. The general structure of an amino acid molecule includes a carboxyl group of atoms, an amine group, and a side chain. The carboxyl group contains one carbon, two oxygen, and one hydrogen atom. The amine group contains one nitrogen atom with two hydrogen atoms attached to it. All 20 amino acids have different side chains, which vary in shape. There are straight chains of atoms, branched chains of atoms, and rings of atoms, plus the side chains may include carbon, hydrogen, sulfur, nitrogen, and oxygen atoms. The configuration and molecules found in the side chain are what differentiates one amino acid from another. The branched-chain amino acids are isoleucine, leucine, and valine and are necessary for muscle structure. Tyrosine, phenylalanine, and tryptophan are called aromatic amino acids and each one contains a side chain with a ring-shaped formation. These three amino acids are needed for neurotransmitter production. Non-Essential and Essential Amino Acids Amino acids are considered either "essential" or "non-essential" based on where they come from—some must be consumed in your diet while others do not. Non-Essential Amino Acids The 11 non-essential amino acids are not called "non-essential" because they are not important. They are important, and your body requires them to perform several functions. These amino acids are called "non-essential" because you don't need to get them from your diet. Your body can build those 11 amino acids from chemicals already present in your body. The non-essential amino acids include: AlanineArginineAsparagineAspartic AcidCysteineGlutamic AcidGlutamineGlycineProlineSerineTyrosine The amino acids arginine, cysteine, glycine, glutamine, proline, and tyrosine are sometimes also considered to be "conditionally essential." That means most people manufacture them on their own, but individuals with certain illnesses or genetic abnormalities can't make them, so they need to get them through their diets. Essential Amino Acids The nine essential amino acids are called "essential" because you can't manufacture them; you have to eat proteins that contain those amino acids. They are: HistidineIsoleucineLeucineLysineMethioninePhenylalanineThreonineTryptophanValine Do you need to worry about essential amino acids when you plan your daily meals? Not really. Animal sources of protein such as meat, eggs, and dairy products are "complete proteins." That means that each protein found in an animal product contains each of the nine essential amino acids. Vegetarians and vegans may need to pay a little more attention to the dietary proteins. Plant proteins are called "incomplete proteins." Each plant protein is missing one or more of the nine essential amino acids. However, every amino acid is found in some plant, so you can combine different plant proteins to get all of the amino acids you need. Protein Function There are many different proteins in your body, and they perform different functions. Primary Functions The primary functions of proteins include: Contributing to enzyme activity that promotes chemical reactions in the bodyHelping blood clotKeeping fluids and pH balanced in the bodyPromoting antibody activity that controls immune and allergy functionsServing as building blocks for hormone productionServing as structural components that give our body parts their shapesSignaling cells what to do and when to do itTransporting substances around the body Protein as Energy You may not think of protein as an energy source, but proteins do contribute calories to your diet. Those calories need to be acknowledged if you are watching your weight. Each gram of protein you eat has four calories. The acceptable macronutrient distribution range created by the Institute of Medicine suggests that you get about 10% to 35% of your calories from protein. Protein Deficiency In underdeveloped countries, malnutrition and protein deficiency is more prevalent. Severe protein deficiency is called kwashiorkor. Children with kwashiorkor tend to have very thin arms and legs and large, distended bellies. Lack of protein can cause growth failure, loss of muscle mass, depressed immune system function, lung problems, heart problems, and death. The Effects of Protein Deficiency Digestion The digestion of protein begins in the mouth with chewing, which makes food easier to swallow. It also helps with digestion by chopping food up into smaller bits. Remember that it's important to chew your food thoroughly; don't gulp your food down in huge bites. Protein digestion continues in the stomach with the release of digestive juices that include hydrochloric acid and pepsinogen. Hydrochloric acid converts pepsinogen to pepsin, which begins to break down the bonds between the amino acids. This process takes place while the muscles surrounding the stomach squeeze and squish the foods and stomach fluids together. The next step occurs in the small intestine where the hydrochloric acid is neutralized with bicarbonates released from the pancreas. The pancreas also releases an enzyme called trypsin. Trypsin continues to break apart the amino acids, which are then absorbed into the bloodstream. Once in the bloodstream, the amino acids are carried to the cells in various parts of your body. Your body uses the individual amino acids to build the proteins needed for the different functions. Protein Sources Meats, fish, seafood, poultry, eggs, and dairy products are significant sources of protein, but you can also get protein from a variety of grains, legumes, nuts, and seeds. It isn't difficult to get enough protein in your diet. Three ounces of chicken will provide you with about 21 grams of protein. Three ounces of salmon has about 18 grams of protein, a cup of oatmeal has six grams of protein, and a cup of asparagus even has 3 grams of protein. Since most people get enough protein from their diet, protein deficiency is rare in developed countries. Sources Verywell Fit uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy. Gropper SS, Smith JL, Groff JL. Advanced Nutrition and Human Metabolism. Sixth Edition. Wadsworth Publishing Company, 2013. Smolin LA, Grosvenor MB. Nutrition: Science and Applications. Third Edition. Wiley Publishing Company, 2013. The National Academies of Sciences, Engineering and Medicine, Health and Medicine Division. Dietary Reference Intakes Tables and Application. United States Department of Agriculture Agricultural Research Service. National Nutrient Database for Standard Reference. Release 28. By Shereen Lehman, MS Shereen Lehman, MS, is a healthcare journalist and fact checker. See Our Editorial Process Meet Our Review Board Share Feedback Was this page helpful? Thanks for your feedback! What is your feedback? Other Helpful Report an Error Submit Related Articles The Effects of Protein Deficiency What Are Amino Acids and Which Ones Are Essential? How Much Protein Do I Need? 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