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PROTEINS Dr. Sakhawat Ali © 2010 Pearson Education, Inc. Proteins and Amino Acids © 2010 Pearson Education, Inc. What Are Proteins? Large molecules Made up of chains of amino acids Are found in every cell in the body Are involved in most of the body’s functions and life processes The sequence of amino acids is determined by DNA © 2010 Pearson Education, Inc. Amino Acid • These Compounds form the structural units of protein. Out of a total of 20 of more, eight ate considered dietary essentials, indispensable to life. • The Various food proteins, when digested, yield their specific constituent amino acids. These amino acids are then available for use by the cells as the cells synthesize specific tissue proteins © 2010 Pearson Education, Inc. The Nature of Proteins The building units , amino acids, are used by the body to construct specific tissue proteins, This process is made possible by the nature of amino acids. Which enables them to form peptide linkages and arrange themselves into peptide chains. © 2010 Pearson Education, Inc. Structure of Proteins Made up of chains of amino acids; classified by number of amino acids in a chain • Peptides: fewer than 50 amino acids - Dipeptides: 2 amino acids - Tripeptides: 3 amino acids - Polypeptides: more than 10 amino acids • Proteins: more than 50 amino acids - Typically 100 to 10,000 amino acids linked together Chains are synthesizes based on specific bodily DNA Amino acids are composed of carbon, hydrogen, oxygen, and nitrogen © 2010 Pearson Education, Inc. Structural Differences Between Carbohydrates, Lipids, and Proteins Figure 6.1 The Anatomy of an Amino Acid Figure 6.2b Peptide Bonds Link Amino Acids Form when the acid group (COOH) of one amino acid joins with the amine group (NH2) of a second amino acid Formed through condensation Broken through hydrolysis © 2010 Pearson Education, Inc. Condensation and Hydrolytic Reactions Figure 6.3 Essential, Nonessential, and Conditional Essential – must be consumed in the diet Nonessential – can be synthesized in the body Conditionally essential – cannot be synthesized due to illness or lack of necessary precursors • Premature infants lack sufficient enzymes needed to create arginine © 2010 Pearson Education, Inc. Structure of the Protein Four levels of structure • Primary structure • Secondary structure • Tertiary structure • Quaternary structure Any alteration in the structure or sequencing changes the shape and function of the protein © 2010 Pearson Education, Inc. Denaturing Alteration of the protein’s shape and thus functions through the use of • Heat • Acids • Bases • Salts • Mechanical agitation Primary structure is unchanged by denaturing © 2010 Pearson Education, Inc. Denaturing a Protein Figure 6.5 Quick Review Proteins are chains of combination of amino acids Amino acids contain carbon, hydrogen, oxygen, nitrogen, and sometimes sulfur Unique amino acids consist of a central carbon with a carboxyl group, a hydrogen, a nitrogen-containing amine group, and a unique side chain There are 20 side chains and 20 unique amino acids • 9 essential amino acids • 11 nonessential amino acids - At time these become conditionally essential Amino acids link together with peptide bonds by condensation and break apart by hydrolysis © 2010 Pearson Education, Inc. Quick Review Attractions and interactions between the side chains cause the proteins to fold into precise three-dimensional shapes Protein shape determines its function Proteins are denatured and their shapes changed by • Heat • Acids • Bases • Salts • Mechanical agitation © 2010 Pearson Education, Inc. Protein Digestion: Part 1 Figure 6.6 Protein Digestion: Part 2 Figure 6.6 Protein Digestion: Part 3 Figure 6.6 Protein Digestion: Part 4 Figure 6.6 Amino Acid Absorption Amino acids are absorbed in the small intestine Amino acids are transported to the liver from the intestines via the portal vein In the liver, amino acids are • Used to synthesize new proteins • Converted to energy, glucose, or fat • Released to the bloodstream and transported to cells throughout the body Occasionally proteins are absorbed intact © 2010 Pearson Education, Inc. Amino Acid Metabolism Liver metabolizes amino acids, depending on bodily needs Most amino acids are sent into the blood to be picked up and used by the cells • Amino acid pool is limited but has many uses Protein turnover – the continual degradation and synthesizing of protein © 2010 Pearson Education, Inc. Protein Synthesis Figure 6.8 Deamination When the amino acid pool reaches capacity the amino acids are broken down to their component parts for other uses First deamination must occur Carbon-containing remnants are • Converted to glucose, if they are glucogenic amino acids, through gluconeogensis • Converted to fatty acids and stored as triglycerides in adipose tissue © 2010 Pearson Education, Inc. Metabolic Fate of Amino Acids Figure 6.7 Quick Review During digestion • Proteins are broken down to amino acids with the help of - Gastric juices - Enzymes in the stomach and small intestine - Enzymes from the pancreas and small intestine lining Limited supply of amino acids exist in the amino acid pool The amino acid pool acts as a reservoir for protein synthesis Surplus amino acids are • Deaminated - Used for glucose or energy - Stored as fat - Nitrogen is converted to urea and excreted in urine © 2010 Pearson Education, Inc. How Does the Body Use Protein? Functions of protein • Provide structural and mechanical support • Maintain body tissues • Functions as enzymes and hormones • Help maintain acid base balance • Transport nutrients • Assist the immune system • Serve as a source of energy when necessary © 2010 Pearson Education, Inc. Functions of Protein Growth and tissue-Building Maintenance The primary function of dietary protein is to supply building material for growth and maintenance of body tissue. It does this by furnishing amino acids in appropriate numbers and types for efficient synthesis of specific cellular tissue proteins. Also, protein supplies amino acids for other essential nitrogen-containing substances such as enzymes and hormones. Specific Physiologic Roles All amin acids supplied by dietary protein participate in growth and tissue maintenance. But some also perform other important physiologic and metabolic roles. For example , methionine is an agent in the formation of choline, which is a precursor of acetylcholine, one of the major neurotransmitters in the brain. In addition, methionine is not only the precursor of the nonessential amino acid cystine, but also of the lesser known ones carnitine and taurine, which are now known to have widespread metabolic functions. 1,2 Tryptophan is the precursor of the B vitamin niacin and of the neurotransmitter serotonin. Phenylalanine is the precursor of the nonessential amino acid tyrosine, which leads to formation of the hormones thyroxine and epinephrine. In addition, protein antibodies provide essential components of the body’s unnybe system, and plasma proteins guard water balance. © 2010 Pearson Education, Inc. Available Energy Protein also contributes to the Body’s overall energy metabolism. This occurs as needed in the fasting state or in extended physical effort such as marathon running, but not in the fed state. After the removal of the nitrogen- containing portion of the constituent amino acid, the amino acid residue, its carbon “ skeleton” , called a keto-acid , may be converted either to glucose or to fat. On the average, 58% of the total dietary protein may become available when needed to be burned for energy. Thus sufficient amounts of non-protein kilocalories from carbohydrate are always needed to spare protein for its primary building purpose and to prevent unnecessary protein breakdown in the process of providing energy. © 2010 Pearson Education, Inc. © 2010 Pearson Education, Inc. Quick Review Protein plays many important roles in the body, including: • Helping facilitate muscular contraction • Promoting satiety and appetite control © 2010 Pearson Education, Inc. Types of Proteins The Proteins illustrate a huge diversity of compounds produced by specific amino acid linkages. As a result, according to their varied specific structures, tissue proteins perform many vital roles in body structure and metabolism. Some of these examples include Structural proteins such as collagen, Contractile proteins such as muscle fibers, Antibodies such as gammaglobulin, Blood proteins such as albumin and Fibrinogin, some hormones such as insulin, and all the enzymes. © 2010 Pearson Education, Inc. How Much Protein Do You Need? Healthy, nonpregnant adults • Should consume enough to replace what is used every day • The goal is nitrogen balance Pregnant woman, people recovering from surgery or injury, and growing children • Should consume enough to build new tissue © 2010 Pearson Education, Inc. Nitrogen Balance and Imbalance Figure 6.12 Not All Protein Is Created Equal High quality protein • Is digestible • Contains all essential amino acids • Provides sufficient protein to synthesize nonessential amino acids It helps to be aware of: • Amino acid score • Limiting protein • Protein digestibility corrected amino acid score (PDCAAS) • Biological value of protein rates absorption and retention of protein for use © 2010 Pearson Education, Inc. Protein Quality Complete proteins • Contain all nine essential amino acids • Usually animal source are complete proteins • Are considered higher quality Incomplete proteins • Low in one or more essential amino acid • Usually plant sources are incomplete © 2010 Pearson Education, Inc. Protein Needs Protein intake recommendations • 10–35% of total daily kilocalories • Adults over 18 - 0.8 g/kg daily American College of Sports Medicine, the American Dietetic Association, and other experts advocate • 50–100% more protein for competitive athletes participating in endurance exercise or resistance exercise • Typically this population eats more and therefore gets additional protein © 2010 Pearson Education, Inc. Quick Review Protein quality is determined by digestibility and types and amounts of amino acids Animal protein is more easily digested and complete Plant proteins are typically incomplete, except soy Plant proteins can be complemented with proteins from other plant sources or animal source to improve their quality Adults should consume 0.8 g/kg/d of protein Men and women in the United States tend to over consume protein © 2010 Pearson Education, Inc. Best Sources of Protein Proteins are abundant in • Dairy foods • Meats • Poultry • Meat alternatives such as dried beans, peanut butter, nuts, and soy 3 oz serving of cooked meat, poultry, or fish • Provides 21–25 grams of protein • About 7 g/oz • About the size of a deck of cards • Adequate amount for one meal © 2010 Pearson Education, Inc. Best Sources of Protein Figure 6.14 Quick Review A well-balanced diet can meet daily protein needs Best source of protein are animal products • Eggs • Lean meats • Low-fat or fat-free dairy products Plant proteins such as soy, grains, and vegetables supply substantial proteins Most people consume adequate protein from their diet and do not need protein supplements © 2010 Pearson Education, Inc. Protein Bars Are marketed as convenient and portable Can be • High in saturated fat and/or sugar • Low in fiber • Expensive A peanut butter sandwich is portable and lower in saturated fat and sugar and higher in fiber than some protein bars © 2010 Pearson Education, Inc. Eating Too Much Protein Risk of heart disease Risk of kidney stones Risk of calcium loss from bones Risk of colon cancer Displacement of other nutrient-rich, disease preventing foods © 2010 Pearson Education, Inc. Eating Too Little Protein Protein-energy malnutrition (PEM) • Protein is used for energy rather than its other functions in the body • Other important nutrients are in short supply • More prevalent in infants and children - 17,000 children die each day as a result © 2010 Pearson Education, Inc. Too Little Protein Without adequate protein • Cells lining the GI tract are not sufficiently replaced as they slough off • Digestive function is inhibited • Absorption of food is reduced • Intestinal bacteria gets into the blood and causes septicemia • Immune system is compromised due to malnutrition and cannot fight infection © 2010 Pearson Education, Inc. Types of PEM: Kwashiorkor Severe protein deficiency • Generally result of a diet high in grains and deficient in protein Symptoms range from • Edema in legs, feet, and stomach • Muscle tone and strength diminish • Hair is brittle and easy to pull out • Appear pale, sad, and apathetic • Prone to infection, rapid heart rate, excess fluid in lungs, pneumonia, septicemia, and water and electrolyte imbalances (Image from http://www.thachers.org/pediatrics.htm) Figure 6.16 Types of PEM: Marasmus Results from a severe deficiency in kilocalories • Frail, emaciated appearance • Weakened and appear apathetic • Many cannot stand without support • Look old • Hair is thin, dry, and lacks sheen • Body temperature and blood pressure are low • Prone to dehydration, infections, and unnecessary blood clotting Figure 6.17 Types of PEM: Marasmic Kwashiorkor Chronic deficiency in kilocalories and protein • Have edema in legs and arms • Have a “skin and bones” appearance • With treatment the edema subsides and appearance becomes more like someone with marasmus © 2010 Pearson Education, Inc. Treatment for PEM Medical and nutritional treatment can dramatically reduce mortality rate Should be carefully and slowly implemented • Step 1 – Address life-threatening factors - Severe dehydration - Fluid and nutrient imbalances • Step 2 – Restore depleted tissue - Gradually provide nutritionally dense kilocalories and high-quality protein • Step 3 – Transition to foods and introduce physical activity © 2010 Pearson Education, Inc. Quick Review High-protein diet may play a role in increasing risk of heart disease, kidney problems, and calcium loss from bones Consuming too much protein from animal sources increase saturated fat intake Too much protein can displace whole grains, fruits, and vegetables, which have been shown to reduce many chronic diseases Low-protein diet can lead to loss of bone mass PEM is caused by inadequate protein and/or kilocalorie intake • Kwashiorkor – severe protein deficiency • Marasmus – severe kilocalorie deficiency © 2010 Pearson Education, Inc. Vegetarian Diet People choose vegetarian diets for a variety of reasons • Ethical • Religious • Environmental • Health Vegetarians must consume adequate amounts of a variety of food and should plan meals well © 2010 Pearson Education, Inc. Potential Benefits, Risks of a Vegetarian Diet Benefits of a healthy vegetarian diet • Reduced risk of - Cancer - Heart disease - Stroke - High blood pressure - Obesity - Diabetes Potential risks of a vegetarian diet • Underconsumption of certain nutrients - Protein - Vitamin B12 © 2010 Pearson Education, Inc. Vegetarian Food Guide Pyramid Figure 6.18 Soy Soy is increasing in popularity in the United States • High-quality protein source • Low in saturated fat • Contains isoflavones • Phytoestrogens • May reduce risk of heart disease • Some research suggests it may reduce the risk of cancer • Some concern it may promote breast cancer © 2010 Pearson Education, Inc. Quick Review Vegetarian diets can be a healthy eating style that may help reduce the risk of some chronic disease Some vegetarians abstain from all animal products Some vegetarians eat eggs and dairy in limited amounts Vegetarians must plan their diets carefully to meet their nutrient needs, especially • Riboflavin • Protein • Vitamin B12 • Iron • Vitamin A • Zinc • Omega-3 fatty acids • Calcium • Vitamin D © 2010 Pearson Education, Inc. Putting It All Together Majority of daily kilocalories should come from carbohydrate-rich foods Fat intake should be no more than about one-third of daily kilocalories Protein should provide the rest of the daily kilocalories © 2010 Pearson Education, Inc. Putting It All Together Best plan for a healthful diet • Eat an abundance of - Grains (at least ½ whole grains) - Vegetables - Fruits • Eat modest amounts of - Commercially made bakery and snack items - Vegetables with creamy sauces or added butter - Sweets • Choose low-fat dairy products, lean meat, poultry, and fish to minimize the intake of heart-unhealthy saturated fats © 2010 Pearson Education, Inc.
