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  • 1. CHAPTER 5 ProteinsEleanor D. Schlenker Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc.
  • 2. Physical and Chemical Nature of Protein In 1838 Dutch chemist Johann Mulder first identified protein as a prime substance in all life forms Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 2
  • 3. Physical and Chemical Nature of Protein – Cont’d Protein enzymes break down our food Antibodies shield us from disease Peptide hormones carry messages Structural proteins in our bodies Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 3
  • 4. The Nature of Amino Acids Amino acids are composed of the elements carbon, hydrogen, oxygen, and nitrogen  Several contain sulfur Amino acids are the building blocks for body tissues Word amino refers to a base or alkaline substance Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 4
  • 5. General Pattern and Structure Common structural pattern holds for all amino acids This pattern is built around a central alpha- carbon with several attached chemical groups:  Amino (base, NH2) group  Carboxyl (acid, COOH) group  Attached radical (R) group Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 5
  • 6. Essential Amino Acids 20 amino acids are used to build body proteins  Indispensable (essential) amino acids: 9 that cannot be synthesized by the body and must be supplied in food  Dispensable (nonessential) amino acids: 5 that can be synthesized by the body in adequate amounts  Conditionally indispensable amino acids: 6 that fall in between; the body is unable to synthesize the amount needed under certain conditions Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 6
  • 7. The Building of Protein Ability of amino acids to form peptide linkages and arrange themselves into peptide chains is basic to all proteins and body tissues Peptide bonds: the joining of the end amino group of one amino acid with the end carboxyl group of the amino acid beside it Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 7
  • 8. The Building of Proteins – Cont’d Polypeptides: specific amino acids joined in a particular sequence to form long chains of amino acids Oligopeptides: short chains of 3 to 15 amino acids Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 8
  • 9. Types of Protein Myosin: fibrous protein found in muscle fiber, built from chains of 153 amino acids that coil and unfold as the muscle contracts and relaxes Collagen: contains three separate polypeptide chains that wind around each other to produce a triple helix Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 9
  • 10. Types of Protein – Cont’d Hemoglobin: globular type of protein that includes four globin polypeptide chains per molecule of hemoglobin Albumin: major plasma protein that has a compact globular shape, consists of a single polypeptide chain of 584 amino acids Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 10
  • 11. Functions of Protein Dietary protein supplies building material for the growth and maintenance of body tissues Dietary protein furnishes amino acids in the appropriate patterns and amounts for efficient synthesis of specific structural molecules Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 11
  • 12. Physiologic Roles All amino acids participate in tissue growth and maintenance Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 12
  • 13. Physiologic Roles – Cont’d Some have important metabolic roles of their own:  Form neurotransmitters for brain and nerve function  Form other amino acids  Form hormones  Support immune function  Maintain fluid balance Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 13
  • 14. Role of Protein in Critical Care Certain amino acids appear to have a particular role in recovery from severe infection, trauma, or life-threatening malnutrition Recovery in these situations may be improved with specific amino acid supplements Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 14
  • 15. Role of Protein in Critical Care – Cont’d Leucine: following surgery and trauma Branched chain amino acids—leucine, isoleucine, and valine: cancer with malnutrition, liver disease Arginine and methionine: burn patients Cysteine: recovery from protein-energy malnutrition Dangers of self-medication Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 15
  • 16. Energy Source Seldom used for energy in the fed state May be used for energy in the fasting state Nitrogen-containing amino group must be removed Remaining keto acid is converted to glucose or fat Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 16
  • 17. Protein and Nitrogen BalanceConcept of Balance Interdependent checks and balances Constant ebb and flow of tissue building and breakdown Coordinated activity to maintain normal function Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 17
  • 18. Body Protein Reserves The average man contains approximately 11 kg of protein  Nearly half this protein (43%) is found in skeletal muscle  The remainder is in skin, blood, kidney, liver, brain, and other organs Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 18
  • 19. Body Protein Reserves – Cont’d Body distribution of protein changes with growth and development Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 19
  • 20. Body Protein Reserves – Cont’d Labile protein reserves  Are easily broken down to meet immediate needs  Make up only about 1% of total body protein  Are intended to provide amino acids for an emergency Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 20
  • 21. Protein Balance Finely tuned mechanisms control protein balance and regulate protein synthesis and breakdown across all body tissues Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 21
  • 22. Protein Balance – Cont’d Sometimes described as nitrogen balance  Net result of nitrogen gain and loss across all body tissues Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 22
  • 23. Protein and Nitrogen Balance Protein balance  The steady state that exists between protein synthesis (anabolism) and protein breakdown (catabolism) Protein turnover  Higher rate: intestinal mucosa, liver, pancreas, kidney, plasma  Lower rate: muscle, brain, skin  Very slow: structural tissues such as collage  Higher in infants and children Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 23
  • 24. Protein and Nitrogen Balance – Cont’d Protein compartments  Tissue protein  Plasma protein Metabolic amino acid pool Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 24
  • 25. Protein QualityEvaluating Food Proteins Protein quality depends on the following two characteristics:  Protein digestibility  Amino acid composition Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 25
  • 26. Protein Quality – Cont’dComparing Food Proteins Nutritive value of a food protein = its amino acid score  A value based on both its digestibility and amino acid composition Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 26
  • 27. Protein Quality – Cont’dComparing Food Proteins – cont’d When evaluating an amino acid score, it is important to identify the limiting amino acid(s)  The limiting amino acid(s) is any indispensable amino acid present in a lower amount than recommended in the Amino Acid Reference Pattern Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 27
  • 28. Protein Quality – Cont’dComparing Food Proteins – cont’d The Amino Acid Reference Pattern indicates the appropriate proportion of each indispensable amino acid needed to produce a body protein Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 28
  • 29. Protein Quality – Cont’dComparing Food Proteins – cont’d Other methods used in animal studies to evaluate protein digestibility and composition:  Biologic value (BV)  Net protein use (NPU)  Protein efficiency ratio (PER) Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 29
  • 30. Amino Acid Content of Plant and Animal Foods Complete proteins: animal foods, which contain all of the indispensable amino acids in the amounts and ratio needed to support protein synthesis  Include eggs, milk, cheese, meat, poultry, and fish Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 30
  • 31. Amino Acid Content of Plant and Animal Foods – Cont’d Incomplete proteins: plant proteins  Supply less than the required amount of one or more indispensable amino acids or are missing an indispensable amino acid Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 31
  • 32. Factors Influencing Protein RequirementsProtein Quality Protein requirements are influenced by the quality of dietary protein and its amino acid patternProtein Digestibility Effects of food preparation Digestibility and absorption is influenced by the time interval between meals, with longer intervals lowering the competition for available enzymes and absorption sites Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 32
  • 33. Factors Influencing Protein Requirements – Cont’dTissue Growth Periods of rapid growth—as for fetal and maternal tissues in pregnancy—require added proteinEnergy Content of the Diet Sufficient amounts of carbohydrates must be available to meet energy needs so dietary protein can be used exclusively for tissue building Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 33
  • 34. Factors Influencing Protein Requirements – Cont’dHealth Status Critical illness and diseases that increase the rate of protein turnover and tissue breakdown (catabolism) raise the protein requirement Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 34
  • 35. Protein Requirements Recommended Dietary Allowance (RDA) for adults  Furnishes the protein and amino acids needed to maintain or replace body tissues and replace nitrogen lost via the urine, feces, and sweat, and lost cells such as those lost from the inner surface of the digestive tract Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 35
  • 36. Protein Requirements – Cont’d Recommended Dietary Allowance (RDA) for adults – cont’d  0.8 g/kg body weight or 56 g/day for men  0.8 g/kg body weight or 46 g/day for women  An additional 25 g/day is needed for pregnancy and lactation Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 36
  • 37. Protein Requirements – Cont’d Protein requirements of infants and children vary according to age and growth patterns Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 37
  • 38. Protein Requirements – Cont’d Acceptable Macronutrient Distribution Range  10% to 35% of total kcalories Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 38
  • 39. Protein Energy Malnutrition Major health problem in many developing countries where protein intakes are low in both quantity and quality  More than 6 million children die each year from protein-related deficiencies  Available protein sources in many developing countries tend to be plant sources with poor amino acid patterns Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 39
  • 40. Protein Energy Malnutrition – Cont’d Marasmus  Chronic extreme starvation  Deficit of energy, protein, and micronutrients  Little or no body fat and extreme wasting Kwashiorkor  Deficient in protein; low in energy and micronutrients  Edema, hypoalbuminemia, skin lesions, fatty liver Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 40
  • 41. Amounts and Types of Protein Extremes  Research to increase indispensable amino acid content of grains and vegetables  Excessive protein intakes in the United States Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 41
  • 42. Low-Protein Diets Children with protein-energy malnutrition (PEM) have the following:  Poor growth with wasting (low weight for height) and stunting (low height for age)  Impaired immune function  Chronic respiratory infections  Diarrhea Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 42
  • 43. Low-Protein Diets – Cont’d Low-protein intake during pregnancy increases risk of a low-birth-weight infant Clinical situations  Parkinson’s disease  Chronic kidney disease  Vegan diets Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 43
  • 44. High-Protein Diets Diets with 25% or more of total kcalories (kcal) from protein appear to promote weight loss, provide satiety, and can restore effective energy regulation Protein suppresses hunger to a greater extent than carbohydrates High protein intakes appear to protect muscle mass Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 44
  • 45. High-Protein Diets – Cont’d Long-term effects on renal function:  Animal proteins increase blood flow to the kidney  Animal and vegetable proteins accelerate renal impairment  Attend to fluid needs Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 45
  • 46. Health Benefits of Plant Foods Soy foods and legumes supply good-quality protein and may help to prevent certain chronic diseases Soy foods are sources of phytochemicals that may help prevent bone loss in older women Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 46
  • 47. Health Benefits of Plant Foods – Cont’d Legumes lower risk of cardiovascular disease Vegetable protein foods are lower in saturated fat Soybeans and chickpeas are good sources of linoleic acid Tree nuts and peanuts are rich in antioxidants, trace minerals, essential fatty acids, and phytochemicals Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 47
  • 48. Nutritional Contributions of Animal and Plant Proteins Animal sources  Provide indispensable amino acids  Meats provide iron and zinc  Dairy foods provide calcium, riboflavin, preformed vitamin A  Vitamin B12 and vitamin D Plant sources  Iron and zinc are less well absorbed  Add fiber and phytochemicals  Essential fatty acids  Fortified soy milk provides calcium and vitamin D Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 48
  • 49. Vegetarian Diets Reasons for following vegetarian diets  Religion  Sustainability of the food supply  Protection of the environment Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 49
  • 50. Vegetarian Diets – Cont’d Vegetarian diets are generally categorized as follows:  Ovolactovegetarian: includes all plant foods, dairy, and eggs  Lactovegetarian: includes all plant foods and dairy  Pescovegetarian: includes plant foods and fish  Vegan: includes plant foods only  Flexitarian: includes predominantly plant foods with animal foods eaten occasionally Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 50
  • 51. Vegetarian Diets – Cont’d Nutritional Implications of Vegetarian Diets  Require careful planning, depending on type of vegetarian diet  Phytates interfere with absorption of iron and zinc  Include vitamin C source to increase iron absorption  Use of fortified foods  Effects of oxalate on calcium absorption Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 51
  • 52. Vegetarian Diets – Cont’d Planning vegetarian diets  Vegetarian Food Guide Vegetarian diets and chronic disease  Lower coronary heart disease, obesity, hypertension, and diabetes  Generally lower risk of cancer, uncertain effects on specific cancers Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 52
  • 53. Digestion-Absorption-Metabolism The food proteins taken into the body must be broken down into ready-to-use building units—the amino acids Mouth  The only digestive action on protein taking place in the mouth is the mechanical effect of chewing Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 53
  • 54. Digestion-Absorption-Metabolism – Cont’dStomach Three chemical agents in the gastric secretions: 1. Pepsin 2. Hydrochloric acid 3. Rennin Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 54
  • 55. Digestion-Absorption-Metabolism – Cont’dSmall Intestine Enzymes secreted by the pancreas and glands in the wall of the intestine help protein digestion:  Pancreatic secretions: (1) trypsin, (2) chymotrypsin, (3) carboxypeptidase  Intestinal secretions: (1) aminopeptidase, (2) dipeptidase Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 55
  • 56. Absorption of Amino Acids Characteristics of amino acid absorption  Requires active transport system  Enter portal blood Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 56
  • 57. Absorption of Amino Acids – Cont’d Absorption of peptides and whole proteins  A few short-chain peptides and smaller intact proteins escape digestion and are absorbed  Most undergo hydrolysis within the cells of the intestinal mucosa and yield their amino acids  Protein molecules that pass into the blood intact may play a part in the development of immunity, protein sensitivity, and food allergies Copyright © 2011, 2007 by Mosby, Inc., an affiliate of Elsevier Inc. 57