HUN1201 LECTURE NOTESCHAPTER 6: Proteins: Crucial Components of All Body TissuesWhat are proteins?• Proteins: large, complex molecules found in cells of all living things• Synthesis is controlled by our DNA • Contain carbon, hydrogen, oxygen, nitrogen• Made from 20 different amino acidsHow do proteins differ from starch?Starch:composed of chains of glucose molecules.Proteins:Composed of chains of amino acids.Amino Acids• Nine essential amino acids• Cannot be produced in sufficient quantities to meet physiological needs • Must be obtained from food• Nonessential amino acids• Can be synthesized in sufficient quantities• We do not need to consume them in the dietNon-Essential Amino Acids• Transamination • The transfer of an amine group from an essential amino acid to a different acid group and side chain (R group)• Used to make nonessential amino acids. • Conditionally essential amino acids:• Nonessential amino acid becomes essential • Phenylketonuria (PKU): • tyrosine becomes a conditionally essential amino acid that must be provided by the diet• Phenylalanine à tyrosine.How are proteins made?• Proteins are long chains of amino acids• Amino acids are connected by peptide bonds• Gene expression is the process by which cells use genes to make proteins• Gene: segment of deoxyribonucleic acid (DNA) that serves as a template for the synthesis (expression) of a particular protein• Transcription: messenger RNA copies the genetic information from DNA • Translation: the genetic information in RNA is converted into the amino acid sequence of a protein• Protein turnover• Existing proteins are degraded to provide the building blocks for new proteins• Amino acid pool includes amino acids from food and cellular breakdown• Protein organization determines function• Sequential order of the amino acids • Spiral shape from twist in amino acid chainProtein Denaturation• Proteins uncoil and lose their shape • Damaging substances: heat, acid, base, heavy metal, alcohol • Protein function is lost• Denatured enzyme• High fever• Blood pH out of normal range• During digestionProtein in the diet• For protein synthesis, all essential amino acids must be available to the cell• Limiting amino acid • Essential amino acid that is missing or in the smallest supply• Slows down or halts protein synthesis• Inadequate energy consumption • Limits protein synthesis• Incomplete protein (low quality): insufficient essential amino acids • Does not support growth and health• Complete protein (high quality): sufficient amounts of all nine essential aminoacids• Derived from animal and soy protein• Mutual supplementation: combine two or more incomplete protein sources tomake a complete protein• Complementary proteins: two or more foods are combined to supply all nine essential amino acids for a complete proteinProtein Digestion• Protein digestion begins in the stomach• Hydrochloric acid denatures protein strands and activates pepsin• Pepsin: enzyme breaks down proteins into short polypeptides and amino acids• Gastrin: hormone controls hydrochloric acid production and pepsin release • Digestion continues in the small intestine• Pancreatic enzymes (proteases) complete protein digestion• Special sites (small intestine) transport amino acids, dipeptides, tripeptides• High doses of individual amino acid supplements can lead to amino acid toxicity and deficienciesProtein Quality• Methods for estimating protein quality• Chemical score• Protein digestibility corrected amino acid score (PDCAAS)• Animal protein and many soy products are highly digestible (90% absorption)Functions of proteins• Cell growth, repair, maintenance • Enzymes and hormones• Fluid and electrolyte balance• Acid−base balance• Immune system• Energy source• Nutrient transport and storageProtein Adequacy• Nitrogen balance determines protein needs• Positive nitrogen balance• Negative nitrogen balance• In nitrogen balanceRDA for protein• RDA = 0.8 g per kg body weight per day• Recommended percentage of energy is 10−35% of total energy intake • Protein needs are higher during growth and development (children, adolescents, and pregnant/lactating women)Too much protein can be harmful;• High cholesterol and heart disease• Animal-protein-rich diets are associated with high blood cholesterol levels (saturated fat)• Contribution to bone loss• High-protein diets increase calcium excretion and possibly lead to bone loss• Kidney disease• High protein intakes are associated with an increased risk among susceptible individuals• People with diabetes have higher rates of kidney disease and may benefit from a lower-protein diet • Maximum of 2 g of protein per kilogram body weight each day is safe for healthy peopleProtein sources• Meats• Milk-based products• Soy products• Legumes• Whole grains• Nuts• Quorn Vegetarian diets• Vegetarianism: restricting the diet to foods of plant origin• People chose vegetarianism for:• Health benefits• Ecological reasons• Religious reasons• Ethical reasons• Concerns over food safetyHealth benefits of Vegetarianism• Lower fat and total energy intake• Lower blood pressure• Reduced risk of heart disease• Fewer digestive problems• Reduced risk of some cancers• Reduced risk of kidney disease, kidney stones, and gallstonesChallenges of Vegetarian diets• Can be low in some nutrients • Associated with disordered eating• Varied and adequate diet planning • Soy and complementary proteins• Vegetarian Food Guide Pyramid• Special attention to vitamins D, B12, and riboflavin (B2); minerals zinc and iron Protein-energy malnutrition: • caused by inadequate protein and energy intake• Common forms:• Marasmus•
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