Chapter 5 - Proteins PDF

Protein: Amino acids PREPARED BY: DR HADEEL ALSUFIANI Chapter outline u The chemists view of proteins u Digestion and absorption of proteins u Protein in the body u Protein in foods u Health effects and recommended intakes of protein  The chemists view of proteins u Chemicall...

Protein: Amino acids PREPARED BY: DR HADEEL ALSUFIANI Chapter outline u The chemists view of proteins u Digestion and absorption of proteins u Protein in the body u Protein in foods u Health effects and recommended intakes of protein  The chemists view of proteins u Chemically, proteins contain nitrogen atoms in addition to the same atoms as carbohydrates and lipids- carbon, hydrogen and oxygen. u These nitrogen atoms give the name amino (nitrogen containing) to the amino acids that make the links in the chains of proteins.  Amino acids u All amino acids have the same basic structure u The side groups vary from one amino acid to the next, making proteins more complex than either carbohydrates or lipids. u A protein is made up of 20 different amino acids. u Essential amino acids: Nine essential amino acids Cannot be produced in sufficient quantities to meet physiological needs Must be obtained from food Also called indispensable amino acids u Nonessential amino acids: Can be synthesized in sufficient quantities Also called dispensable amino acids Ø Conditionally Essential Amino Acids: Sometimes a nonessential amino acid becomes essential under special circumstances. For example, the body normally uses the essential amino acid phenylalanine to make tyrosine (a nonessential amino acid). But if the diet fails to supply enough phenylalanine, or if the body cannot make the conversion for some reason (as happens in the inherited disease phenylketonuria), then tyrosine becomes a conditionally essential amino acid  proteins u Cells link amino acids end-to-end in a variety of sequences to form thousands of different proteins u A peptide bond unites each amino acid to the next.  Digestion of proteins u Proteins In foods do not become body proteins directly. u Instead, dietary proteins supply the amino acids from which the body makes its own proteins. u Enzymes break the long polypeptides into short peptides Tripeptides dipeptides individual amino acids.  Protein absorption Active u A number of specific carriers transport transport amino acids (and some dipeptides and tripeptides) into the intestinal cells u Once inside the intestinal cells, amino acids may be used for energy or to synthesize needed compounds. u Amino acids that are not used by the intestinal cells are transported across the cell membrane into the surrounding fluid where they enter the capillaries on their way to the liver. Proteins in the body Protein synthesis:  Roles of proteins Act As As a source Act As Act As Act As Act As regulator of Act As Act As of energy structural acid-base enzymes hormones fluid transporters antibodies and materials regulators balance glucose  Protein In Foods Protein Quality: The protein quality of the diet Protein determines, in large part, how well quality children grow and how well adults maintain their health. Two factors influence protein Amino acid composition quality: 1- the protein's digestibility. 2- Its amino acid composition. Protein digestibility  Two factors influence protein quality: Protein quality 1) protein digestibility: a measure of the amount of amino acids absorbed from a given protein intake. Amino acid composition Factors affect protein digestibility: 1- The protein's source. 2- The other foods eaten with it. Protein digestibility u The digestibility of most animal proteins is high (90 to 99 percent); u plant proteins are less digestible (70 to Protein Other 90 percent for most, but over 90 source foods percent for soy and legumes).  Two factors influence protein quality: 2) Amino Acid Composition: u To make proteins, a cell must have all the needed amino acids available simultaneously. u The liver can produce any nonessential amino acid that Protein quality may be in short supply so that the cells can continue linking amino acids into protein strands. u If an essential amino acid is missing, though, a cell must dismantle its own proteins to obtain it. u Therefore, to prevent protein breakdown, dietary protein must supply at least the nine essential amino acids plus Amino acid composition enough nitrogen-containing amino groups and energy for the synthesis of the others. u If the diet supplies too little of any essential amino acid, protein synthesis will be limited. Protein digestibility u The body makes whole proteins only. If one amino acid is missing, the others cannot form a "partial" protein.  Protein quality High quality Low quality proteins proteins dietary proteins containing all the essential amino acids in relatively the same amounts dietary proteins limiting in one that human beings require. or more essential amino acids. They may also contain nonessential amino acids. Foods derived from animals (meat, fish, poultry, cheese, Food from plants (vegetables, eggs, yogurt, and milk) provide nuts, seeds, grains, and high-quality proteins, except legumes) except, soy protein. gelatin (It lacks tryptophan) Complementary proteins: two or more dietary proteins whose amino acid assortments complement each other in such a way that the essential amino acids missing from one are supplied by the other. isoleucine lysine methionine tryptophan legumes grains together  Health Effects of Protein u It should come as no surprise that protein deficiency can have devastating effects on people’s health. But like the other nutrients, protein in excess can also be harmful. Protein deficiency: u Develops when the diet consistently supplies too little protein or lacks essential amino acids. u When this occur, the synthesis of body proteins decrease and degradation increases to provide cells with amino acids they need. u Without proteins to perform their critical roles, many of the body’s activities come to halt. Ø The consequences of protein deficiency include: slowed growth, impaired brain and kidney functions, poor immunity and inadequate nutrient absorption. protein-energy malnutrition (PEM), also called protein-kcalorie malnutrition (PCM): a deficiency of protein, energy, or both, including kwashiorkor, marasmus. Acute PEM: protein-energy malnutrition caused by recent severe food restriction; characterized in children by thinness for height (wasting). Chronic PEM: protein-energy malnutrition caused by long-term food deprivation; characterized in children by short height for age (stunting). Marasmus (ma-RAZ-mus): a form of PEM that results from a severe deprivation, or impaired absorption, of energy, protein, vitamins, and minerals. u The extreme loss of muscle and fat characteristic of marasmus is apparent in this child's "matchstick" arms. Kwashiorkor (kwash-ee-OR-core): a form of PEM that results either from inadequate protein intake or, more commonly, from infections. u The edema characteristic of kwashiorkor is apparent in this child's swollen belly. Malnourished children commonly have an enlarged abdomen from parasites as well. Overconsumption of protein: u offers no benefits and may pose health risks. u High-protein diets have been implicated in several chronic diseases, including: Heart Kidney cancer Osteoporosis obesity disease stones Heart Disease : u Foods rich in animal protein also tend to be rich in saturated fats. u On the other hand, vegetable protein (legumes and nuts), low-fat milk and fish may improve blood pressure and blood lipids and decrease heart disease mortality. Cancer u Population studies suggest a correlation between high intakes of red meats and processed meats and some types of cancer (cancer of the colon, pancreas, and ovaries). u In contrast, protein-rich legumes, fish and milk may lower the risk of some cancers. Adult Bone Loss (Osteoporosis): When protein intake is high, calcium excretion increases. Weight Control: Protein-rich foods are often fat-rich foods that contribute to weight gain with its accompanying health risks. Kidney Disease: u Excretion of the end products of protein metabolism depends on an adequate fluid intake and healthy kidneys. u A high protein intake increases the work of the kidneys, but does not appear to diminish kidney function u Restricting dietary protein, may help to slow the progression of kidney disease and limit the formation of kidney stones in people who have chronic kidney diseases.  Recommended intakes of protein RDA for protein: 0.8 g/kg of healthy body weight/day 10 to 35 % of energy intake