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Composition of Proteins & forms Amino Acids Structure Denaturing Proteins Digestion Just the basics Role & functions Protein requirements Too little and too much protein PEM Marasmus Kwashiokor The building blocks of protein Each has an amine group at one end ◦ The nitrogen-containing part An acid group at the other end A distinctive side chain attached to the carbon at the center of the backbone ◦ Gives identity and chemical nature to each amino acid Essential amino acids ◦ Amino acids that either cannot be synthesized at all by the body or cannot be synthesized in amounts sufficient to meet physiological needs So, where must these essential amino acids come from? ◦ Can only be replenished from foods Conditionally essential amino acid ◦ An amino acids that is normally nonessential ◦ Under special circumstances, when the need for it exceeds that body’s ability to produce it, it must be supplied by the diet Peptide bond ◦ Connects one amino acid to another ◦ Formed between the amine of one amino acid and the acid group of the next amino acid ◦ Forms a straight chain of amino acids with side chains bristling out from the backbone © 2006 Thomson-Wadsworth Denaturation ◦ The irreversible change in a protein’s shape ◦ The first step in the destruction of a protein ◦ Brought about by Heat Acids Bases Alcohols Salts of heavy metals Important to the digestion of food protein ◦ Stomach acids open up a protein’s structure Allows digestive enzymes to make contact with the peptide bonds and cleave them Mouth ◦ Protein is crushed by chewing and moistened with saliva ◦ Nothing happens to the protein until the strong acid of the stomach denatures it Stomach ◦ Acid helps to uncoil the protein’s tangled strands so that molecules of the stomach’s proteindigesting enzyme can attack the peptide bonds The stomach’s protein-digesting enzyme works best in an acidic environment ◦ Lining is protected from acid and enzymes by a mucus coating secreted by stomach cells Small intestine ◦ Receives small denatured pieces of protein from the stomach A few are single amino acids Most are polypeptides Protein fragments of 10+ amino acids bonded together A strand of 4 to 10 amino acids is called an oligopeptide Cells along the small intestine absorb single amino acids ◦ Cells that line the small intestine have enzymes on their surfaces that split most tri- and dipeptides into single amino acids Which are then absorbed Some di- and tripeptides are absorbed into cells ◦ Where they are split into single amino acids before being released into the bloodstream Amino acids in the bloodstream ◦ Carried to the liver Used by the liver or released into the blood to be taken up by other cells ◦ Cells can then use the amino acids to Make their proteins for their own use Make proteins that are released into the lymph or blood for other uses ◦ When necessary body cells can use amino acids for energy Amino acids must be continuously available to build the proteins of new tissue ◦ ◦ ◦ ◦ ◦ ◦ Embryo Muscles of an athlete in training Growing child New blood to replace losses Scar tissue New hair and nails Protein helps replace ◦ Worn-out cells and internal cell structures Red blood cells live only 3-4 months Cells lining the intestinal tract live only 3 days Skin cells are constantly being shed and replaced An amino acid that arrives at a cell can be used is one of several ways ◦ Used to build part of a growing protein ◦ Altered to make another need compound ◦ Dismantled so as to use its amine group to build another amino acid Remainder can be used For fuel Converted to glucose or fat ◦ If the cell is starved for energy and is lacking glucose and fatty acids The amine group will be removed and the remainder used for energy The amine group will ultimately be excreted as urine ◦ If the body has a surplus of amino acids and energy The amino group will be excreted The remainder can be Used for energy Converted to glucose or fat for storage Digestibility of a protein varies from food to food ◦ In general, the amino acids from animal sources are more easily digested and absorbed than those from plant sources Animal sources: 90+% digested and absorbed Legumes: ≈80%-90% digested and absorbed Grains and other plant foods:≈70%-90% digested and absorbed Food preparation and protein digestibility ◦ Cooking with moist heat improves protein digestibility ◦ Cooking with dry heat can impair protein digestibility High-quality proteins ◦ Dietary proteins containing all essential amino acids ◦ May also contain nonessential amino acids Complementary proteins ◦ Two or more proteins whose amino acids complement each other; the essential amino acids missing from one are supplied by the other Mutual supplementation ◦ The strategy of combining two incomplete proteins sources so that the amino acids in one food make up for those lacking in the other food Supporting growth and development Building Enzymes, Hormones and other compounds Maintaining Fluid and Electrolyte Balance Maintaining Acid-Base Balance Clotting of blood Providing energy and Glucose Proteins ◦ Composed of carbon, hydrogen, oxygen & nitrogen; arranged as strands of amino acids ◦ Versatile and vital cellular working molecules Enzymes Antibodies Cellular transport Hormones Cellular “pumps” Oxygen carriers ◦ Some are structural molecules Tendons Ligaments Scars Fibers of muscles Cores of bone and teeth Filaments of hair Materials of nails DRI ◦ Designed to cover the need to replace proteincontaining tissue that healthy adults lose and wear out every day Depends on body size: larger people have a higher protein need ◦ For adults: 0.8g/kg body weight/day ◦ Minimum: 10% of total calories ◦ Athletes may need slightly more Increased need is well covered by a regular diet Nitrogen balance ◦ The amount of nitrogen consumed compared with the amount excreted in a given time period ◦ Studies of nitrogen balance underlie the DRI Committee’s recommendations Under normal circumstances, healthy adults are in nitrogen equilibrium = zero balance Positive nitrogen balance Nitrogen in > nitrogen out Negative nitrogen balance Nitrogen in < nitrogen out Protein deficiencies, along with energy deficiencies, are the world’s leading form of malnutrition Protein-energy malnutrition (PEM) ◦ World’s most widespread malnutrition problem ◦ Includes both marasmus and kwashiorkor and states in which they overlap Over 500 million children face imminent starvation, severe malnutrition and hunger ◦ Most of the 33,000 children who die each day are malnourished ◦ Prevalent in Africa, central America, South America, the Middle East, East and Southeast Asia Developing countries, including those in North America, are not immune to it ◦ Chronic inadequate food intake Inadequate energy vitamin mineral protein intake ◦ Person is shriveled ◦ Lean all over Marasmus ◦ Occurs most commonly in children from 6 – 18 mo ◦ Children in impoverished nations subsist on a weak cereal drink with scant energy and protein of low quality A starving child often looks like a little old person Just skin and bones ◦ Without adequate nutrition Muscles, including the heart, waste and weaken Brain development is stunted; learning is impaired Metabolism is so slow that body temperature is subnormal Little or no fat under the skin to insulate against the cold ◦ Child engages in as little physical activity as possible Not even crying for food ◦ Body cuts down on any expenditure of protein not needed for the heart, lungs, and brain to function ◦ Growth ceases ◦ Skin loses its elasticity and moisture Tends to crack, sores develop and fail to heal ◦ Digestive enzymes are in short supply Digestive tract lining deteriorates and absorption fails ◦ Blood proteins, including hemoglobin, are no longer produced Child becomes anemic and weak ◦ Lack the protein needed to heal a broken bone ◦ Antibodies are degraded to provide amino acids for other uses ◦ Leaving the child an easy target for infection Dysentery An infection of the digestive tract hat causes diarrhea Depletes the body of nutrients, especially minerals Measles Might make a healthy child sick for 1-2 weeks Kills a child with PEM within 2-3 days Ultimately, marasmus progresses to the point of no return ◦ The body’s machinery for protein synthesis, itself made of protein, has been degraded ◦ At this point, attempts to prevent death by giving food or protein fail Physical Presentation ◦ Swollen belly ◦ Skin rash ◦ Severe acute malnutrition Too little protein In countries where kwashiorkor is prevalent, each baby is weaned from breast milk as soon as the next one comes along ◦ The older baby no longer receives breast milk, which contains high-quality protein ◦ Instead, is given a watery cereal with scant protein of low quality Rare in the United States, but not totally unknown Kwashiorkor Symptoms ◦ Resemble those of marasmus Often without severe wasting of body fat ◦ Proteins and hormones that previously maintained fluid balance are diminished Fluids leak out of the blood and accumulate in the belly and legs Causing edema Kwashiorkor Symptoms ◦ Belly often bulges with a fatty liver Caused by lack of the protein carriers that transport fat out of the liver Fatty liver loses some of its ability to clear poisons from the body, prolonging their toxic effects ◦ Without sufficient tyrosine to make melanin, the child’s hair loses its normal color ◦ Inadequate protein synthesis leaves the skin patchy and scaly ◦ Sores fail to heal Once an infection sets in kwashiorkor often follows and the immune system weakens further Infections that occur with malnutrition are responsible for two-thirds of the deaths of young children in developing nations If caught in time, the starvation of a child can be reversed by careful nutrition therapy ◦ Fluid balances are most critical Diarrhea will have depleted the body’s potassium And upset other electrolyte balances Electrolyte imbalances, anemia, fever, and infection often lead to heart failure and sudden death Correction of fluid and electrolyte balances usually raises blood pressure and strengthens the heartbeat within a few days PEM at Home ◦ Occurs among some groups in the United States and Canada The poor living On U.S. Indian reservations In inner cities In rural areas Many elderly people Hungry and homeless children Those suffering from anorexia nervosa At risk for PEM: ◦ Those with wasting diseases Such as AIDS and cancer ◦ Those addicted to drugs and alcohol PEM and serious illnesses worsen each other ◦ Treating the PEM often reduces medical complications and suffering even when the underlying disease is untreatable Overconsumption of protein ◦ No health benefits ◦ May pose health risks for the Heart Kidneys Bones ◦ Often associated with obesity ◦ Animal protein sources can be high in saturated fat atherosclerosis and heart disease Protein-Rich Foods ◦ Protein is critical in nutrition Too many protein-rich foods can displace other important foods from the diet ◦ Foods richest in protein carry with them a characteristic array of vitamins and minerals Including vitamin B12 and iron ◦ Foods rich in protein tend to lack, for example, vitamin C and folate The Advantages of Legumes ◦ The protein of some legumes is of a quality almost comparable to that of meat For practical purposes, the quality of soy protein can be considered the equivalent to that of meat The Advantages of Legumes ◦ Excellent sources of B vitamins Iron Calcium and other minerals ◦ Lack Vitamin A Vitamin C Vitamin B12