Chapter 6: Proteins & Amino Acids

CHAPTER 6: PROTEINS & AMINO ACIDS LEARNING OBJECTIVES At the end of this chapter, you should be able to: Describe the structure of proteins and amino acids and explain their role in health and disease Plan a diet to meet protein recommendations

THINK about this – then share within a PAIR – then SHARE with the class What do you know about proteins and amino acids? What are benefits of eating plant foods over animal foods?

Sources of protein Animal: meat, eggs, dairy products High in saturated fat and cholesterol; provide B vitamins and some absorbable minerals Plant: grains, nuts, legumes Low in saturated fat and cholesterol; high in fiber, phytochemicals, and unsaturated fat Legumes: starchy plant seeds producing bean pods, including peas, peanuts, beans, soybeans, lentils

Sources of protein a. Animal products are high in protein, iron, zinc, and calcium but add saturated fat and cholesterol to the diet.

Sources of protein Michael Newman/Photo Edit b. Plant sources of protein are rich in fibre, phytochemicals, and monounsaturated and polyunsaturated fats.

Amino acids Amino acids: building blocks of proteins Carbon, hydrogen, amino group (contains nitrogen), acid group, and a unique side chain 20 different side chains make 20 different amino acids

Amino acids link to form proteins We obtain amino acids from eating plant and animal foods. Our bodies reassemble the amino acid building blocks into our body proteins.

Match the building blocks with the nutrient: ____ complex carbohydrates a) amino acids ____ triglycerides b) fatty acids and glycerol ____ proteins c) monosaccharides

Essential vs. non-essential amino acids Essential/indispensable: Cannot be made in the body so “essential” to eat them If one is missing, body proteins are broken down to make new proteins Non-essential/dispensable: Body can make them from essential amino acids so not essential to eat them

Essential vs. non-essential amino acids b. Twenty amino acids are commonly found in proteins. The table shown here lists them based on whether they are essential or nonessential and indicates those that are conditionally essential.

Essential vs. non-essential amino acids Food A Food B

Essential vs. non-essential amino acids Can you use only the amino acids from Food A to make this protein?

Essential vs. non-essential amino acids Can you use only the amino acids from Food A to make this protein? The yellow triangle is a non-essential amino acid

Essential vs. non-essential amino acids Can you use only the amino acids from Food A to make this protein? The blue square is a limiting amino acid

Essential vs. non-essential amino acids Can you use only the amino acids from Food A to make this protein?

Essential vs. non-essential amino acids Can you use only the amino acids from Food B to make this protein?

Essential vs. non-essential amino acids Can you use only the amino acids from Foods A and B to make this protein?

Essential vs. non-essential amino acids Did you use all of the amino acids from Foods A and B to make this protein? It not, what did the body do with the extras?

Essential vs. non-essential amino acids Foods A and B are complementary: each supplies some amino acids for the full set of needed amino acids

Essential vs. non-essential amino acids Cannot be made Need to be eaten Non-essential amino acids Can be made from others

Conditionally essential amino acids Under certain conditions some of the nonessential amino acids cannot be made in the body Example: in phenylketonuria (PKU) phenylalanine cannot be converted to tyrosine thus tyrosine, a nonessential amino acid in healthy individuals, becomes an essential amino acid in people with PKU

Protein structure Peptide bonds: join the acidic group of one amino acid with the amino group of another amino acid Polypeptides: a chain of amino acids linked by peptide bonds Proteins: made of one or more polypeptide chains folded into specific three-dimensional structures

Amino acids link to form proteins

Protein structure vs. function Shape of proteins determines their functions If protein shape is changed, then protein function may change

Protein denaturation Denaturation: change in a protein’s three-dimensional shape Occurs with: Heat from cooking Acidity (example: low pH in the stomach) Mechanical agitation

Protein denaturation When an egg is cooked, the heat denatures the protein. The protein in a raw egg white forms a clear, viscous liquid, but when cooking denatures it, the egg white becomes white and firm and cannot be restored to its original form. Other factors that denature the proteins in food include mechanical agitation, such as when cream is whipped to make whipped cream, and the addition of acid, such as when milk is curdled by the addition of an acid such as lemon juice. © Can Stock Photo Inc./ glorcza

Concept check Which chemical element is found in protein but not in carbohydrate or lipid? What determines whether an amino acid must be consumed in the diet? What determines the shape of a protein? How does denaturation affect protein function?

Protein digestion Proteins have to be broken down to tripeptides, dipeptides and amino acids, before they can be absorbed into the mucosal cells of the small intestine Mechanical digestion of proteins starts in the mouth, continues in the stomach and the small intestine Chemical digestion of proteins begins in the stomach but most of it occurs in the small intestine

Protein absorption In the mucosal cells of the small intestine dipeptides and tripeptides are broken down to single amino acids Amino acids are moved from the mucosal cells to blood by the transporting system Since the transporting system has a limited capacity, amino acid supplements may reduce the absorption of some dietary amino acids

Protein digestion

Application Insulin is a protein. Why do diabetics need to inject insulin rather than take it orally?

Concept check The purple amino acids are absorbed by the same transport system as the green amino acids. If you consume a sports drink that is supplemented with the green amino acids what will happen to the absorption of the purple ones. nothing the larger amount of green ones will limit the absorption of the purple ones the small amount of the purple ones will be absorbed first both will be absorbed equally because the body needs them

Concept check Where does the chemical digestion of protein begin? Why might supplementing one amino acid reduce the absorption of other amino acids?

Amino acid pool Amino acid pool: amino acids available for use in the body From the diet or breakdown of body proteins Used to make other proteins or chemicals (for example: DNA, RNA, histamine, etc.) Used to provide energy or make glucose or fatty acids

Amino acid pool Amino acids enter the available pool from the diet and from the breakdown of body proteins. Of the approximately 300 grams of protein synthesized by the body each day, only about 100 grams are made from amino acids consumed in the diet. The other 200 grams are produced by the recycling of amino acids from protein broken down in the body. Amino acids in the pool can be used to synthesize body proteins and other nitrogen-containing molecules, to provide energy, or to synthesize glucose or fatty acids.

Protein synthesis Protein synthesis is regulated by genes Depending whether a gene is turned on (expressed) or turned off, the protein is synthesized or its synthesis stops Chemicals can turn genes on and off For example, high iron turns on the ferritin gene to make more ferritin which is needed for iron storage

Limiting amino acid A shortage of just one essential amino acid can stop the process of protein synthesis Limiting amino acid is the essential amino acid present in short supply relative to body’s need Nonessential amino acids can be made in the body via transamination Transamination: amino acid group from one amino acid is transformed to carbon-containing molecule to form another amino acid

Protein synthesis

Genes contain instructions for proteins

Protein functions Structure (e.g., skin, hair, muscle) Speed up chemical reactions (enzymes) Chemical signaling (e.g., hormones) Transportation of substances (e.g., blood proteins) Movement (e.g., muscles) Immunity (e.g., antibodies) Blood clotting Fluid balance Maintenance of pH

Protein functions Structure: Collagen, which is the most abundant protein in the body, plays important structural roles. It is the major protein in ligaments, which hold our bones together, and in tendons, which attach muscles to bones, and it forms the protein framework of bones and teeth.

Protein functions Catalyzing reactions: Enzymes, such as this one that speeds up the breakdown of starch, are protein molecules. All chemical reactions occurring within the body require the help of enzymes. The specific structure or shape of each enzyme allows it to interact with the molecules in the specific reaction it accelerates. Without enzymes, metabolic reactions would occur too slowly to support life.

Protein functions Transport: Proteins help transport materials throughout the body and into and out of cells. The protein hemoglobin, which gives these red blood cells their colour, shuttles oxygen to body cells and carries away carbon dioxide. © Can Stock Photo Inc./Eraxion

Protein functions Protection from disease: Young women can now be immunized against the human papilloma virus (HPV). The vaccine contains a small amount of dead or inactivated HPV virus. It does not make a person sick, but it does stimulate the immune system to make proteins called antibodies, which specifically attack the virus and prevent HPV from being contracted. © Can Stock Photo Inc./BVDV

Protein functions Movement: The proteins actin and myosin in the arm and leg muscles of this rock-climber are able to slide past each other to contract the muscles. A similar process causes contraction in the heart muscle and in the muscles of the digestive tract, blood vessels, and body glands © Can Stock Photo Inc./gregepperson

Protein functions Fluid balance: Blood proteins contribute to the number of dissolved particles in the blood. Since proteins are charged particles, they help attract water to keep it in the blood. If protein levels in the blood fall too low, osmosis causes fluid to leak out of the blood vessels and accumulate in the tissues, causing swelling known as edema, shown here. Proteins also regulate fluid balance because some are membrane transporters, which pump dissolved substances from one side of a membrane to the other.

Functions of amino acids Amino acids from the diet are used to make proteins and nitrogen-containing molecules Extra amino acids cannot be stored as proteins Extra amino acids are used for energy or stored as fat

Using proteins for energy When we do not consume enough calories, body proteins are broken down and amino acids used to provide energy Deamination removes amino group (NH2) Produces urea which is excreted in urine Carbon, hydrogen, and oxygen from amino acids can be converted to glucose and can be broken down to produce ATP

Producing ATP from amino acids

Protein deficiency Protein-energy malnutrition: loss of fat and muscle and decreased immunity from long-term protein and calorie deficiencies Types: Kwashiorkor: pure protein deficiency Marasmus: overall protein-energy deficiency

Protein deficiency: Kwashiorkor Kwashiorkor, seen in this child from Haiti, is characterized by a swollen belly, which results from fluid accumulating in the abdomen and fat accumulating in the liver. Growth is impaired, but because energy intake is not necessarily low, the child may not appear extremely thin. The lack of protein also causes poor immune function and an increase in infections, changes in hair colour, and impaired nutrient absorption.

Protein deficiency: Marasmus Marasmus, seen in this Somalian boy, is characterized by depletion of fat stores and wasting of muscle. It has devastating effects on infants because most brain growth takes place in the first year of life; malnutrition in the first year causes decreases in intelligence and learning ability that persist throughout the individual’s life.

Concept check Why does protein synthesis stop when the supply of an amino acid is limited? How does the body know in what order to assemble the amino acids when making a protein? When is protein used as an energy source?

High-protein diets Excess dietary amino acids can be deaminated and converted to fatty acids and stored as triglycerides in adipose tissue Increased urea output Increased demands on the kidneys Increased loss of water from the body Possible increased loss of calcium Increased risk of kidney stones

High-protein diets Often: high in animal proteins with high saturated fat and cholesterol and low in fiber low in grains, fruits, and vegetables high in calories and fat Increased risk of heart disease, cancer, obesity, diverticulosis (outpouching of the colon wall) and diabetes mellitus

Diverticulosis CNRI/ Science Source simononly

Concept check What are consequences of low or high protein diets?

Food allergies Food allergy: an adverse immune response to a specific food product

Protein allergies When a protein is absorbed intact, the immune system can recognize it as an antigen and start cellular reactions The second time immune system detects that same allergen (allergy-causing antigen), there is an allergic reaction Major food allergens: milk, eggs, peanuts, tree nuts, fish, shellfish, soy, and wheat

Food sensitivities Food intolerance or food sensitivity: an adverse reaction to food; usually does not involve production of antibodies Triggers various symptoms after consumption Examples: MSG symptom complex or Chinese restaurant syndrome Gluten intolerance or Celiac disease Ingestion of gluten in rye, wheat or barley triggers an autoimmune reaction against villi in the small intestines

Food allergy labelling

Protein balance Protein intake recommendations are based on nitrogen balance Nitrogen balance: the amount of nitrogen consumed versus nitrogen excreted

Nitrogen balance Nitrogen balance: nitrogen intake equals nitrogen loss Maintenance of body protein and weight Negative nitrogen balance: more nitrogen lost than consumed From illness, injury, or decreased consumption Positive nitrogen balance: more nitrogen consumed than lost During growth, pregnancy, or weight training

Nitrogen balance Negative nitrogen Nitrogen balance Positive nitrogen

Recommended daily allowance (RDA): 0.8 g/kilogram of body weight for adults With more weight, more protein is needed for maintenance and repair 70 kg (154 lb) adult = 56 g of protein/day Typical Canadian adult consumes less than 80 g of protein/day Higher needs in infants, children, during pregnancy and lactation (breast feeding), after injury, and in athletes Pregnant/lactating women: add 25 g of protein/day

Recommended protein intake

Protein intake in athletes Endurance athletes, like triathlete Simon Whitfield, need more protein than others because some protein is used for energy and to maintain blood glucose during endurance events, such as triathlons. Endurance athletes may benefit from the daily consumption of 1.2 to 1.4 g of protein/kilogram of body weight. Andy Lyons/Staff/Getty Images Sport Strength athletes, such as weight lifter Marie-Eve Beauchemin-Nadeau, need extra protein to provide the raw materials needed for muscle growth; 1.2 to 1.7 g kilogram/day is recommended. Laurence Griffiths/Staff/Getty Images Sport

Protein supplements

How much protein does this person need? Calculate 100 pound adult weight in kilograms equals weight in pounds x0.45 kg/lb Protein RDA = 0.8 g/kilogram of body weight How much protein does this person need?

Choosing correct protein intake During the first year of life, growth is rapid, so a large amount of protein is required per unit of body weight. As growth rate slows, requirements per unit of body weight decrease but continue to be greater than adult requirements until age 19. What is the RDA for a 2-year-old child who weighs 14 kilograms? 14 g/day 15.4 g/day 11.2 g/day 21 g/day

Recommended protein intake Acceptable Macronutrient Distribution Range for protein:10% to 35% of calories Protein quality: measure content of essential amino acids compared to body needs Protein digestibility corrected amino acid score (PDCASS): most commonly used for determining protein quality

Protein quality High-quality, or complete dietary proteins Contain all amino acids to meet body’s needs Higher PDCASS More easily digested Animal proteins and soy Incomplete dietary proteins Lower in one or more essential amino acids Most plant proteins Combine complementary proteins

Complementary proteins Protein complementation: combining proteins from different sources to assure presence of all essential amino acids Example: legumes with grains

Complementary proteins The amino acids that are most often limited in plant proteins are lysine (lys), methionine (met), and cysteine (cys). As a general rule, legumes are deficient in methionine and cysteine but high in lysine. Grains, nuts, and seeds are deficient in lysine but high in methionine and cysteine.

Complementary proteins Many traditional diets take advantage of complementary plant proteins, such as lentils and rice or chickpeas and rice in India, rice and beans in Mexico and South America, hummus (chickpeas and sesame seeds) in them Middle East, and bread and peanut butter (peanuts are a legume) in Canada. Complementary proteins do not need to be consumed in the same meal, so eating an assortment of plant foods throughout the day can provide enough of all the essential amino acids. Melanie Acevedo/FoodPix, Getty Images, Inc.

Canada Food Guide Recommendations Choose variety of foods to meet body needs of the essential amino acids

Choosing healthy protein sources

What are some menu items Get protein without too much saturated fat Eat both animal and plant proteins Go with beans What are some menu items you could consume?

Which foods provide the cheapest source of protein? Calculate Which foods provide the cheapest source of protein?

Types of vegetarian diets Vegetarian diets: plant-based diets that eliminate some or all animal foods

Vegetarian diets Reasons to be vegetarian: Limited land and/or resources Health Religion Personal ethics Environmental concerns Benefits: lower cost and healthier Risks: amino acid, mineral, and B vitamin deficiency

Meeting dietary needs with a vegan diet

Debate Should you switch to soy? 

Good sources of soy

Concept check How can nitrogen balance be maintained? What are benefits and risks of consuming a plant-based diet? How can vegetarians avoid nutrient deficiencies?

Think critically Sickle cell anemia is an inherited disease caused by an abnormality in the gene for the protein hemoglobin. It causes red blood cells to take on a sickle shape. Sickle cell hemoglobin differs from normal hemoglobin by one amino acid. Why might this difference change the shape of the hemoglobin? Do you think sickle-shaped red blood cells can travel easily through narrow capillaries? How might this disorder affect the ability to get oxygen to the body’s cells?

What are similarities and differences between: Proteins and amino acids? Plant and animal proteins? Kwashiorkor and marasmus? Vegetarian and vegan diets?

Applications What advice could you give to a loved one about protein consumption to decrease disease risk?

Checking student learning outcomes How do proteins contribute to health and disease? What advice would you give to a loved one about protein consumption?

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