CH 6: Proteins

Proteins Consider last as protein is the body’s least desirable source of energy 4 kcal/gram When body uses protein for energy it is not available to perform other critical functions No protein stores – all protein has a function in the body We’ll come back to this soon

Chemical Nature of Proteins Proteins are made from amino acids (aa) There are 20 different aa Amino acids all contain: C, H, O, N Carbs and lipids do not contain N Cannot make protein from carbs and lipids Each aa has an amino group, an acid group, a hydrogen atom, and a side group. It is the side group that makes each amino acid unique.

© 2008 Thomson - Wadsworth

Chemical Nature of Proteins Proteins are chains of amino acids (a.a.) joined by peptide bonds Order of the a.a. is determined by your DNA The sequence of amino acids in each protein determines its unique shape and function. For the protein to function the amino acids must be in the correct order and the chain must fold up properly

© 2008 Thomson - Wadsworth

© 2008 Thomson - Wadsworth

Amino Acids The body cannot make 9 of the amino acids – these are the essential amino acids These amino acids must be supplied by the diet The body can make 11 of the amino acids These are the nonessential a.a. Some amino acids are conditionally essential, required under certain conditions Page 212

Amino Acids Diet must provide all 9 of the essential a.a. on a regular/daily basis for proteins to be made Need all 20 a.a. to make most proteins Animal sources of proteins contain all 9 essential aa (one exception, gelatin) Plant sources are missing or low in at least one essential aa (one exception, soy)

Protein Intake Recommended level of intake is expressed 2 ways: 10 – 25 % daily caloric intake 0.8 grams protein per kg body weight (RDA) Computer programs use this value Most meet this level EASILY

Protein Function page 223 Growth, maintenance, and repair of body tissue Need protein to make muscles, skin, hair, blood vessels…….. Cannot grow without protein Replace lining of GI tract every 3 days (maintenance example)

Protein Function Enzymes Enzymes are proteins that catalyze (speed up) chemical reactions Every reaction in the body requires a specific enzyme

Protein Function Hormones Hormones are chemical messengers Travel to target cells/organs and ilicit a response Examples: Insulin and glucagon Growth hormone Thyroxin – regulates metabolic rate

Protein Function Antibodies Component of immune system Attack foreign substances in the body

Protein Function Transportation of Substances Proteins are needed to transport lipids in the blood Proteins are needed to transport substances across cell membranes Need for nerve and muscle contraction Proteins are needed to absorb calcium and iron

Protein Function Fluid Balance – page 222 Proteins attract water Proteins are too big to pass through plasma membranes or out of capillaries Water moves between the compartment If protein levels in the blood drop, water leaks out of the blood into surrounding tissues  edema

Protein Function Edema occurs when there are inadequate plasma proteins This occurs when: Liver disease Inadequate protein/food intake Kidney disease  blood proteins excreted Injury breaks open cells

Protein Function Acid Base Balance Proteins can act as acids and bases Help keep body fluids at a safe pH Serve as buffers in the body

Protein Function Energy 4 kcal/gram Use as a source of energy when glucose/glycogen stores are empty

Amino Acids The body regularly breaks down proteins and remakes them or uses them for energy as needed – page 224 If an essential a.a. is missing the body cannot make all of the proteins it needs Hair and nails may grow more slowly Immune system compromised (antibodies are proteins)

Nitrogen Balance pg 225 Positive Nitrogen Balance Making more protein than breaking down Anabolism > catabolism Pregnant women Infants and children Athletes (building muscle) Recovering from surgery, injury, or illness

Nitrogen Balance Negative Nitrogen Balance Breaking down more protein than you are making Catabolism > anabolism Illness, fever Burn victims Starvation/anorexia

Nitrogen Balance/Equilibrium In Zero Nitrogen Balance Protein made = protein broken down Anabolism = catabolism Most healthy adults are in nitrogen balance/equilibrium

Protein in the Diet Complete Proteins Food Sources Contain all 9 essential a.a. in adequate amounts Food Sources Most animal sources of protein are complete proteins (exception is gelatin) Meat, poultry, fish, eggs, dairy Soy products (plant source of complete protein)

Protein in the Diet Incomplete Proteins Food Sources Lack or are low in 1 or more of the essential a.a. Food Sources Most plant sources are incomplete proteins Nuts, seeds, legumes, grains, and vegetables Gelatin

Protein in the Diet Complementary Proteins Two or more protein sources that together provide all 9 of the essential a.a. Most combinations of at least 2 categories of plant proteins will complement each other Seeds and nuts (with legumes) Grains (with legumes) Legumes (with seeds, nuts, grains) Vegetables (with grains, nuts, seeds)

© 2008 Thomson - Wadsworth

Protein in the Diet Examples of meals/foods containing complementary proteins: Rice and beans …..

Protein Digestion Digestion Mouth Stomach Protein is denatured by hydrochloric acid. Pepsinogen is converted into its active form pepsin by hydrochloric acid. Pepsin chemically breaks proteins into smaller polypeptides

Protein Digestion In the Small Intestine Pancreatic enzymes (proteases) digest protein into short peptide chains called oligopeptides, which contain four to nine amino acids. SI enzymes (peptidases) split proteins into amino acids.

Protein Absorption Amino acids are absorbed into the cells of the SI and enter the blood Amino acids are transported to the liver for processing