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

1. 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

2. Enzymes• Enzymes are proteins that catalyze (speed up)

chemical reactions• Every reaction in the body requires a specific

enzyme

Protein Function

3. 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

4. Antibodies• Component of immune system• Attack foreign substances in the body

Protein Function

5. 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

6. 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

7. 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

8. 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 Balancepg 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– 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– 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