CH 6: Proteins and Amino Acids

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Transcript CH 6: Proteins and Amino Acids

CH 6: Proteins and Amino Acids
• 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
Chemical Nature of Proteins
• Proteins are made from 20 different amino acids 9 of
which are essential.
• Each amino acid 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.
• Amino acids all contain the elements: C, H, O, N
• Carbs and lipids do not contain N
• Cannot make protein from carbs and lipids
© 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
© 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
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)
– Plant sources are missing or low in at least one
essential aa (one exception) – page 188
Protein Intake
• Recommended level of intake is expressed 2
– 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
1. Growth, maintenance, and repair of body
Need protein to make muscles, skin, hair, blood
• 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
Protein Function
3. Hormones
Hormones are chemical messengers
Travel to target cells/organs and ilicit a response
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
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
Proteins attract water
Proteins are too big to pass through plasma
membranes or out of capillaries
The water moves in and out of the blood
If protein levels in the blood drop, water leaks
out of the blood into surrounding tissues 
Protein Function
• Fluid Balance
– Generally proteins do not leave their
– Fluids can move among compartments
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 – see page 199
• 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
• 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
• In Zero Nitrogen Balance
– Protein made = protein broken down
• Anabolism = catabolism
– Most adults are in zero nitrogen balance
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
• Page 188
– 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
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