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 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
– 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