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Chapter 6
The Proteins and
Amino Acids
Ask Yourself
TRUE OR FALSE?
1. When more protein is eaten than the body
needs, it is stored intact in the body (the way
fat is stored) so that it can be used when a
person’s diet falls short of supplying the
day’s need for essential proteins.
2. No new living tissue can be built without
protein.
3. Whenever cells are lost, protein is lost.
4. All enzymes and hormones are made of
protein.
5. When antibodies enter the body, they
produce illness.
Ask Yourself
6. When a person doesn’t eat enough food to
meet the body’s energy needs, the body
devours its own protein tissue.
7. Once the body has assembled its proteins
into body structures, it never lets go of
them.
8. Milk protein is the standard against which
the quality of other proteins is usually
measured.
9. It is impossible to consume too much
protein.
10.People who eat no meat have to eat a lot of
special foods to get enough protein.
What Proteins Are Made Of
• Proteins
Compounds—composed of
atoms of carbon, hydrogen,
oxygen, and nitrogen—
arranged as strands of amino
acids. Some amino acids also
contain atoms of sulfur.
 Amino acids
Building blocks of protein;
each is a compound with an
amine group at one end, an
acid group at the other, and a
distinctive side chain.
 Amine group
The nitrogen-containing
portion of an amino acid.
An Amino Acid: glycine
Acid
group
Amine
group
Side
group
An Amino Acid: phenylalanine
What Proteins Are Made Of
Nine essential amino
acids:
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• Essential amino acids:
Amino acids that cannot be
synthesized by the body or
that cannot be synthesized in
amounts sufficient to meet
physiological need.
Histidine
Isoleucine
Leucine
Lysine
Methionine
Phenylalanine
Threonine
Tryptophan
Valine
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What Proteins Are Made Of
The nonessential amino acids are
also important in nutrition:
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Alanine
Arginine
Asparagine
Aspartic acid
Cysteine
Glutamic Acid
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Glutamine
Glycine
Proline
Serine
Tyrosine
What Proteins Are Made Of
• Protein synthesis: the
process by which cells
assemble amino acids into
proteins.
 Each individual is unique
because of minute
differences in the ways his
or her body proteins are
made.
 The instructions for
making every protein in a
person’s body are
transmitted in the genetic
information the person
receives at conception.
• Peptide bond:
A bond that connects one
amino acid with another.
An Insulin Molecule
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Peptide
bond
A Molecule of Insulin
What Proteins Are Made Of
• Proteins are made of many different amino
acid units hooked to each other.
 Strands of proteins are tangled chains,
globular in structure.
• The differing shapes of proteins enable them
to perform different tasks in the body.
 Proteins may repel or attract water.
 Some proteins contain minerals or vitamins.
 Several proteins may gather to form a
functional group.
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What Proteins Are Made Of
• Denaturation:
The change in shape of a
protein brought about by
heat, alcohol, acids,
bases, salts of heavy
metals, or other agents.
 First step in the protein’s
breakdown.
 Useful to the body in
digestion.
 Stomach acid opens up
the protein’s structure,
allowing digestive
enzymes to cleave peptide
bonds.
Cooking an egg
denatures its
proteins
Functions of Body Proteins
• No living tissue can be built
without protein.
• Protein is part of every living cell.
• Proteins account for about 20% of
our body weight.
• Proteins come in many forms and
perform many vital functions.
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Functions of Body Proteins
• Amino acids are constantly needed for the
body to build the proteins of new tissue
during growth and maintenance.
 Examples of growth: a developing embryo; a
growing child.
 Examples of maintenance: replacing blood lost to
burns, hemorrhage, or surgery; developing scar
tissue that heals wounds; replacing hair or nails;
replacing cells that are worn out.
• Amino acids must constantly be resupplied
by food for new growth to occur.
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Functions of Body Proteins
• Proteins form vital parts of most of
our body structures.
Examples of body structures
include skin, hair, nails,
membranes, muscles, teeth,
bones, organs, ligaments, and
tendons.
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Functions of Body
Proteins
• Enzyme Action
Each enzyme facilitates
a specific chemical
reaction.
A
B
Enzyme plus two compounds, A and B
A
B
Enzyme complexed with A and B
AB
Enzyme plus new compound AB
Functions of Body Proteins
• Hormones
Chemical messengers.
Hormones are secreted by a
variety of glands in the body in
response to altered conditions.
Each affects one or more target
tissues or organs and elicits
specific responses to restore
normal conditions.
Functions of Body Proteins
• Antibodies
Large proteins of the blood and body fluids,
produced by one type of immune cell in
response to invasion of the body by unfamiliar
molecules (mostly foreign proteins).
 Antibodies inactivate the foreign substances
and so protect the body.
 The foreign substances are called antigens.
• Immunity
Specific disease resistance derived from the
immune system’s memory of prior exposure
to specific disease agents and its ability to
mount a swift response against them.
 Malnutrition injures the immune system.
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Functions of Body Proteins
• Fluid balance:
Distribution of fluid
among body
compartments.
• Shown here are the
fluids within and
surrounding a cell.
Body proteins help
hold fluid within cells,
tissues, and blood
vessels.
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Fluid between cells
(intercellular or
interstitial fluid)
Fluid within cell
(intracellular fluid)
Nucleus
Fluid within
blood vessel
(intravascular
fluid)
Cell
Blood vessels
Functions of Body Proteins
• Acid-Base Balance
Equilibrium between acid and base
concentrations in the body fluids.
 Acid-base balance of blood is carefully controlled.
• Normal body processes continually
produce acids and bases.
 Acids
Compounds that release hydrogens in a watery
solution; acids have a low pH.
 Bases
Compounds that accept hydrogens from
solutions; bases have a high pH.
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pH Values of Selected Fluids
A fluid’s acidity or alkalinity is
measured in pH units.
pH
The concentration of
hydrogen ions.
The lower the pH, the
stronger the acid
•pH 2 is a strong acid,
•pH 7 is neutral, and
•pH above 7 is alkaline.
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Functions of Body Proteins
Acid-Base Balance
• Acidosis
Blood acidity above normal, indicating excess acid.
• Alkalosis
Blood alkalinity above normal.
• Buffers
Compounds that help keep a solution’s acidity
(amount of acid) or alkalinity (amount of base)
constant.
 Some proteins act as buffers to maintain normal
blood pH.
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Functions of Body Proteins
Transport Proteins specialize in
moving nutrients and molecules
into and out of cells.
The “sodium-potassium pump” is
switched on and off by hormones.
Special proteins carry vitamins,
minerals.
Lipoproteins carry lipids.
Functions of Body Proteins
Protein As Energy:
• In the absence of adequate energy, the
body will sacrifice protein to provide
energy.
• The amine group will be degraded,
incorporated by the liver into urea, and
sent to the kidneys for excretion in urine.
 Urea
The principal nitrogen excretion product of
metabolism, generated mostly by the removal
of amine groups from unneeded amino acids or
from those amino acids being sacrificed to a
need for energy.
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Functions of Body Proteins
Protein As Energy:
• After the amine group is removed, the
remaining carbon, hydrogen, and oxygen
will be used for immediate energy.
 Protein sparing
A description of the effect of carbohydrate and fat,
which, by being available to yield energy, allow
amino acids to be used to build body proteins.
• Excess amino acids are not stored by the
body.
 After removing the amine group, the excess is
converted to glycogen or fat for energy storage.
Mouth
Salivary
glands
1. In the mouth, chewing
crushes and softens
protein-rich foods and
mixes them with saliva.
2. Stomach acid works
to uncoil (denature)
Liver
protein strands and
activate stomach
enzymes. The enzyme
pepsin breaks the
protein strands into
dipeptides, tripeptides,
Gallbladder
and polypeptides. A
mucous coating on
Small
the stomach wall
intestine
protects the
stomach’s own
proteins from both
the harsh stomach
Large
acid and the protein
intestine
digesting enzymes.
Stomach
Pancreas
3. In the small intestine,
the fragments of protein
are split into free amino
acids, dipeptides, and
tripeptides with the help
of enzymes from the
pancreas and small
intestine. Enzymes on
the surface of the small
intestinal cells break
these peptides into
amino acids, and they
are absorbed through
the microvilli of the small
intestine into the blood.
4. The large intestine
carries any undigested
protein residue out of
the body. Normally,
practically all the
protein is digested
and absorbed.
How the Body Handles Protein
• By the time proteins slip
into the small intestine,
they are already broken
into different-sized
pieces.
 Some single amino acids
and many strands of
two, three, or more
amino acids.
 There are dipeptides,
tripeptides, and longer
chains.
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How the Body Handles Protein
• Amino acids pass from the absorptive cells in
the small intestine into the bloodstream,
where they can be taken up by cells and used
to make proteins.
• Amino acid strands are assembled to make
proteins.
 Nonessential amino acids can be
manufactured by the body from fragments of
other amino acids.
 If essential amino acids are missing, protein
building will halt.
Protein Quality of Foods
• Important characteristics of dietary protein:
 Should supply at least the essential amino
acids.
 Should supply enough other amino acids to
make nitrogen available for synthesizing other
nonessential amino acids.
 Should provide adequate food energy to
prevent sacrifice of amino acids for energy.
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Protein Quality of Foods
• Complete proteins
Proteins containing all the essential amino acids in
the right proportion relative to need. The quality of a
food protein is judged by the proportions of essential
amino acids that it contains relative to our needs.
Animal proteins are the highest in quality.
• Incomplete protein
A protein lacking or low in one or more of the
essential amino acids.
• Limiting amino acid
A term given to the essential amino acid in shortest
supply (relative to the body’s need) in a food
protein; it therefore limits the body’s ability to make
its own proteins.
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Protein Quality of Foods
• Complementary
proteins
Two or more food
proteins whose amino
acid assortments
complement each other
in such a way that the
essential amino acids
limited in or missing
from each are supplied
by the others.
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Both meals shown supply an
adequate assortment of amino
acids
HOW TWO PLANT PROTEINS COMBINE TO
YIELD A COMPLETE PROTEIN:
• Two incomplete proteins (for
example, legumes plus grains)
can be combined to equal a
complete protein (peanut
butter sandwich). In this
example,
 The peanut butter provides
adequate amounts of the amino
acid lysine, but is lacking in
methionine.
 The bread “complements” the
peanut butter because it contains
adequate methionine, but is
lacking in lysine.
• When combined as a sandwich,
all essential amino acids are
present.
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Protein Quality of Foods
• Protein quality
A measure of the essential amino acid content of
a protein relative to the essential amino acid
needs of the body.
• Protein digestibility-corrected amino acid score
(PDCAAS)
• A measuring tool for determining protein quality.
PDCAAS reflects both a protein’s digestibility and
its proportion of amino acids relative to human
needs.
 Foods earning a perfect score of “100” include
egg white, fat-free milk, tuna, beef, and chicken.
 Soy earned a score of 94.
 Most legumes have scores of 50-60.
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Protein Quality of Foods
• Biological value (BV)
A measure of protein quality,
assessed by determining how well
a given food or food mixture
supports nitrogen retention.
• Reference protein
Egg white protein, the standard
with which other proteins are
compared to determine protein
quality.
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Recommended Protein Intakes
• Recommended protein intakes can be
stated by two methods.
• As a percentage of total calories:
 Protein should provide 10%-35% of
total calories.
• As an absolute number (grams per
day).
 A healthy adult should consume 0.8
gram per kilogram of desirable body
weight per day.
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Recommended Protein Intakes
To calculate the percentage of calories you
derive from protein:
1. Use your total calories as the denominator
(example: 1,900 cal).
2. Multiply your total protein intake in grams by 4
cal/g to obtain calories from protein as the
numerator (example:
70 g protein  4 cal/g = 280 cal).
3. Divide to obtain a decimal, multiply by 100,
and round off (example:
280/1,900  100 = 15% cal from protein).
Recommended Protein Intakes
To figure your recommended protein intake (RDA)
1. Find the desirable weight for a person your height
(see Appendix A). Assume this weight is appropriate
for you.
2. Change pounds to kilograms (divide pounds by 2.2;
one kilogram = 2.2 pounds).
3. Multiply kilograms by 0.8 g/kg.
 Example (for a 5’8” male):
1. Desirable weight: about 150 lb.
2. 150 lb. divide by 2.2 lb. = 68 kg (rounded off).
3. 68 kg  0.8 g/kg = 54 g protein (rounded off).
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Protein and Health
• Protein deficiency and energy deficiency
go hand in hand so often that public
health officials have given a nickname
to the pair.
 Protein-energy malnutrition (PEM),
also called protein-calorie
malnutrition (PCM)
The world’s most widespread
malnutrition problem, including both
kwashiorkor and marasmus.
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Protein and Health
• Kwashiorkor
A deficiency disease
caused by inadequate
protein in the presence of
adequate food energy.
• Edema
Swelling of body tissue
caused by leakage of fluid
from the blood vessels,
seen in (among other
conditions) protein
deficiency.
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Protein and Health
• Marasmus
An energy deficiency disease;
starvation.
• Dysentery
An infection of the digestive
tract that causes diarrhea.
• Acquired immune
deficiency syndrome
(AIDS)
An immune system disorder
caused by the human
immunodeficiency virus
(HIV).
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Protein and Health
Too Much Protein
• The problems of protein excess can be found
in developed countries.
 Possible to overload the liver and kidneys.
 Can promote calcium excretion.
 Excess protein can be converted to energy and
stored as body fat.
 No apparent benefit to consuming too much
protein when caloric intake is adequate.
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Protein and Health
• Foods that supply
protein in abundance
are shown here in the
Milk Group and the
Meat & Beans Group of
the MyPyramid Food
Guide (top two
photos).
• Servings of foods from
the Vegetable Group
and the Grains Group
can also contribute
protein to the diet
(bottom two photos).
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Protein and Health
• American women consume about 60-65 grams of
protein per day.
 Recommended intake is about 46 grams a day.
• Young American men consume about 100 grams
of protein per day and drop to about 75-85
grams as older adults.
 Recommended protein intake is 52-56 grams.
• More than 65% of this protein comes from animal
sources, which are higher in saturated fat.
• To better balance food choices, aim to select onethird or less of protein from animal sources and
the rest from plant sources.
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Eat More Beans
1. Enjoy adding
more legumes to
your weekly
meals.
2. Explore the many
varieties of
legumes used in
cooking
3. Learn more about
legumes online
Protein and Health
• Legumes
Plants of the bean and pea family
having roots with nodules that contain
bacteria that can trap nitrogen from the
air in the soil and make it into
compounds that become part of the
seed.
• The seeds are rich in high-quality
protein compared with those of most
other plant foods.
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Eat Well Be Well
Make Over Your Plate:
• In the Kitchen:
 Small meat portions tend to
work best mixed into dishes with
lots of vegetables and grains…
• In the Lunch Box:
 Take a thermos filled with chili,
vegetable soup, or a milk-based
soup, such as cream of tomato,
prepared with nonfat milk
instead of a sandwich…
• At the Table:
 Make whole grains, vegetables,
and legumes the main event of
your meals…
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The Vegetarian Diet
• Well-planned, plant-based
meals consisting of:
 A variety of whole grains
 Legumes and nuts
 Vegetables and fruits
 Eggs and dairy products
(for some vegetarians)
• Can offer sound nutrition
and health benefits to
vegetarians and nonvegetarians alike.
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The Vegetarian Diet
• Vitamin D, Iron, Zinc are concerns
in the vegetarian diet
• Vegetarian diets are high in fiber,
richer in some vitamins/minerals,
and lower in fat
• Vegan diets require special efforts
to meet calcium needs and
possibly Riboflavin needs
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Food Allergy—Nothing to Sneeze At
• The top eight foods
causing adverse
reactions in some
individuals
•Milk
•Eggs
•Peanuts
•Nuts
•Fish
•Shellfish
•Soy
•Wheat
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Wonder Bean--The Benefits of Soy
Wonder Bean: The Benefits of Soy
• A legume, high in fiber and
protein, somewhat low in
carbohydrates
• Contain isoflavones possibly
a key factor in the diseasefighting potential
• The new information about
soy seems promising.
• No recommended daily
amount of soy yet
• One serving of soy per day
may improve your health.
• Only vegetable that is a
complete protein
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