Chapter 6
The
Proteins
and Amino
Acids
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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 (a-MEEN-o) 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 (a-MEEN) group: the
nitrogen-containing portion of
an amino acid.
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An Amino Acid: glycine
Acid
group
Amine
group
Side
group
An Amino Acid: phenylalanine
p. 172
What Proteins Are Made Of
Nine essential amino
acids:
•
•
•
•
•
•
•
•
•
Histidine
Isoleucine
Leucine
Lysine
Methionine
Phenylalanine
Threonine
Tryptophan
Valine
• Essential amino
acids: amino acids
that cannot be
synthesized by the body
or that cannot be
synthesized in amounts
sufficient to meet
physiological need.
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What Proteins Are Made Of
The nonessential amino acids are
also important in nutrition:
•
•
•
•
•
•
Alanine
Arginine
Asparagine
Aspartic acid
Cysteine
Glutamic Acid
•
•
•
•
•
Glutamine
Glycine
Proline
Serine
Tyrosine
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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.
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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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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.
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A
B
Enzyme plus two compounds, A and B
A
B
Enzyme complexed with A and B
AB
Enzyme plus new compound AB
p. 174
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.
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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
p. 175
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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Functions of Body Proteins
Acid-Base Balance
• Acidosis (a-sih-DOSE-sis): blood acidity
above normal, indicating excess acid.
• Alkalosis (al-kah-LOH-sis): 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.
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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 (yoo-REE-uh): 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.
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A summary of protein digestion & absorption
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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
Both meals shown supply an
in such a way that the
essential amino acids
adequate assortment of
limited in or missing from
amino acids
each are supplied by the
others.
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Protein Quality of Foods
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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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).
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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 (kwashee-OR-core): a
deficiency disease caused
by inadequate protein in
the presence of adequate
food energy.
• Edema (eh-DEEM-uh):
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 (ma-RAZ-
mus): an energy deficiency
disease; starvation.
• Dysentery (DISS-enterry): 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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Eat More Beans
1. Enjoy adding
more legumes to
your weekly
meals.
2. Enjoy a variety
of dried, beans,
peas, and lentils
when dining out.
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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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