Proteins
Functions of Protein:
Building Body Constituents
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All cells contain protein
Many are primarily protein
All body cells break down
Protein is needed for continual repair
and rebuilding
Functions of Protein:
Maintaining Fluid Balance
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Fluid flows from capillary bed into
extracellular space (spaces between cells) to
nourish those cells
Protein stays in the blood vessels (too large
to move through vessel walls)
Protein in the capillaries attracts fluid back
into the blood
If there is inadequate protein, extremities
swell as fluid leaves bloodstream
Functions of Protein:
Acid-Base Balance
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Proteins help regulate acid-base balance in
the blood
pH measures acidity or alkalinity (base)
– pH of 7 = neutral
– pH<7 = acid
– pH>7 = alkaline or base
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Proteins pump ions in and out of cells to
regulate pH
Proteins serve as buffers – compounds that
maintain acidity within a narrow range
Functions of Protein:
Hormones
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The body’s
messenger system
Regulate body
functions including
metabolic rate
(thyroid hormone)
and blood glucose
(insulin and
glucacon)
Functions of Protein:
Enzymes
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Speed chemical reactions
Genetic errors may result in lack of a
key enzyme
– Example: galactosemia, PKU
Functions of Proteins:
Immune Function
White blood
cells
 Antibodies
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Functions of Protein: Can
be Converted to Glucose
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Maintain blood sugar for brain and red
blood cells
Brain uses 19% of the body’s energy
at rest
Amino acids can be converted to
glucose (gluconeogenesis)
Functions of Protein:
Energy
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Provides little energy to the body
except in prolonged exercise
Only carbon backbone used
Deamination
– Liver removes amine group
– Nitrogen converted to urea and excreted
by the kidneys
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Inefficient and expensive energy
source
Protein Structure
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Carbohydrates and lipids: contain
carbon, hydrogen, oxygen
Proteins: contain carbon, hydrogen,
oxygen and nitrogen
Building blocks of proteins are amino
acids
Amino Acid
R
NH2
O
C
C
OH
H
R group
Acid group
Amine group
Features Common to all
Amino Acids
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Central carbon
Amino or amine group
Acid group
Amino Acids
Differentiated by
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Side group (“R” group)
20 different side chains
20 different amino acids
Amino Acids
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Nine “essential” amino acids
– The body cannot make in sufficient
quantities
– Must consume in diet
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Eleven “non-essential” amino acids
– Body can produce from other amino acids
Essential AA
Nonessential AA
Histidine
Alanine
Isoleucine
Arginine
Leucine
Asparagine
Lysine
Aspartic Acid
Methionine
Cysteine
Phenylalanine
Glutamic acid
Threonine
Glutamine
Tryptophan
Glycine
Valine
Proline
Serine
Tyrosine
Protein Turnover
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Protein undergo breakdown and
synthesis
Responds to change
Amino acid can be recycled
Proteins
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Very large molecules
1000’s of amino acids joined
together with peptide bonds
Myglobin →
Protein Shape
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Determined by amino acid sequence
Different amino acids joined →
different shapes
Different shapes → different
functions
Protein Synthesis
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DNA contains coded instructions
Copies of codes are transferred to the
cytoplasm (via mRNA)
Amino acids added one at a time with
aid of tRNA
Protein Synthesis
Protein Organization
H
H
O
N
C OH
CH
SH
SH
CH2
CH2
CH
H2O
H N C OH
H
H
H N
O
CH3
H2O
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Dipeptide
Tripeptide
Oligopeptide
CH
C N C
CH
Peptide bond
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O
CH3
H O
OH
Denaturation
Changes in protein shape
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Non-functioning protein
Not biologically active
Denaturing agents
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Acid
Alkaline substances (bases)
Heat
Agitation
Enzymes
Denaturation of Proteins
Heat/acid/alkaline/enzymes
Results in alteration of the protein’s three
dimensional structure
Protein Deficiency: ProteinEnergy Malnutrition
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Kwashiorkor: the disease that the first
child gets when the new child comes
Older child is converted from breast
milk to protein-deficient diet
Moderate energy deficit with severe
protein deficit
Often overlaid with infections
Symptoms of Kwashiorkor
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Apathy, listlessness, withdrawal
– Protein lacking for brain development
– Lack energy for interaction
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Failure to grow, poor weight gain
– Protein unavailable for building tissue
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Change in hair color
– Melanin (a protein) gives hair color, not
made
Symptoms of
Kwashiorkor (cont)
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Fatty infiltration in the liver
– Lipoproteins are not made, fats
accumulate, liver enlarges, abdomen
protrudes
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Massive edema in the abdomen and
legs
– Protein unavailable to maintain fluid
balance
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Some subcutaneous fat tissue
Kwashiorkor Treatment
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Cure infections, parasites
Adequate protein, energy, vitamins,
minerals
Successful if in time
Fig. 6.10
Marasmus
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Starving to death
Often younger infants
Insufficient protein, energy, nutrients
“skin and bones” appearance
Little or no subcutaneous fat
Reduced brain growth
Marasmus
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Little or no breast-feeding
Formula overdiluted, unsanitary water
Can occur in adults as well as infants
and children
Marasmus: Treatment
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Large amounts of
energy and
protein
Permanent
retardation
common
– Poor brain
growth in the
first year of life
Effect of High Protein Diet
Generally low in
 Dietary fiber
 Some vitamins
 Some minerals
 Phytochemicals
Generally high in
 Saturated fat
Is a High-Protein Diet
Harmful?
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Low in plant foods (fiber), vitamins,
phytochemicals
Intake of animal protein increases risk for
heart disease (high in saturated fat)
Excessive intake of red meat is linked with
colon cancer
Burden on the kidney
Increase calcium loss
National Academy of Sciences recommend
no more than 2 x RDA for protein
Protein in Foods
Plant Protein
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Provide protein, minerals, and dietary
fiber
Contain no cholesterol
Limited saturated fats
Allow a few weeks for the GI tract to
adjust to the higher fiber
Availability of Beano®
Protein Digestion:
Cooking
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Cooking food can be a first step in
protein digestion
Cooking denatures and softens
proteins and softens tough connective
tissue in meat
Cooking makes protein-rich foods
easier to chew, swallow, digest
Also makes protein-rich foods safer to
eat
Digestion of Protein in
the Stomach
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Proteins are denatured by cooking and acid in
the stomach
Gastrin stimulates the release of acid and
pepsin
Pepsin is activated and breaks down proteins
into peptones (shorter chains of amino acids)
Digestion of Protein in
the Small Intestine
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Proteins stimulate the release of CCK
Pancreas release the protein splitting
enzymes: trypsin, chymotrypsin,
and carboxypeptidase into the
duodenum
The enzymes will break peptones into
smaller peptides and amino acids
Peptides and amino acids are ready for
absorption
Protein Absorption
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Active absorption
Whole proteins are broken down at the
microvilli surface and within the
absorptive cells
Whole proteins are eventually broken
down to amino acids
Many different amino acid transport
mechanisms
Amino acids are sent to the liver via
portal circulation
Amino Acid Metabolism
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Amino acids are used to synthesize
new body proteins
If not used to synthesize new proteins
– Deamination: liver removes amino group
– Nitrogen is converted to urea
– Kidneys flush nitrogen from the body
RDA for Protein (adults)
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Promotes equilibrium
0.8 gm of protein / kg of healthy body
weight
154 lb.
= 70 kg
2.2 kg/lb.
70 kg x
0.8 g protein
= 56 g protein
kg healthy body wt
RDA for Protein
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Increased by ~10-15 gm /day for
pregnancy
Endurance athletes may need 1.5 - 2
gm/kg healthy weight
About 8-10% of total kcals
Most of us eat more than the RDA for
protein
Excess protein cannot be stored as
protein
New DRI for protein coming
Fig. 6.9
Individual Amino Acid
Supplement
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Supplement may cause imbalances
and toxicities
Body is designed to handle whole
proteins
Supplement can overwhelm the
absorptive mechanism
Excess of one AA can hamper
absorption of other AAs
Dietary Protein
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High-quality
– complete
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Low-quality
– incomplete
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All-or-none principle in protein
synthesis
Limiting amino acid
Complementary protein
Limiting Amino Acid
C is the limiting amino acid in this example
CCCCC
AAAAAAAA
RRRRRR
CAR CAR CAR
CAR CAR
R
A A A
Complementary Protein
Food 1
Food 2
CC
AAAA
RRR
CAR
CCCC
AA
RRR
CAR CAR CAR
CAR CAR
Complementary Protein
Beans (legumes)
Grains
Nuts/seeds
Vegetables
Soy
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Soy protein is similar to animal protein
(used in school lunches)
High in linoleic acid and some linolenic acid
Calcium source and bone health
Isoflavones
Heart disease
Cancer
Vegetarian Diets
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Why become a vegetarian?
– Ethical reasons
– Health reasons
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As a group, vegetarians are thinner and
healthier than meat eaters
– Religious reasons
Types of Vegetarians
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Vegans: eat only plant foods
Fruitarians: eat only the seed-bearing
parts of plants
Lactovegetarians: use dairy products
Lactoovovegetarians: use dairy
products and eggs
The Vegetarian Diet
Pyramid