Transcript Essential amino acids

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Composition of Proteins & forms
 Amino Acids
 Structure
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Denaturing Proteins
Digestion
 Just the basics
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Role & functions
Protein requirements
Too little and too much protein
 PEM
 Marasmus
 Kwashiokor
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The building blocks of protein
Each has an amine group at one end
◦ The nitrogen-containing part
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An acid group at the other end
A distinctive side chain attached to the
carbon at the center of the backbone
◦ Gives identity and chemical nature to each amino
acid
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Essential amino acids
◦ Amino acids that either cannot be synthesized at all
by the body or cannot be synthesized in amounts
sufficient to meet physiological needs
So, where must these essential amino acids come
from?
◦ Can only be replenished from foods
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Conditionally essential amino acid
◦ An amino acids that is normally nonessential
◦ Under special circumstances, when the need for it
exceeds that body’s ability to produce it, it must be
supplied by the diet
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Peptide bond
◦ Connects one amino acid to another
◦ Formed between the amine of one amino acid and
the acid group of the next amino acid
◦ Forms a straight chain of amino acids with side
chains bristling out from the backbone
© 2006 Thomson-Wadsworth
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Denaturation
◦ The irreversible change in a protein’s shape
◦ The first step in the destruction of a protein
◦ Brought about by
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Heat
Acids
Bases
Alcohols
Salts of heavy metals
Important to the digestion of food protein
◦ Stomach acids open up a protein’s structure
 Allows digestive enzymes to make contact with the peptide
bonds and cleave them
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Mouth
◦ Protein is crushed by chewing and moistened with
saliva
◦ Nothing happens to the protein until the strong
acid of the stomach denatures it
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Stomach
◦ Acid helps to uncoil the protein’s tangled strands
so that molecules of the stomach’s proteindigesting enzyme can attack the peptide bonds
 The stomach’s protein-digesting enzyme works best in
an acidic environment
◦ Lining is protected from acid and enzymes by a
mucus coating secreted by stomach cells
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Small intestine
◦ Receives small denatured pieces of protein from the
stomach
 A few are single amino acids
 Most are polypeptides
 Protein fragments of 10+ amino
acids bonded together
 A strand of 4 to 10 amino acids
is called an oligopeptide
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Cells along the small intestine absorb single
amino acids
◦ Cells that line the small intestine have enzymes on
their surfaces that split most tri- and dipeptides
into single amino acids
 Which are then absorbed
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Some di- and tripeptides are absorbed into
cells
◦ Where they are split into single amino acids before
being released into the bloodstream
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Amino acids in the bloodstream
◦ Carried to the liver
 Used by the liver or released into the blood to be taken
up by other cells
◦ Cells can then use the amino acids to
 Make their proteins for their own use
 Make proteins that are released into the lymph or
blood for other uses
◦ When necessary body cells can use amino acids for
energy
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Amino acids must be continuously available to
build the proteins of new tissue
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Embryo
Muscles of an athlete in training
Growing child
New blood to replace losses
Scar tissue
New hair and nails
Protein helps replace
◦ Worn-out cells and internal cell structures
 Red blood cells live only 3-4 months
 Cells lining the intestinal tract live only 3 days
 Skin cells are constantly being shed and replaced
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An amino acid that arrives at a cell can be
used is one of several ways
◦ Used to build part of a growing protein
◦ Altered to make another need compound
◦ Dismantled so as to use its amine group to build
another amino acid
 Remainder can be used
 For fuel
 Converted to glucose or fat
◦ If the cell is starved for energy and is lacking
glucose and fatty acids
 The amine group will be removed and the remainder
used for energy
 The amine group will ultimately be excreted as urine
◦ If the body has a surplus of amino acids and energy
 The amino group will be excreted
 The remainder can be
 Used for energy
 Converted to glucose or fat for storage
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Digestibility of a protein varies from food to
food
◦ In general, the amino acids from animal sources are
more easily digested and absorbed than those from
plant sources
 Animal sources: 90+% digested and absorbed
 Legumes: ≈80%-90% digested and absorbed
 Grains and other plant foods:≈70%-90% digested and
absorbed
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Food preparation and protein digestibility
◦ Cooking with moist heat improves protein
digestibility
◦ Cooking with dry
heat can impair
protein digestibility
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High-quality proteins
◦ Dietary proteins containing all essential amino acids
◦ May also contain nonessential amino acids
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Complementary proteins
◦ Two or more proteins whose amino acids
complement each other; the essential amino acids
missing from one are supplied by the other
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Mutual supplementation
◦ The strategy of combining two incomplete proteins
sources so that the amino acids in one food make
up for those lacking in the other food
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Supporting growth and development
Building Enzymes, Hormones and other
compounds
Maintaining Fluid and Electrolyte Balance
Maintaining Acid-Base Balance
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Clotting of blood
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Providing energy and Glucose
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Proteins
◦ Composed of carbon, hydrogen, oxygen & nitrogen; arranged as
strands of amino acids
◦ Versatile and vital cellular working molecules
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Enzymes
Antibodies
Cellular transport
Hormones
Cellular “pumps”
Oxygen carriers
◦ Some are structural molecules
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Tendons
Ligaments
Scars
Fibers of muscles
Cores of bone and teeth
Filaments of hair
Materials of nails
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DRI
◦ Designed to cover the need to replace proteincontaining tissue that healthy adults lose and wear
out every day
 Depends on body size: larger people have a higher
protein need
◦ For adults: 0.8g/kg body weight/day
◦ Minimum: 10% of total calories
◦ Athletes may need slightly more
 Increased need is well covered by a regular diet
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Nitrogen balance
◦ The amount of nitrogen consumed compared with
the amount excreted in a given time period
◦ Studies of nitrogen balance underlie the DRI
Committee’s recommendations
 Under normal circumstances, healthy adults are in
nitrogen equilibrium = zero balance
 Positive nitrogen balance
 Nitrogen in > nitrogen out
 Negative nitrogen balance
 Nitrogen in < nitrogen out
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Protein deficiencies, along with energy
deficiencies, are the world’s leading form of
malnutrition
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Protein-energy malnutrition (PEM)
◦ World’s most widespread malnutrition problem
◦ Includes both marasmus and kwashiorkor and
states in which they overlap
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Over 500 million children face imminent
starvation, severe malnutrition and hunger
◦ Most of the 33,000 children who die each day are
malnourished
◦ Prevalent in Africa, central America, South America,
the Middle East, East and Southeast Asia
 Developing countries, including those in North
America, are not immune to it
◦ Chronic inadequate food intake
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Inadequate energy
vitamin
mineral
protein intake
◦ Person is shriveled
◦ Lean all over
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Marasmus
◦ Occurs most commonly in children from 6 – 18 mo
◦ Children in impoverished nations subsist on a weak
cereal drink with scant energy and protein of low
quality
 A starving child often looks
like a little old person
 Just skin and bones
◦ Without adequate nutrition
 Muscles, including the heart, waste and weaken
 Brain development is stunted; learning is impaired
 Metabolism is so slow that body temperature is
subnormal
 Little or no fat under the skin to insulate against the
cold
◦ Child engages in as little physical activity as
possible
 Not even crying for food
◦ Body cuts down on any expenditure of protein not
needed for the heart, lungs, and brain to function
◦ Growth ceases
◦ Skin loses its elasticity and moisture
 Tends to crack, sores develop and fail to heal
◦ Digestive enzymes are in short supply
 Digestive tract lining deteriorates and absorption fails
◦ Blood proteins, including hemoglobin, are no longer
produced
 Child becomes anemic and weak
◦ Lack the protein needed to heal a broken bone
◦ Antibodies are degraded to provide amino acids for
other uses
◦ Leaving the child an easy target for infection
 Dysentery
 An infection of the digestive tract hat causes diarrhea
 Depletes the body of nutrients, especially minerals
 Measles
 Might make a healthy child sick for 1-2 weeks
 Kills a child with PEM within 2-3 days
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Ultimately, marasmus progresses to the point
of no return
◦ The body’s machinery for protein synthesis, itself
made of protein, has been degraded
◦ At this point, attempts to prevent death by giving
food or protein fail
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Physical Presentation
◦ Swollen belly
◦ Skin rash
◦ Severe acute malnutrition
 Too little protein
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In countries where kwashiorkor is prevalent,
each baby is weaned from breast milk as
soon as the next one comes along
◦ The older baby no longer receives breast milk,
which contains high-quality protein
◦ Instead, is given a watery cereal with scant protein
of low quality
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Rare in the United States, but not totally
unknown
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Kwashiorkor Symptoms
◦ Resemble those of marasmus
 Often without severe wasting of body fat
◦ Proteins and hormones that previously maintained
fluid balance are diminished
 Fluids leak out of the blood and accumulate in the
belly and legs
 Causing edema
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Kwashiorkor Symptoms
◦ Belly often bulges with a fatty liver
 Caused by lack of the protein carriers that transport fat
out of the liver
 Fatty liver loses some of its ability to clear poisons from
the body, prolonging their toxic effects
◦ Without sufficient tyrosine to make melanin, the
child’s hair loses its normal color
◦ Inadequate protein synthesis leaves the skin patchy
and scaly
◦ Sores fail to heal
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Once an infection sets in kwashiorkor often
follows and the immune system weakens
further
Infections that occur with malnutrition are
responsible for two-thirds of the deaths of
young children in developing nations
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If caught in time, the starvation of a child
can be reversed by careful nutrition therapy
◦ Fluid balances are most critical
 Diarrhea will have depleted the body’s potassium
 And upset other electrolyte balances
 Electrolyte imbalances, anemia, fever, and infection often
lead to heart failure and sudden death
 Correction of fluid and electrolyte
balances usually raises blood pressure
and strengthens the heartbeat within
a few days
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PEM at Home
◦ Occurs among some groups in the United States
and Canada
 The poor living
 On U.S. Indian reservations
 In inner cities
 In rural areas
 Many elderly people
 Hungry and homeless children
 Those suffering from anorexia nervosa
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At risk for PEM:
◦ Those with wasting diseases
 Such as AIDS and cancer
◦ Those addicted to drugs and alcohol
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PEM and serious illnesses worsen each other
◦ Treating the PEM often reduces medical
complications and suffering even when the
underlying disease is untreatable
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Overconsumption of protein
◦ No health benefits
◦ May pose health risks for the
 Heart
 Kidneys
 Bones
◦ Often associated with obesity
◦ Animal protein sources can be high in saturated fat
 atherosclerosis and heart disease
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Protein-Rich Foods
◦ Protein is critical in nutrition
 Too many protein-rich foods can displace other
important foods from the diet
◦ Foods richest in protein carry with them a
characteristic array of vitamins and minerals
 Including vitamin B12 and iron
◦ Foods rich in protein tend to lack, for example,
vitamin C and folate
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The Advantages of Legumes
◦ The protein of some legumes is of a quality almost
comparable to that of meat
 For practical purposes, the quality of soy protein can
be considered the equivalent to that of meat
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The Advantages of Legumes
◦ Excellent sources of
 B vitamins
 Iron
 Calcium and other minerals
◦ Lack
 Vitamin A
 Vitamin C
 Vitamin B12