Chapter 6
Protein: Amino Acids
CHON
Objectives for Chapter 6
• Describe how proteins are created from
amino acids.
• Explain the process of protein digestion
and absorption.
• List roles of protein in the body.
• Calculate your personal protein needs.
• Identify sources of protein, including two
non-animal sources.
• Discuss the potential harm caused by
either too much or too little dietary protein.
What are Proteins?
• Proteins are made of amino acids (CHON)
structural and working substance in all cells not
just muscle cells
• Contain carbon, hydrogen, oxygen (like
carbohydrates and fats)
• In addition, also contain nitrogen
• Each amino acid has:
– Acid group (-COOH)
– Amine group (-NH2)
– Side chain (unique)
Protein: Amino Acids
Amino Acids
Proteins are made up of 20 AAs
• 9 are essential
• 11 are nonessential
– Can be made in the body from other amino
acids or by adding nitrogen to carboncontaining structures
Conditionally essential amino acids
– Under certain conditions, some nonessential
amino acids cannot be made by body.
Proteins
• Proteins are a link of Amino acid with
peptide bonds through condensation
– Dipeptides - have two amino acids bonded together.
– Tripeptides - have three amino acids bonded together.
– Polypeptides - have more than two amino acids bonded
together
Proteins
• Amino acid sequence – determines
shape and function and what protein is
made.
Amino Acid Sequence of Human Insulin
Protein
shape
and
function
Hydrophilic side
groups are
attracted to water.
Hydrophobic side
groups repel water.
Coiled and twisted
chains help to
provide stability.
Proteins
Protein Denaturing
• unfolding of protein shape by heat, acids,
bases or salts that changes its ability to
function.
• After a certain point, denaturation cannot
be reversed.
• For example - hard boiled egg
• Stomach acid untangles proteins to aid in
digestion
Denaturation
Protein Digestion and Absorption
Mouth
– Crushed and moisten
– No digestion of protein in the mouth
Stomach
• protein is denatured
• Enzymes start to break the peptide
bonds
• HCl activates pepsinogen to pepsin
• Pepsin cleaves proteins into smaller
polypeptides.
Protein Digestion
• small intestine
– In the small intestine, polypeptides broken down into
tri-, dipeptides and amino acids by pancreatic and
intestinal proteases
– Cells in the small intestine absorb amino acids
– Enzymes on the surface split dipeptides and
tripeptides to amino acids
– Amino acids enter blood and travel to liver
• Absorption
– Used by intestinal cells for energy
– Transported and stored in the liver
Synthesis of New Proteins
• There are about 50,000 proteins in the
human body made from the 20 AAs
• DNA in the cell nucleus contains instructions
for protein synthesis.
• Sequencing – order of the AA in a chain
– Sequencing error may occur when the protein
contain the wrong AA for a particular gene to be
expressed
Synthesis of New Proteins
• Gene: DNA segment that codes for
specific protein
• Specialized RNA molecules carry out
instructions for protein synthesis.
• Gene mutations can result in errors in
protein synthesis.
– Example: sickle-cell anemia
Protein Synthesis
Proteins in the Body
Roles of Proteins
• Building Materials for Growth and Maintenance
– A matrix of collagen is filled with minerals (such as
calcium) to provide strength to bones and teeth.
– Replaces tissues including the skin, hair, nails, and GI
tract lining
• Enzymes are proteins that facilitate anabolic
(building up) and catabolic (breaking down)
chemical reactions.
• Hormones regulate body processes and some
hormones are proteins. An example is insulin.
Proteins in the Body
• Roles of Proteins
– Regulators of Fluid Balance
• Plasma proteins attract water
• Maintain the volume of body fluids to prevent
edema which is excessive fluid
• Maintain the composition of body fluids
– Acid-Base Regulators
• Act as buffers by keeping solutions acidic or
alkaline
– Transporters
• Carry lipids, vitamins, minerals and oxygen in the
body
Proteins in the Body
• Roles of Proteins
– Antibodies
• Fight antigens, such as bacteria and viruses, that invade the
body
• Provide immunity to fight an antigen more quickly the second
time exposure occurs
– Source of energy and glucose if needed
• Some amino acids can be used to make
proteins or nonessential amino acids
– Neurotransmitters are made from the amino acid
tyrosine.
– Tyrosine can be made into the melanin pigment or
thyroxine.
– Tryptophan makes niacin and serotonin.
Protein Metabolism
• Protein turnover
– Protein turnover is the continual making and
breaking down of protein.
•
Amino acid pool
– is the supply of amino acids that are
available
Protein Metabolism
Nitrogen balance - The body maintains the
same amount of protein in its tissues.
• Zero nitrogen balance - Amount of protein
consumed = amount of protein used (nitrogen
excreted)
• Positive nitrogen balance – amount of protein
consumed is greater than nitrogen excreted.
• Negative nitrogen balance – amount of nitrogen
excreted is greater than protein consumed.
– Examples: starvation, serious injury or illness
Protein Metabolism
• Deaminating Amino Acids
– Nitrogen-containing amino groups are
removed.
– Ammonia is released into the bloodstream.
– Ammonia is converted into urea by the liver.
– Kidneys filter urea out of the blood.
• Using Amino Acids to Make Fat
– Excess protein is deaminated and converted
into fat.
– Nitrogen is excreted.
Protein Quality
• Protein Digestibility
• Amino acid
composition
• High-quality proteins
• Complementary
Protein
Protein in Foods
Protein Quality
• Digestibility - Depends on protein’s food source
– Animal proteins are 90-99% absorbed.
– Plant proteins are 70-90% absorbed.
– Soy and legumes are 90% absorbed.
• Amino Acid Composition
– The liver can produce nonessential amino acids.
– Cells must dismantle to produce essential amino acids if
they are not provided in the diet.
– Limiting amino acids are those essential amino acids that
are supplied in less than the amount needed to support
protein synthesis.
Protein in Foods
Protein Quality
• High-Quality Proteins
– Contains all the essential amino acids
– Animal foods contain all the essential amino acids.
– Plant foods are diverse in content and tend to be
missing one or more essential amino acids.
• Complementary Proteins
– Combining plant foods that together contain all the
essential amino acids
– Used by vegetarians
How do Vegetarians Meet
Protein Needs?
Vegetarians can meet protein needs by
consuming:
– Variety of plant foods
– Protein-rich meat alternatives:
•
•
•
•
Soy
Dried beans and other legumes
Nuts
Eggs, dairy (lacto-ovo-vegetarians)
Health Effects of Protein
• Protein deficiency and excesses can be
harmful to health.
• Protein deficiencies arise from low protein
and low calorie diets.
• This is a worldwide malnutrition problem,
especially for young children.
• High-protein diets have been implicated in
several chronic diseases.
Health Effects of Protein
• Protein-Energy Malnutrition (PEM) – also
called protein - kcalorie malnutrition (PCM)
– Classifying PEM
• Chronic PEM - caused by long term food
deprivation (marasmus – dying away)
• Acute PEM – caused by recent severe food
restriction (kwashiorkor)
• Marasmus - Kwashiorkor Mix
– Both malnutrition and infections
– Edema of kwashiorkor
– Wasting of marasmus
Health Effects and Recommended Intakes
of Protein
• PEM - Marasmus
– Infancy, 6 to 18 months of
age
– Severe deprivation or
impaired absorption of
protein, energy, vitamins and
minerals
– Develops slowly
– Severe weight loss and
muscle wasting, including
the heart
– < 60% weight-for-age
– Good appetite is possible
– Hair and skin problems
Health Effects and Recommended Intakes
of Protein
• PEM - Kwashiorkor
– Older infants and young children,
18 months to 2 years of age
– Inadequate protein intake,
infections
– Rapid onset
– Some muscle wasting, some fat
retention
– Growth is 60-80% weight-for-age
– Edema and fatty liver
– Apathy, misery, irritability and
sadness
– Loss of appetite
– Hair and skin problems
Comparison of Kwashiorkor and
Marasmus
A classic sign of
Kwashiorkor is edema.
Marasmus results in an
emaciated appearance.
Health Effects of Protein
• Heart disease
– Foods high in animal protein also tend to be high in
saturated fat.
– Homocysteine levels increase cardiac risks.
– Arginine may protect against cardiac risks.
• Cancer
• A high intake of animal protein is associated
with some cancers.
• Osteoporosis
• High protein intake associated with increased calcium
excretion.
• Inadequate protein intake affects bone health also
Health Effects of Protein
• Weight Control
• High-protein foods are often high-fat foods.
• Protein at each meal provides satiety.
• Adequate protein, moderate fat and sufficient
carbohydrate better support weight loss.
• Kidney Disease
• High protein intake increases the work of the
kidneys.
Health Effects of Protein
• Protein and Amino Acid Supplements
– are not beneficial, Amino Acid Supplements can
be harmful.
• Branched-chain amino acids can be toxic to the brain.
• Tryptophan has been used experimentally for sleep
and pain, but may result in a rare blood disorder.
– Protein Powders have not been found to
improve athletic performance.
• Purified protein preparations increase the work of the
kidneys
Recommended Intakes
RDA
• 10% - 35% of energy
• 0.8 g of protein/kg of body weight needed
daily
Calculating your daily protein needs:
• Convert weight to pounds by dividing by
2.2 lbs/kg:
If you weigh: 130 lbs ÷ 2.2 = 59 kg
59 kg X 0.8 g = 47 g of protein/day
Extra credit – 5 points
• Gather information on 5 protein
supplements marketed to body builders.
Evaluate the safety of the supplements.
Note the recommended intake, the amount
of protein in grams, cost of these
supplement and compare to the cost of
food sources or protein