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Chapter 6
Protein: Amino Acids
© 2008 Thomson - Wadsworth
The Chemist’s View of
Proteins
• Proteins are made from 20 different amino
acids, 9 of which are essential.
• Each amino acid has an amino group, an
acid group, a hydrogen atom, and a side
group.
• It is the side group that makes each amino
acid unique.
• The sequence of amino acids in each
protein determines its unique shape and
function.
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© 2008 Thomson - Wadsworth
The Chemist’s View of
Proteins
• Amino Acids
Have unique side groups that result
in differences in the size, shape and
electrical charge of an amino acid
Nonessential amino acids, also called
dispensable amino acids, are ones the
body can create.
• Nonessential amino acids include alanine,
arginine, asparagines, aspartic acid,
cysteine, glutamic acid, glutamine,
glycine, proline, serine, and tyrosine.
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© 2008 Thomson - Wadsworth
The Chemist’s View of
Proteins
• Amino Acids
 Essential amino acids, also called
indispensable amino acids, must be
supplied by the foods people consume.
• Essential amino acids include histidine, isoleucine,
leucine, lysine, methionine, phenyalanine,
threonine, tryptophan, and valine.
 Conditionally essential amino acids refer to
amino acids that are normally nonessential
but essential under certain conditions.
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The Chemist’s View of
Proteins
• Proteins
 Amino acid chains are linked by peptide
bonds in condensation reactions.
• Dipeptides have two amino acids bonded
together.
• Tripeptides have three amino acids bonded
together.
• Polypeptides have more than two amino acids
bonded together.
 Amino acid sequences are all different,
which allows for a wide variety of possible
sequences.
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© 2008 Thomson - Wadsworth
© 2008 Thomson - Wadsworth
The Chemist’s View of
Proteins
• Proteins
Protein Shapes
• Hydrophilic side groups are attracted to
water.
• Hydrophobic side groups repel water.
• Coiled and twisted chains help to provide
stability.
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© 2008 Thomson - Wadsworth
The Chemist’s View of
Proteins
• Proteins
 Protein Functions
• Some carry and store materials.
• Some provide strength.
• Some require minerals for activation (example:
hemoglobin and the mineral iron).
 Protein denaturation is the uncoiling of
protein that changes its ability to function.
• Proteins can be denatured by heat and acid.
• After a certain point, denaturation cannot be
reversed.
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Digestion and Absorption
of Protein
• Stomach acid and enzymes facilitate
the digestion of protein.
• It is first denatured, then broken
down to polypeptides.
• The small intestine continues to
break down protein into smaller
peptides and amino acids so it can be
absorbed.
© 2008 Thomson - Wadsworth
© 2008 Thomson - Wadsworth
Digestion and Absorption
of Protein
• Protein Digestion
In the Stomach
• Protein is denatured by hydrochloric acid.
• Pepsinogen (a proenzyme) is converted
into its active form pepsin in the
presence of hydrochloric acid.
• Pepsin cleaves proteins into smaller
polypeptides.
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Digestion and Absorption
of Protein
• Protein Digestion
In the Small Intestine
• Proteases hydrolyze protein into short
peptide chains called oligopeptides, which
contain four to nine amino acids.
• Peptidases split proteins into amino
acids.
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Digestion and Absorption
of Protein
• Protein Absorption
Used by intestinal cells for energy or
synthesis of necessary compounds
Transported to the liver
Taking enzyme supplements or
consuming predigested proteins is
unnecessary
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Proteins in the Body
• Proteins are versatile and unique. The
synthesis of protein is determined by genetic
information.
• Protein is constantly being broken down and
synthesized in the body.
• Researchers measure nitrogen balance to
study synthesis, degradation and excretion of
protein.
• Protein has many important functions in the
body.
• Protein can be used for energy if needed; its
excesses are stored as fat.
• The study of proteins is called proteomics.
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Proteins in the Body
• Protein Synthesis
 Synthesis is unique for each human being
and is determined by the amino acid
sequence.
 Delivering the instructions through
messenger RNA
• Carries a code to the nuclear membrane and
attaches to ribosomes
• Presents a list to make a strand of protein
 Transfer RNA lines up the amino acids and
brings them to the messenger
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© 2008 Thomson - Wadsworth
Proteins in the Body
• Protein Synthesis
 Sequencing errors can cause altered
proteins to be made.
 An example is sickle-cell anemia where an
incorrect amino acid sequence interferes
with the cell’s ability to carry oxygen.
 Nutrients and Gene Expression - Cells
regulate gene expression to make the type
of protein needed for that cell.
• Epigenetics refers to a nutrient’s ability to activate
or silence genes without interfering with the
genetic sequence.
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© 2008 Thomson - Wadsworth
Proteins in the Body
• Roles of Proteins
 Building Materials for Growth and
Maintenance
• A matrix of collagen is filled with minerals 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.
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© 2008 Thomson - Wadsworth
B
A
A
B
New
compound
A B
Enzyme
Enzyme
Enzyme
Stepped Art
Fig. 6-9, p. 190
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
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Proteins in the Body
• Roles of Proteins
 Acid-Base Regulators
• Act as buffers by keeping solutions acidic or
alkaline
• Acids are compounds that release hydrogen ions
in a solution.
• Bases are compounds that accept hydrogen ions
in a solution.
• Acidosis is high levels of acid in the blood and
body fluids.
• Alkalosis is high levels of alkalinity in the blood
and body fluids.
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Proteins in the Body
• Roles of Proteins
Transporters
• Carry lipids, vitamins, minerals and
oxygen in the body
• Act as pumps in cell membranes,
transferring compounds from one side of
the cell membrane to the other
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© 2008 Thomson - Wadsworth
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
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Proteins in the Body
• Roles of Proteins
Source of energy and glucose if
needed
Other Roles
• Blood clotting by producing fibrin which
forms a solid clot
• Vision by creating light-sensitive
pigments in the retina
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Proteins in the Body
• A Preview of Protein Metabolism
 Protein Turnover and the Amino Acid Pool
• Protein turnover is the continual making and
breaking down of protein.
• Amino acid pool is the supply of amino acids that
are available.
 Amino acids from food are called
exogenous.
 Amino acids from within the body are called
endogenous.
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Proteins in the Body
• A Preview of Protein Metabolism
Nitrogen Balance
• Zero nitrogen balance is nitrogen
equilibrium, when input equals output.
• Positive nitrogen balance means nitrogen
consumed is greater than nitrogen
excreted.
• Negative nitrogen balance means
nitrogen excreted is greater than
nitrogen consumed.
© 2008 Thomson - Wadsworth
Proteins in the Body
• A Preview of Protein Metabolism
 Using Amino Acids to Make Proteins or Nonessential
Amino Acids – Cells can assemble amino acids into
the protein needed.
 Using Amino Acids to Make Other Compounds
• Neurotransmitters are made from the amino acid
tyrosine.
• Tyrosine can be made into the melanin pigment
or thyroxine.
• Tryptophan makes niacin and serotonin.
 Using Amino Acids for Energy and Glucose
• There is no readily available storage form of
protein.
• Breaks down tissue protein for energy if needed
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Proteins in the Body
• A Preview of 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.
© 2008 Thomson - Wadsworth
Protein in Foods
• Eating foods of high-quality protein is the
best assurance to get all the essential
amino acids.
• Complementary proteins can also supply
all the essential amino acids.
• A diet inadequate in any of the essential
amino acids limits protein synthesis.
• The quality of protein is measured by its
amino acid content, digestibility, and ability
to support growth.
© 2008 Thomson - Wadsworth
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.
• Other foods consumed at the same time
can change the digestibility
© 2008 Thomson - Wadsworth
Protein in Foods
• Protein Quality
 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.
 Reference Protein is the standard by which
other proteins are measured.
• Based on their needs for growth and
development, preschool children are used to
establish this standard.
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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
© 2008 Thomson - Wadsworth
© 2008 Thomson - Wadsworth
Protein in Foods
• Protein Quality
A Measure of Protein Quality PDCAAS (protein digestibilitycorrected amino acid score)
• Compares amino acid composition of a
protein to human amino acid
requirements
• Adjusts for digestibility
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Protein in Foods
• Protein Regulation for Food Labels
List protein quantity in grams
% Daily Values is not required but
reflects quantity and quality of
protein using PDCAAS.
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Health Effects and Recommended
Intakes of Protein
• Protein deficiency and excesses can be
harmful to health.
• Protein deficiencies arise from proteindeficient diets and energy-deficient
diets.
• This is a worldwide malnutrition
problem, especially for young children.
• High-protein diets have been implicated
in several chronic diseases.
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Health Effects and Recommended
Intakes of Protein
• Protein-Energy Malnutrition (PEM) –
also called protein-kcalorie
malnutrition (PCM)
Classifying PEM
• Chronic PEM and acute PEM
• Maramus, kwashiorkor, or a combination
of the two
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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
• Anxiety and apathy
• Good appetite is possible
• Hair and skin problems
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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
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Health Effects and Recommended
Intakes of Protein
• PEM
Marasmus-Kwashiorkor Mix
• Both malnutrition and infections
• Edema of kwashiorkor
• Wasting of marasmus
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Health Effects and Recommended
Intakes of Protein
• PEM
 Infections
•
•
•
•
•
Lack of antibodies to fight infections
Fever
Fluid imbalances and dysentery
Anemia
Heart failure and possible death
 Rehabilitation
• Nutrition intervention must be cautious, slowly
increasing protein.
• Programs involving local people work better.
© 2008 Thomson - Wadsworth
Health Effects and Recommended
Intakes of Protein
• 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.
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Health Effects and Recommended
Intakes of Protein
• Health Effects of Protein
Cancer
• A high intake of animal protein is
associated with some cancers.
• Is the problem high protein intake or
high fat intake?
Adult Bone Loss (Osteoporosis)
• High protein intake associated with
increased calcium excretion.
• Inadequate protein intake affects bone
health also.
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Health Effects and Recommended
Intakes of Protein
• 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.
• Does not seem to cause kidney disease
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Health Effects and Recommended
Intakes of Protein
• Recommended Intakes of Protein
10-35% energy intake
Protein RDA
• 0.8 g/kg/day
• Assumptions
– People are healthy.
– Protein is mixed quality.
– The body will use protein efficiently.
© 2008 Thomson - Wadsworth
Health Effects and Recommended
Intakes of Protein
• Recommended Intakes of Protein
Adequate Energy
• Must consider energy intake
• Must consider total grams of protein
Protein in abundance is common in
the U.S. and Canada.
© 2008 Thomson - Wadsworth
Health Effects and Recommended
Intakes of Protein
• Protein and Amino Acid Supplements
Many reasons for supplements
Protein Powders have not been found
to improve athletic performance.
• Whey protein is a waste product of
cheese manufacturing.
• Purified protein preparations increase the
work of the kidneys.
© 2008 Thomson - Wadsworth
Health Effects and Recommended
Intakes of Protein
• Protein and Amino Acid Supplements
Amino Acid Supplements are not
beneficial and can be harmful.
• Branched-chain amino acids provide little
fuel and can be toxic to the brain.
• Lysine appears safe in certain doses.
• Tryptophan has been used experimentally
for sleep and pain, but may result in a
rare blood disorder.
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Nutritional Genomics
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Nutritional Genomics
• In the future, genomics labs may be
used to analyze an individual’s genes
to determine what diseases the
individual may be at risk for
developing.
• Nutritional genomics involves using a
multidisciplinary approach to
examine how nutrition affects genes
in the human genome.
© 2008 Thomson - Wadsworth
A Genomics Primer
• Human DNA contains 46 chromosomes
made up of a sequence of nucleotide
bases.
• Microarray technology is used to analyze
gene expression.
• Nutrients are involved in activating or
suppressing genes without altering the
gene itself.
• Epigenetics is the study of how the
environment affects gene expression.
• The benefits of activating or suppressing a
particular gene are dependent upon the
gene’s role.
© 2008 Thomson - Wadsworth
© 2008 Thomson - Wadsworth
Cell
1. Nucleus
2. Chromosome
4.
Gene
3. DNA
4.
Stepped Art
Fig. H6-1, p. 208
Genetic Variation and
Disease
• Small differences in individual genomes
• May affect a disease’s ability to respond to
dietary modifications
• Nutritional genomics would allow for
personalization of recommendations.
• Single-Gene Disorders
 Mutations cause alterations in single genes.
 Phenylketonuria is a single-gene disorder
that can be affected by nutritional
intervention.
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Genetic Variation and
Disease
• Multigene Disorders
 Multiple genes are responsible for the
disease.
 Heart disease is a multigene disorder that is
also influenced by environmental factors.
 Genomic research may be helpful in guiding
treatment choices.
 Variations called single nucleotide
polymorphisms (SNPs) may influence an
individual’s ability to respond to dietary
therapy.
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Clinical Concerns
• An increased understanding of the human
genome may impact health care by:
 Increasing knowledge of individual disease
risks
 Individualizing treatment
 Individualizing medications
 Increasing knowledge of nongenetic causes
of disease
• Some question the benefit of identifying
individual genetic markers.
• Even if specific recommendation can be
made based on genes, some may choose
not to follow recommendations.
© 2008 Thomson - Wadsworth