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BMS208 Human Nutrition
Topic 6: Protein & Amino acids
Chris Blanchard
Learning objectives
• Describe how the chemical structure of proteins differs
from the structures of carbohydrates and fats.
• List the 9 essential amino acids.
• Trace the digestion of protein and list the enzymes
needed to complete the process.
• Explain the process used by the body to synthesize new
proteins.
• List the 8 major functions of protein in the body.
• Describe nitrogen balance and provide examples of
positive nitrogen balance, negative nitrogen balance,
and equilibrium.
• Describe deamination, where it occurs in the body, the
products produced, and the fate of these products.
Learning objectives
• Discuss the factors used to evaluate protein quality.
• Describe the diseases that result from inadequate intake
of protein and protein-kcalories.
• Discuss the health effects of over-consumption of
protein.
• Calculate the protein needed daily using the RDA for
protein.
• Discuss the health risks of protein and amino acid
supplements.
• Define nutritional genomics and explain its potential uses
in health care.
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.
Amino acid structure
Amino acid structure
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,
asparagine, aspartic acid, cysteine, glutamic acid,
glutamine, glycine, proline, serine, and tyrosine.
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.
Peptides
• 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.
Peptides bonds
Protein Shapes
• Hydrophilic side groups are attracted to
water.
• Hydrophobic side groups repel water.
• Coiled and twisted chains help to provide
stability.
Protein Shapes
Protein Shapes
Protein Functions
• Some carry and store materials.
• Some provide strength.
• Some require minerals for activation
(example: hemoglobin and the mineral
iron).
Protein denaturation
• 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.
Digestion and Absorption
• 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.
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.
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.
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
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.
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
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.
• 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.
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.
Enzymes
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
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.
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
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
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
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.
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.
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
Protein Metabolism
• Deaminating Amino Acids
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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 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.
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
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.
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
Protein Quality
Protein Quality
• A Measure of Protein Quality - PDCAAS
(protein digestibility-corrected amino acid
score)
– Compares amino acid composition of a
protein to human amino acid requirements
– Adjusts for digestibility
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.
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.
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
Protein-Energy Malnutrition
• 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
Protein-Energy Malnutrition
• 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
Protein-Energy Malnutrition
• Marasmus-Kwashiorkor Mix
– Both malnutrition and infections
– Edema of kwashiorkor
– Wasting of marasmus
Protein-Energy Malnutrition
• Infections
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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.
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.
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.
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
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.
Recommended Intakes of Protein
• Adequate Energy
– Must consider energy intake
– Must consider total grams of protein
• Protein in abundance is common in first
world countries
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.
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.
Nutritional Genomics
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.
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.
A Genomics Primer
Genetic Variation and Disease
• Small differences in individual genomes
• May affect 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.
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.
Clinical Concerns
• An increased understanding of the human
genome may impact health care by:
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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.