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Proteins
Learning Outcomes
• Describe how amino acids form proteins
• Define essential and nonessential amino acids and
explain why adequate amounts of each of the
essential amino acids are required for protein
synthesis
• Distinguish between high quality and low quality
proteins and list sources of each
• Describe how 2 low quality proteins can be
complementary to each other to provide the
required amounts of essential amino acids
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Learning Outcomes
• Explain the methods used to measure the protein
quality of foods
• List the factors that influence protein needs.
• Calculate the RDA for protein for a healthy adult
with a given body weight
• Explain positive nitrogen balance, negative
nitrogen balance and nitrogen equilibrium and list
conditions under which they may occur
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Learning Outcomes
• Describe how protein is digested and absorbed in
the body
• List the primary functions of protein in the body
• Describe types of protein-energy malnutrition
• Describe the symptoms and treatment of food
allergies
• Develop a vegetarian diet plan that meets the
body’s protein needs
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Structure of Protein
• Contains hydrogen, oxygen, carbon and
nitrogen
• Comprised of amino acids
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Nitrogen (amino) group
Carboxyl (acid) group)
Hydrogen
Side chain (R)
• Determines protein function and name
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Amino Acids
• Nonessential (dispensable) amino acids
– Body can produce
• Essential (indispensable) amino acids
– Must be taken in via food
• Conditionally essential amino acids
– Essential during infancy, disease or trauma
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Synthesis of Non Essential
Amino Acids
• Transamination
– Transfer of an amino group from an amino acid
to a carbon skeleton to form a new amino acid
• Deamination
– Amino acid losing an amino group
– Amino Group is incorporated into urea in the
liver
– Excreted in urine
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Complete and Incomplete
Proteins
• Complete Proteins
– Adequate amounts of all the essential amino acids
– Animal proteins, except gelatin
• Incomplete Proteins
– Inadequate amounts of 1 or more of the essential amino
acids
– Plant proteins, except soybeans
• Complementary Proteins
– Combining plant proteins to compensate for limiting
amino acids
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Synthesis of Proteins
• Amino acids are linked by peptide bonds to form
proteins
• Synthesis of protein determined through gene
expression
• DNA transcription phase
– DNA code transferred from the nucleus to the cytosol
via messenger RNA (mRNA)
• mRNA translation phase
– tRNA and ribosomes
• DNA-coded instructions determine shape, and
thus function of proteins
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Protein Organization
• Primary Structure
– Order of amino acids determines shape
• Secondary Structure
– Weaker bonds between nearby amino acids
form spiral-lie or pleated sheet shape
• Tertiary Structure
– 3D folding determines function
• Quaternary Structure
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Denaturation and Adaptation
• Denaturation of Protein
– Altering protein’s 3 dimensional structure
– Acid, alkaline, heat, enzymes, or agitation
• Adaptation of Protein Synthesis
– Constant state of breakdown, rebuilding and
repair (protein turnover)
– In response to diet, exercise, etc.
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Sources of Protein
• Diet and recycling of body protein
• North America: 70% supplied by meat,
poultry, fish, milk and milk products,
legumes and nuts
• Worldwide: 35% of protein comes from
animal
• How do we incorporate more plant protein?
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Evaluation of
Food Protein Quality
• Biological Value (BV)
– Egg white is highest
• Protein Efficiency Ratio (PER)
• Chemical Score
• Protein Digestibility Corrected Amino Acid
Score (PDCAAS)
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Recommended Intakes of Protein
• Positive Nitrogen Balance
– Protein intake exceeds protein losses
• Negative Nitrogen Balance
– Protein losses exceed protein intake
• Equilibrium
– Protein intake equals protein losses
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Protein Needs
• AMDR (IOM): 10-35% kcal
• Adult RDA
– 0.8g/kg healthy body weight
• Recovery states
– 0.8 –2 g/kg body weight
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Protein Digestion and Absorption
• Cooking denatures protein
• Stomach
– Hydrochloric acid denatures
– Pepsin begins enzymatic digestion
• Gastrin controls the release of pepsin
• Small intestine
– Secretin and CCK released; stimulate release of
pancreatic proteases (trypsin, chymotrypsin,
carboxypeptidases)
• Amino acids absorbed into the portal vein
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Functions of Proteins
• Producing Vital Body Structures
• Maintaining Fluid Balance
– Edema
• Contributing to Acid Base Balance
– Buffers
• Forming Hormones, Enzymes, and
Neurotransmitters
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Functions of Proteins
• Contributing to Immune Function
– Anergy is immune incompetence
• Transporting Nutrients
• Forming Glucose
– Gluconeogenesis
– Muscle wasting is cachexia
• Providing Energy
– 4 kcal/g
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Health Concerns
Related to Protein Intake
• Protein- Energy Malnutrition (PEM)
– Marasmus
• Minimal amounts of energy, protein and other
nutrients
– Kwashiorkor
• Minimal amounts of protein and moderate energy
deficit
• High-Protein Diets
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Food Protein Allergies
• Food proteins (allergens) cause an immune
response, creating an allergic reaction
– IgE most commonly produced
– anaphylaxix
• 8 foods account for 90% of all food allergies
– Peanuts, tree nuts, milk, eggs, fish, shellfish, soy,
and wheat
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Vegetarian Diets
• Types of Vegetarians
– Vegan, Lacto-vegetarians, Lacto-ovo-vegetarians
• Nutrient Concerns
– B12, Calcium, iron, zinc, vitamin D, high-quality
protein, riboflavin
• Special Concerns for Infants and Children
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