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Chapter 6
Proteins and
Amino Acids:
Function Follows
Form
Why Is Protein Important?
• Major component of all plant and animal issues
• Vital to many aspects of health
• Provide energy and help keep skin, hair, and nails
healthy
• Deficiency leads to wasting
• Excess protein intake a greater problem in the
U.S.
Amino Acids Are the
Building Blocks of Proteins
• Proteins are sequences of amino acids (AA)
• 20 different amino acids
– 9 essential amino acids
• Body cannot make them
• Must get them through diet
– 11 nonessential amino acids
• Do not need to be supplied by diet
• If body cannot make them, categorized as
conditionally essential amino acids
Amino Acids as Building Blocks
• Amino acid structure
– One central carbon atom
– One hydrogen atom (H)
– One carboxylic acid group
(-COOH)
– One amino group (-NH2)
– One side group unique to
each amino acid
• Side group gives the amino acid
its identity
Protein Structure
• Amino Acid Sequence
– Each amino acid is
joined to the next by a
peptide bond
– Peptide bonds
• Dipeptide (2 AA)
• Oligopeptide (3 AA)
• Polypeptide (4-10
AA)
Protein Structure
• Protein Shape
– Sequence and properties of AA determines
protein shape
– Shape determines protein’s function
Protein Denaturation
• Denaturation
– Disrupts shape
– Protein unfolds and
loses its shape
(denature)
– Caused by acidic or
alkalinity, high
temperature,
alcohol, oxidation,
and agitation
Functions of Body Proteins
• Each protein has a
specific function
determined by its
unique shape
Functions of Body Proteins
• Structural and mechanical functions
– Collagen
• Most abundant
• Gives skin and bone their elastic strength
– Keratin
• Primary constituent of hair and nails
– Motor proteins
• Turn energy into mechanical work
Functions of Body Proteins
• Immune functions
– Antibodies
• Blood proteins that attack and inactivate
bacteria and viruses
• Part of the body’s immune response
Functions of Body Proteins
• Enzymes
– Catalyze, or speed up, chemical reactions
– Every cell contains thousands of types of
enzymes
Functions of Body Proteins
• Hormones
– Regulate body processes
• Acid-base balance
– Proteins help maintain stable pH levels by
acting as buffers
– Acidosis or alkalosis occurs when proteins
unable to fulfill buffer function
Functions of Body Proteins
• Transport functions
– Proteins act as
• Channels and pumps, allowing substances to
flow through membranes
• Carriers, transporting important substances
in the blood stream
Functions of Body Proteins
• Fluid Balance
– Fluids found inside and outside cells
• Inside cells: intracellular fluid
• Outside cells: extracellular
–Two types:
»Interstitial fluid (between cells)
»Intravascular fluid (in the blood)
Functions of Body Proteins
• Fluid Balance
– Blood proteins maintain appropriate fluid
levels in vascular system
– Edema results when diet lacks enough protein
to maintain normal levels of blood proteins
• Edema: Swelling caused by buildup of fluid
between cells
Functions of Body Proteins
• Source of Energy and Glucose
– Carbohydrates and fat are protein-sparing
• Body prefers burning them for energy
• Will resort to burning protein for energy if
necessary
– Body readies protein for use as energy through
deamination
• Removal of amino group from an amino acid
Protein Digestion and Absorption
• Protein digestion
– In the stomach
• Proteins are denatured by hydrochloric acid
• Pepsin begins digestion
– 10–20% of digestion
– In the small intestine
• Proteases and intestinal lining cells break
down large peptides into smaller peptides
Protein Digestion and Absorption
• Undigested Protein
– Pass out of the body in feces
– Diseases of the intestinal tract cause problems
with digestion by decreasing absorption
efficiency
• Celiac disease
• Cystic fibrosis
Protein Digestion and Absorption
• Amino Acid and Peptide Absorption
– Travel via portal vein to liver  released into
general circulation
Protein in the Body
• Protein Synthesis
– AA assembled in specific sequence
– Draws on AA pool as needed
• Dispensable AA missing
–Cell will make that AA or obtain it from
the liver
Protein in the Body
• Protein Synthesis
– Draws on AA pool as needed
• Indispensable AA missing
–Body may break its own protein down to
supply the missing AA
• Indispensable AA unavailable
–Protein synthesis halts and partially
completed protein used elsewhere in
the body
Protein in the Body
• Amino Acid Pool and Protein
Turnover
– The AA pool
• Available AA
–Protein synthesis;
produce energy and
glucose
– Protein turnover
• Constant recycling of
protein
Protein in the Body
• Synthesis of Nonprotein Molecules
– Amino acids as precursors of DNA, RNA, and
coenzymes
• Also used to make neurotransmitters:
–Chemicals that send signals from nerve
cells to other parts of the body
Protein in the Body
• Protein and Nitrogen Excretion
– Breakdown of amino acids
• Removes nitrogen (amino) group
• Amino groups converted to urea for
excretion
Protein in the Body
• Nitrogen Balance
– Nitrogen intake vs. nitrogen output
• Nitrogen equilibrium
– Nitrogen intake = nitrogen output
– Healthy adults
Protein in the Body
• Positive nitrogen balance
– Nitrogen intake > nitrogen output
– Growth; recovery from illness
• Negative nitrogen balance
– Nitrogen intake < nitrogen output
– Injury and illness
Protein in the Diet
• Recommended Intakes of Protein
– Adult RDA = 0.8 gram per kilogram of body
weight
– Severe physical stress can increase body’s need
for proteins
• Infections, burns, fevers, surgery increase
protein losses
• Diet must replace lost protein
Protein in the Diet
• Protein Consumption in the United States
– Generally higher than recommended range
Protein in the Diet
• Protein Quality
– Complete Proteins
• Supply all indispensable amino acids
• Animal proteins; soy proteins
– Incomplete and Complementary Proteins
• Low in one or more indispensable amino
acids
• Most plant proteins
Protein in the Diet
• Protein Quality
– Complementary proteins
• Two incomplete proteins can equal a
complete protein
• One makes up for the other’s lack of specific
essential amino acids
• Combination then provides sufficient
amounts of all essential amino acids
Protein in the Diet
• Protein Quality
– Evaluating Protein Quality and Digestibility
• Provides all the indispensable amino acids at
amounts the body needs
• Provides enough other amino acids to serve as
nitrogen sources for making dispensable amino
acids
• Should be easy to digest
• Consider the protein digestibility-corrected
amino acid score (PDCAAS)
Protein in the Diet
• Proteins and AAs as Additives and Supplements
– Protein and AA additives
• Structure, texture, and taste of foods
– Protein and AA supplements
• Used for a variety of reasons: dieters,
athletes, certain diseases
• Risks unknown
Vegetarian Eating
• Why People Become Vegetarians
– Various reasons, including
• Religious beliefs
• Environmental concerns
• Aversion to eating another living creature
• Concerns about animal cruelty concerns
– In some cases a necessity
Vegetarian Eating
• Health Benefits of Vegetarian Diets
– Less fat, saturated fat, and cholesterol intake
– More magnesium and folate
– More antioxidants
– More fiber and phytochemicals
• Reduces risk for heart disease, obesity,
hypertension, and cancer
Vegetarian Eating
• Health Risks of Vegetarian Diets
– Vegan diets may be low in some nutrients
• Calcium, iron, zinc, vitamin D, and vitamin B12
– Higher intake of phytates
– More restrictive food choices = less nutrients
– Careful planning needed for growth and overall
health
Vegetarian Eating
• Dietary Recommendations for Vegetarians
– Choose a variety of foods
– Choose whole, unrefined foods
– Choose a variety of fruits and vegetables
– Choose lower-fat dairy products and eggs in
moderation
– Consume a regular source of vitamins B12 and
D
• Fortified foods or supplements
Health Effects of
Too Little or Too Much Protein
• Protein-Energy Malnutrition
– Kwashiorkor
• Edema and other signs
• Between ages of 18 and 24 months
• Associated with extreme poverty
Health Effects of
Too Little or Too Much Protein
• Protein-Energy Malnutrition
– Marasmus
• Chronic PEM
• Infants and 6- to 18-month-old children
• Adults with cancer or starvation
Health Effects of
Too Little or Too Much Protein
• Health Effects of Excess Dietary Protein
– Kidney function
• Increases kidney filtration rate, straining
function
– Mineral losses
• Link between high-protein diet and
osteoporosis
Health Effects of
Too Little or Too Much Protein
• Health Effects of Excess Dietary Protein
– Obesity
• Correlation with body fat
– Heart disease
• Increased saturated fat and cholesterol
intake
– Cancer
• Increased risk for certain types of cancer