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Proteins and Amino Acids:
Function Follows Form
Chapter 7
Proteins and Amino Acids
• A part of every cell
• Needed in chemical reactions
• Keep us “together”
Why Is Protein Important?
• Constantly assemble, break down, and
use proteins
– Diets replace what is being used
– Excess
• Make energy
• Store as fat
Why Is Protein Important?
• Food sources
– Beef
– Chicken
– Fish
– Milk
– Plant foods
• Beans, peas, grains, nuts, seeds, and
vegetables
© PhotoDisc
Why Is Protein Important?
• Protein deficiency
– Wasting
– Immune system
• More vulnerable to infection
• Impairs digestion and absorption of
nutrients
Amino Acids Are the Building Blocks
of Protein
• Identified by side chains
– Carbon, carboxylic acid, amino acid, and
side group
– Side group gives AA its identity
Amino Acids Are the Building Blocks
of Protein
• Proteins are sequences of amino acids
• Types of amino acids
– Indispensable: must come from diet
– Dispensable: can be made in the body
– Conditionally dispensable
Amino Acids Are the Building Blocks
of Protein
• Protein structure
– Amino acids sequence
• Peptide bonds
– Protein shape
• Sequence of AA determines shape
• Shape of protein determines function
Amino Acids Are the Building Blocks
of Protein
– Denaturing protein structure
• Disrupts function
• Caused by acidic or alkalinity,
alcohol, high temperature
oxidation, and agitation
• Denaturation
Functions of Body Protein
• Structural and mechanical functions
– Collagen
– Keratin
– Motor proteins
• Immune functions
– Antibodies
– Immune response
Functions of Body Protein
• Enzymes
– Catalyze reactions
• Hormones
– Regulate body processes
• Acid-base balance
– Proteins as buffers
– Acidosis
– Alkalosis
Functions of Body Protein
• Transport functions
– Channels and pumps
– Carriers
• Fluid balance
– Intracellular—interstitial
– Extracellular—intravascular
– Edema
Functions of Body Protein
• Source of energy and glucose
– Carbohydrates and fat are protein-sparing
– Protein for energy
• Deamination
Protein Digestion and Absorption
• Protein digestion
– Proteases and proenzymes
– Stomach
• Proteins are denatured by hydrochloric
acid
• Pepsin begins digestion
–10–20% of digestion
Protein Digestion and Absorption
• Protein digestion
– Small intestine
• Proteases and intestinal lining cells
• Amino acids and peptide absorption
– Small intestine
• Proteases and intestinal lining cells
Protein Digestion and Absorption
• Undigested protein
– Feces
– Problems with digestion
• Celiac disease
• Cystic fibrosis
Proteins in the Body
• Protein synthesis
– AA assembled in specific sequence
– Draws on AA pool as needed
• Dispensable AA missing
• Indispensable AA missing
• Indispensable AA unavailable
Proteins in the Body
• AA pool
– Available AA
• Protein synthesis; produce energy and
glucose
• Protein turnover
– Constant recycling of protein
• Synthesis of nonprotein molecules
– Precursors
• Neurotransmitters
Proteins in the Body
• Protein turnover
Proteins in the Body
• Protein and nitrogen excretion
– Breakdown of amino acids
• Removes nitrogen (amino) group
• Amino groups converted to urea for
excretion
Proteins in the Body
• Nitrogen balance
– Nitrogen intake vs. nitrogen output
• Nitrogen equilibrium
– Nitrogen intake = nitrogen output
– Healthy adults
• Positive nitrogen balance
– Nitrogen intake > nitrogen output
– Growth; recovery from illness
• Negative nitrogen balance
– Nitrogen intake < nitrogen output
– Injury and illness
Proteins in the Diet
• Recommended protein intake
– Adult RDA = 0.8 gram per kilogram body
weight
– Infants, children, and teens
• See Figure 7.2
• Physical stress
– Infection, burns, fevers, surgery, and intensive
weight training
• Consumption in the United States
– Generally protein intake > protein needs
Proteins in the Diet
• Protein quality
– Complete proteins
• Supply all indispensable
amino acids
• Animal proteins; soy
proteins
– Incomplete proteins
• Low in one or more
indispensable amino acids
• Most plant proteins
Photos courtesy of the USDA
Proteins in the Diet
• Protein quality
– Two incomplete proteins = complete protein
– Complementary proteins
Photo © PhotoDisc
Proteins in the Diet
• Evaluating protein quality
– Amino acid composition
– Digestibility
– Protein Digestibility-Corrected Amino Acid
Score (PDCAAS)
• Used to determine %DV
Proteins in the Diet
• Protein and AA additives
– Structure, texture, and taste
of foods
• Protein and AA
supplements
– Generally not needed
– Risks unknown
Photo © Jones and Bartlett Publishers
The Pros and Cons
of Vegetarian Eating
• Types of vegetarian diets
– Lacto-ovo vegetarian
• Includes milk, eggs
– Vegan
• Plant foods only
© AbleStock
The Pros and Cons
of Vegetarian Eating
– Semi-vegetarian
• Avoid red meat; eat some chicken and
fish
– Zen macrobiotic diet
• Mostly vegans; stress whole grains and
locally grown food
The Pros and Cons
of Vegetarian Eating
• Health benefits
– 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
The Pros and Cons
of Vegetarian Eating
• Health risks
– 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
The Pros and Cons
of Vegetarian Eating
• Dietary recommendations
– 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
The Health Effects
of Too Little Protein
• Dietary protein = indispensible amino
acids
• Protein deficiency
• Protein-energy malnutrition (PEM)
– Kwashiorkor
• Edema and other signs
• Between ages of 18–24 months
• Associated with extreme poverty
The Health Effects
of Too Little Protein
• Protein-energy malnutrition (PEM)
– Marasmus
• Chronic PEM
• Infants and 6- to18-month-old children
• Adults with cancer or starvation
– Nutritional rehabilitation
• Gradual and careful refeeding
The Health Effects
of Too Much Protein
The Health Effects
of Too Much Protein
• Strains kidney function
– Excretes products of protein breakdown
• Mineral losses
– Link between high-protein diet and
osteoporosis
• Obesity
– Correlation with body fat
The Health Effects
of Too Much Protein
• Heart disease
– Increased saturated fat and cholesterol
intake
• Cancer
– Increased risk for certain types of cancer