Download Proteins

Chapter 6 Lecture
The Science of Nutrition
Third Edition
Proteins: Crucial
Components of All
Body Tissues
© 2014 Pearson Education, Inc.
What Are Proteins?
• Proteins: large, complex molecules found in cells
of all living things
– Dictated by genetic material (DNA)
– Contain carbon, hydrogen, oxygen, nitrogen
– Made from 20 different amino acids
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Amino Acids
• Nine essential amino acids
– Cannot be produced in sufficient quantities to
meet physiological needs
– Must be obtained from food
• Nonessential amino acids
– Can be synthesized in sufficient quantities
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
Amino Acids
• Transamination
– Transfer amine group from an essential amino
acid to a different acid group and R group
• Conditionally essential amino acid
– Nonessential amino acid becomes essential
– Phenylketonuria (PKU): tyrosine becomes a
conditionally essential amino acid that must
be provided by the diet
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
How Are Proteins Made?
• Proteins are long chains of amino acids
• Peptide bonds join amino acids together
• Gene expression is the process by which cells
use genes to make proteins
– Gene: segment of deoxyribonucleic acid
(DNA) that serves as a template for the
synthesis (expression) of a particular protein
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
How Are Proteins Made?
• Transcription: messenger RNA copies the
genetic information from DNA
• Translation: the genetic information in RNA is
converted into the amino acid sequence of a
protein
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
How Are Proteins Made?
• Protein turnover
– Existing proteins are degraded to provide the
building blocks for new proteins
– Amino acid pool includes amino acids from
food and cellular breakdown
• Protein organization determines function
– Sequential order of the amino acids
– Spiral shape from twist in amino acid chain
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
Protein Denaturation
• Proteins uncoil and lose their shape
• Damaging substances: heat, acid, base, heavy
metal, alcohol
• Protein function is lost
– Denatured enzyme
– High fever
– Blood pH out of normal range
– During digestion
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Protein in the Diet
• For protein synthesis, all essential amino acids
must be available to the cell
• Limiting amino acid
– Essential amino acid that is missing or in the
smallest supply
– Slows down or halts protein synthesis
• Inadequate energy consumption
– Limits protein synthesis
© 2014 Pearson Education, Inc.
Protein in the Diet
• Incomplete protein (low quality): insufficient
essential amino acids
– Does not support growth and health
• Complete protein (high quality): sufficient
amounts of all nine essential amino acids
– Derived from animal and soy protein
© 2014 Pearson Education, Inc.
Protein in the Diet
• Mutual supplementation: combine two or more
incomplete protein sources to make a complete
protein
• Complementary proteins: two or more foods are
combined to supply all nine essential amino
acids for a complete protein
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
Protein Digestion
• Protein digestion begins in the stomach
• Hydrochloric acid denatures protein strands and
activates pepsin
• Pepsin: enzyme breaks down proteins into short
polypeptides and amino acids
• Gastrin: hormone controls hydrochloric acid
production and pepsin release
© 2014 Pearson Education, Inc.
Protein Digestion
• Digestion continues in the small intestine
• Pancreatic enzymes (proteases) complete
protein digestion
• Special sites (small intestine) transport amino
acids, dipeptides, tripeptides
• High doses of individual amino acid
supplements can lead to amino acid toxicity and
deficiencies
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
Protein Quality
• Methods for estimating protein quality
– Chemical score
– Protein digestibility corrected amino acid
score (PDCAAS)
• Animal protein and many soy products are
highly digestible (90% absorption)
© 2014 Pearson Education, Inc.
Functions of Proteins
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Cell growth, repair, maintenance
Enzymes and hormones
Fluid and electrolyte balance
Acid−base balance
Immune system
Energy source
– Deamination
– Urea excretion
• Nutrient transport and storage
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
Protein Adequacy
• Nitrogen balance determines protein needs
– Positive nitrogen balance
– Negative nitrogen balance
– In nitrogen balance
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
RDA for Protein
• RDA = 0.8 g per kg body weight per day
• Recommended percentage of energy is 10−35%
of total energy intake
• Protein needs are higher during growth and
development (children, adolescents, and
pregnant/lactating women)
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
Too Much Protein Can Be Harmful
• High cholesterol and heart disease
– Animal-protein-rich diets are associated with
high blood cholesterol levels (saturated fat)
• Contribution to bone loss
– High-protein diets increase calcium excretion
and possibly lead to bone loss
© 2014 Pearson Education, Inc.
Too Much Protein Can Be Harmful
• Kidney disease
– High protein intakes are associated with an
increased risk among susceptible individuals
– People with diabetes have higher rates of
kidney disease and may benefit from a lowerprotein diet
– Maximum of 2 g of protein per kilogram body
weight each day is safe for healthy people
© 2014 Pearson Education, Inc.
Protein Sources
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Meats
Milk-based products
Soy products
Legumes
Whole grains
Nuts
Quorn
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
Vegetarian Diets
• Vegetarianism: restricting the diet to foods of
plant origin
• People chose vegetarianism for:
– Health benefits
– Ecological reasons
– Religious reasons
– Ethical reasons
– Concerns over food safety
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
Health Benefits of Vegetarianism
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Lower fat and total energy intake
Lower blood pressure
Reduced risk of heart disease
Fewer digestive problems
Reduced risk of some cancers
Reduced risk of kidney disease, kidney stones,
and gallstones
© 2014 Pearson Education, Inc.
Challenges of Vegetarian Diets
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Can be low in some nutrients
Associated with disordered eating
Varied and adequate diet planning
Soy and complementary proteins
Special attention to vitamins D, B12, and
riboflavin (B2); minerals zinc and iron
© 2014 Pearson Education, Inc.
Protein-Energy Malnutrition
• Protein-energy malnutrition: caused by
inadequate protein and energy intake
• Common forms:
– Marasmus
– Kwashiorkor
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
Marasmus
• Grossly inadequate energy and nutrient intake
• Consequences of marasmus:
– Wasting and weakening of muscles (heart)
– Stunted brain development and learning
– Depressed metabolism
– Stunted physical growth
– Deterioration of the intestinal lining (anemia)
– Severely weakened immune system
– Fluid and electrolyte imbalances
© 2014 Pearson Education, Inc.
Kwashiorkor
• Disease resulting from low protein intake
• Kwashiorkor symptoms include:
– Some weight loss and muscle wasting
– Retarded growth and development
– Edema resulting in distention of the belly
– Fatty degeneration of the liver
– Loss of appetite, sadness, irritability, apathy
– Skin problems and hair loss
© 2014 Pearson Education, Inc.
Genetic Disorders
• Numerous disorders are caused by defective
DNA
• Genetic disorders include:
– Phenylketonuria
– Sickle cell anemia
– Cystic fibrosis
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.