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© 2011 Pearson Education, Inc.
6Proteins: Crucial
Components of All Body Tissues
© 2011 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
The Building Blocks of Proteins
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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
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 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
Deanimation/Transanimation
© 2011 Pearson Education, Inc.
© 2011 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
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 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
© 2011 Pearson Education, Inc.
© 2011 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
Protein Synthesis
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 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
© 2011 Pearson Education, Inc.
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
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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
© 2011 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
© 2011 Pearson Education, Inc.
© 2011 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
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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
Protein Digestion
© 2011 Pearson Education, Inc.
© 2011 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)
Protein Absorption
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Functions of Proteins
• Cell growth, repair, maintenance • Enzymes and hormones• Fluid and electrolyte balance• Acid−base balance• Immune system• Energy source• Nutrient transport and storage
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Protein Adequacy
• Nitrogen balance determines protein needs• Positive nitrogen balance• Negative nitrogen balance• In nitrogen balance
Nitrogen Balance
© 2011 Pearson Education, Inc.
© 2011 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)
© 2011 Pearson Education, Inc.
© 2011 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
© 2011 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 lower-protein diet • Maximum of 2 g of protein per kilogram body
weight each day is safe for healthy people
Fat Synthesis from Excess Protein
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Protein Sources
• Meats• Milk-based products• Soy products• Legumes• Whole grains• Nuts• Quorn
© 2011 Pearson Education, Inc.
© 2011 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
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Health Benefits of Vegetarianism
• 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
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Challenges of Vegetarian Diets
• Can be low in some nutrients • Associated with disordered eating• Varied and adequate diet planning • Soy and complementary proteins• Vegetarian Food Guide Pyramid• Special attention to vitamins D, B12, and
riboflavin (B2); minerals zinc and iron
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Protein-Energy Malnutrition
• Protein-energy malnutrition: caused by inadequate protein and energy intake
• Common forms:• Marasmus• Kwashiorkor
© 2011 Pearson Education, Inc.
© 2011 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
© 2011 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
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Genetic Disorders
• Numerous disorders are caused by defective DNA
• Genetic disorders include:• Phenylketonuria• Sickle cell anemia• Cystic fibrosis
© 2011 Pearson Education, Inc.