© 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

© 2011 Pearson Education, Inc.

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

© 2011 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

© 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

© 2011 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

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

© 2011 Pearson Education, Inc.

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

© 2011 Pearson Education, Inc.

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

© 2011 Pearson Education, Inc.

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

© 2011 Pearson Education, Inc.

Genetic Disorders

• Numerous disorders are caused by defective DNA

• Genetic disorders include:• Phenylketonuria• Sickle cell anemia• Cystic fibrosis

© 2011 Pearson Education, Inc.