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Chapter 6 – Protein: Amino Acids
Learning Objectives
After completing Chapter 6, the student will be able to:
1. Describe how the chemical structure of proteins differs from the structures of carbohydrates and fats.
2. List the 9 essential amino acids.
3. Trace the digestion of protein and list the enzymes needed to complete the process.
4. Explain the process used by the body to synthesize new proteins.
5. List the major functions of protein in the body.
6. Describe nitrogen balance and provide examples of positive nitrogen balance, negative nitrogen balance, and
equilibrium.
Assignments and Other Instructional Materials
The following ready-to–use assignments are available in this chapter of the instructor’s manual:
• New! Case study
• Worksheet 6-1: Nitrogen Balance Calculations1
Other instructional materials in this chapter of the instructor’s manual include:
• Answer key for How To (p. 193) activity
• Classroom activities
• Worksheet answer keys (as appropriate)
Lecture Presentation Outline4
“Of special interest to…” symbol key: = Hot Topic = Personal Health
= Health Care Professionals = Science Majors
1 Worksheets 6-1, 6-2, and 6-5 contributed by Daryle Wane
2 Worksheet 6-3 and Handouts 6-2 and 6-3 contributed by Sharon Rady Rolfes
3 Contributed by Mary A. Wyandt, Ph.D., CHES
4 Contributed by Melissa Langone.
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Key to instructor resource annotations (shown to the right of or below outline topics):
PL = Available on Power Lecture DVD-ROM (ISBN 0538797592): V = video
TRA = Transparency acetates: 12e TRA = 12th edition, 11e TRA = 11th edition, 10e TRA = 10th edition
Introductory/whole chapter resources: PL figure JPEGs; Test Bank; IM WS 6-4, CA 6-1
I. The Chemist’s View of Proteins
Proteins are made from 20 different amino acids, 9 of which are essential. Each amino acid has an amino
group, an acid group, a hydrogen atom, and a side group. It is the side group that makes each amino acid
unique. The sequence of amino acids in each protein determines its unique shape and function.
A. Amino Acids 10e TRA 58, 59; IM WS 6-2
1. Amino acids have unique side groups that result in differences in the size, shape, and electrical charge
of an amino acid.
2. Nonessential amino acids, also called dispensable amino acids, are ones the body can create.
Nonessential amino acids include alanine, arginine, asparagines, aspartic acid, cysteine, glutamic acid,
B. Proteins 10e TRA 60
1. Amino acid chains are linked by peptide bonds in condensation reactions.
2. The sequence of amino acids that determines the structure of a protein varies greatly. Unlike
3. Weak electrical attractions within a polypeptide chain determine the secondary structure of proteins.
4. Polypeptide Tangles – Tertiary Structure
5. Multiple Polypeptide Interactions – Quaternary Structures
6. Protein denaturation is the uncoiling of protein that changes its ability to function.
a. Proteins can be denatured by heat and acid.
b. After a certain point, denaturation cannot be reversed.
II. Digestion and Absorption of Protein
Stomach acid and enzymes facilitate the digestion of protein. It is first denatured, then broken down to
polypeptides. The small intestine continues to break down protein into smaller peptides and amino acids so it
can be absorbed.
A. Protein Digestion
1. In the Stomach
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a. Protein is denatured by hydrochloric acid.
2. In the Small Intestine
a. Proteases hydrolyze protein into short peptide chains called oligopeptides, which contain four to
nine amino acids.
B. Protein Absorption
1. Used by intestinal cells for energy or synthesis of necessary compounds.
III. Proteins in the Body
Proteins are versatile and unique. The synthesis of protein is determined by genetic information. Protein is
constantly being broken down and synthesized in the body. Researchers measure nitrogen balance to study
synthesis, degradation, and excretion of protein. Protein has many important functions in the body. Protein can
be used for energy if needed; its excesses are stored as fat. The study of proteins is called proteomics.
A. Protein Synthesis 10e TRA 61
1. Synthesis is unique for each human being and is determined by the amino acid sequence.
2. Delivering the instructions through messenger RNA
a. Carries a code to the nuclear membrane and attaches to ribosomes.
b. Presents a list to make a strand of protein.
B. Roles of Proteins
1. Building Materials for Growth and Maintenance
2. Enzymes are proteins that facilitate anabolic (building up) and catabolic (breaking down) chemical
reactions. 10e TRA 62; Website HN 6-3, CA 6-2
3. Hormones regulate body processes and some hormones are proteins. An example is insulin.
4. Regulators of Fluid Balance
a. Plasma proteins attract water.
5. Acid-Base Regulators
a. Act as buffers by keeping solutions acidic or alkaline.
6. Transporters 10e TRA 63; IM CA 6-3
7. Antibodies
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8. Source of energy and glucose if needed.
9. Other Roles
a. Blood clotting by producing fibrin, which forms a solid clot.
C. A Preview of Protein Metabolism
1. Protein Turnover and the Amino Acid Pool
a. Protein turnover is the continual making and breaking down of protein.
2. Nitrogen Balance IM WS 6-1
a. Zero nitrogen balance is nitrogen equilibrium, when input equals output.
3. Using Amino Acids to Make Other Compounds
a. Neurotransmitters are made from the amino acid tyrosine.
4. Using Amino Acids for Energy and Glucose
a. There is no readily available storage form of protein.
b. Breaks down tissue protein for energy if needed.
5. Using Amino Acids to Make Fat
6. Deaminating Amino Acids
7. Using Amino Acids to Make Proteins or Nonessential Amino Acids
a. Cells can assemble amino acids into the protein needed.
8. Converting Ammonia to Urea – Ammonia and carbon dioxide are combined in the liver to make urea.
10e TRA 78
9. Excreting Urea 10e TRA 79
a. Ammonia is converted to urea in the liver.
IV. Protein in Foods
Eating foods of high-quality protein is the easiest way to get all the essential amino acids. Complementary
proteins can also supply all the essential amino acids. A diet inadequate in any of the essential amino acids
limits protein synthesis. The quality of protein is measured by its amino acid content, digestibility, and ability to
support growth.
A. Protein Quality
1. Digestibility
a. Depends on protein’s food source
1. Animal proteins are 90%-99% absorbed.
b. Other foods consumed at the same time can change the digestibility.
2. Amino Acid Composition
a. The liver can produce nonessential amino acids.
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3. Reference protein is the standard by which other proteins are measured. Based on their needs for
growth and development, preschool children are used to establish this standard.
4. High-Quality Proteins
5. Complementary Proteins 10e TRA 64; IM HN 6-1
a. Combining plant foods that together contain all the essential amino acids.
b. Used by vegetarians.
B. Protein Regulation for Food Labels IM CI 6.3
1. List protein quantity in grams.
V. Health Effects and Recommended Intakes of Protein IM WS 6-5
Protein deficiency and excesses can be harmful to health. Protein deficiencies arise from protein-deficient diets
and energy-deficient diets. This is a worldwide malnutrition problem, especially for young children. High–
protein diets have been implicated in several chronic diseases.
A. Protein-Energy Malnutrition (PEM) – Those with chronic PEM are short for their age, whereas those
with acute PEM are underweight for their height. 10e TRA 65
1. Classifying PEM – a. May have maramus, kwashiorkor, or a combination of the two.
2. Marasmus
a. Infancy, 6 to 18 months of age.
b. A severe deprivation or impaired absorption of protein, energy, vitamins and minerals.
3. Kwashiorkor
a. Older infants and young children, 18 months to 2 years of age.
b. Inadequate protein intake, infections.
4. Marasmus-Kwashiorkor Mix
a. Both malnutrition and infections.
b. Edema of kwashiorkor.
c. Wasting of marasmus.
5. Infections
a. Lack of antibodies to fight infections.
6. Rehabilitation
a. Nutrition intervention must be cautious, slowly increasing protein.
B. Health Effects of Protein PL V “Red/Processed Meat and Mortality Risk”
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1. Heart Disease
a. Foods high in animal protein also tend to be high in saturated fat.
2. Cancer – A high intake of animal protein is associated with some cancers.
3. Adult Bone Loss (Osteoporosis)
4. Weight Control
5. Kidney Disease
a. High protein intake increases the work of the kidneys.
b. Does not seem to cause kidney disease.
C. Recommended Intakes of Protein IM WS 6-3, CI 6.1, 6.2; Website HN 6-2
1. 10%-35% energy intake.
2. Protein RDA
a. 0.8 g/kg/day for most adults.
D. Protein and Amino Acid Supplements IM CA 6-4, CI 6.4
1. Many reasons for supplements.
2. Protein powders have not been found to improve athletic performance.
3. Amino acid supplements are not beneficial and can be harmful.
a. Branched-chain amino acids provide little fuel and can be toxic to the brain.
VI. Highlight: Nutritional Genomics IM CI Highlight
In the future, genomics labs may be used to analyze an individual’s genes to determine what diseases the
individual may be at risk for developing. Nutritional genomics involves using a multidisciplinary approach to
examine how nutrition affects genes in the human genome.
A. A Genomics Primer 12e TRA 12, 13; 11e TRA 15
1. Human DNA contains 46 chromosomes made up of a sequence of nucleotide bases.
2. Microarray technology is used to analyze gene expression.
B. Genetic Variation and Disease
1. Small differences in individual genomes.
2. May affect a disease’s ability to respond to dietary modifications.
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5. Multigene Disorders
a. Multiple genes are responsible for the disease.
C. Clinical Concerns
1. An increased understanding of the human genome may impact health care by:
a. Increasing knowledge of individual disease risks.
2. Some question the benefit of identifying individual genetic markers.
3. Even if specific recommendation can be made based on genes, some may choose not to follow
recommendations.
Case Study5
Erin is a 28-year-old professional woman who is 5 feet 8 inches tall and vigilantly maintains her weight at 118
pounds by following a lacto-ovo (non-fat milk and egg whites only) vegetarian diet that supplies approximately
1200 calories a day. With her understanding that protein should provide between 10 and 35 percent of her daily
calories, she reasons that her daily intake of 40 grams of protein from milk, eggs, legumes, and nuts is adequate for
her needs. She is concerned, however, that she has been sick more than usual and has experienced two stress
fractures in her leg over the past three years while exercising.
1. Explain why Erin’s assumptions about her protein needs are unrealistic based on her current weight.
2. Assuming a healthy weight for Erin is 141 pounds, use the information from Box 6-1 of this chapter to calculate
her recommended daily protein requirement. Show your calculations.
3. What percentage of Erin’s energy comes from protein? Is this adequate? Why or why not?
4. Erin’s energy needs for a healthy weight are closer to 1600 calories a day. What are some consequences of her
low calorie intake on her body’s need for protein?
5. How does Erin’s low intake of calories and protein contribute to her risk for osteoporosis?
6. Assuming Erin consumes 20 grams of protein from whole grains, vegetables, and legumes each day, calculate
how she can meet the remainder of her protein needs with dairy foods and egg whites.
Answer Key
1. Protein needs are based on “healthy body weight.” Erin is underweight so her actual weight is not a good
parameter for calculating her protein needs.
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Suggested Classroom Activities
Newcomers to nutrition too often think that improving nutritional status is simply a matter of introducing more
nutrients into the body. Some people think it is like dropping pennies into a jar—the more you put in, the more will
be there. They need to appreciate the enormous compensating ability of the body before they can understand how
little (or how much) nutrients affect nutritional status.
Classroom Activity 6-1: Chapter Opening Quiz
Objective: Introduction to chapter Class size: Any
Classroom Activity 6-2: Illustration of Enzymatic Action6
Key concept: Action of enzymes Class size: Any
Instructions: Compare a chemical reaction in the body to the union of a bride and a groom. The union requires a
catalyst such as a priest or justice of the peace (JOP). The JOP has the power to unite the bride and groom and
performs the ceremony, but afterwards, goes on unchanged.
Classroom Activity 6-3: Demonstration of Diffusion and Active Transport7
Key concept: Transfer of molecules across a membrane Class size: Any
Classroom Activity 6-4: Protein Content of Over-the-Counter Protein and Amino Acid Supplements
Key concepts: Protein sources; protein/amino acid supplements Class size: Any
Instructions: Protein powder supplements are marketed as providing substantial amounts of protein and enhancing
How To “Try It” Activities Answer Key
How to Calculate Recommended Protein Intakes
The student should first determine the weight to use: her or his weight in kg if within the healthy BMI range, or if
Critical Thinking Questions8
These questions will also be posted to the book’s website so that students can complete them online and e-mail their
answers to you.
1. Discuss the three differences between proteins and carbohydrates/fats. Articulate why these differences are
important.
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Answer: 1.) Proteins contain a nitrogen group along with carbons, oxygen, and hydrogen. These nitrogen
2. List the roles of proteins in the body and detail the importance of each role identified.
Answer: Growth and Maintenance of Body Tissues: One of the major roles for which proteins are noted or
known is providing for growth and development of body tissues such as the muscles, skin, hair, etc. Proteins are
also important in the repair of body tissues that are damaged or require replacement. Most consumers identify
Maintenance of Acid-Base Balance: In the normal course of daily events of the body, the body works hard to
maintain homeostasis. Proteins are an important part of this process through their work in maintaining acid-base
balance. Proteins do so by attracting positively charged ions from hydrogen molecules to their negatively
charged surfaces. Proteins can release these negative charges elsewhere in a basic environment, protecting
themselves from denaturation.
Role in Energy: As discussed in earlier chapters, the body can break down protein to serve as a source of
glucose and energy when needed. While there are many other more important roles for protein and utilizing
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protein for energy can be costly to those other roles, protein will be supplied as an energy source in times of
need. In these cases, protein can be taken from the cells, body tissues, etc.
3. Many Americans consume beyond the RDA for protein. However, there are many individuals that have
difficulty acquiring enough food to feed themselves and their families, or are simply unable to eat enough to
meet the RDA for protein. Many times, individuals having difficulty financially may consume inexpensive
staples or leftovers from others. Discuss how the RD would go about assessing such an individual’s protein
status. What suggestions would you have for this person to improve his/her protein intake and balance?
Answer: The average American eats about 100 grams of protein/day compared to a recommended 50-60 grams,
depending on gender and activity level (for those over 25 years). While protein intake is generally not a
problem in American culture, there are enough cases of protein malnutrition and individuals that are unable to
4. Protein consumption in adults could be considered a form of substance abuse in the U.S. Why might this
statement be true, and what are the health consequences of protein overconsumption?
Answer: As stated above, in the United States, consumers eat much more protein than is required for their
bodies. There might be many reasons for this. One is that most consumers do not understand what an
appropriate portion size for a “protein” is in the United States. Another is that most Americans believe that
In this manner, protein overconsumption might be analogous to a drug addiction because the individual does not
see the problem as an addiction, the individual will continue to pursue eating high volumes of proteins
regardless of their health history, most individuals cannot go without meat proteins if given an ultimatum, most
individuals cannot cut down on their consumption, and individuals that are meat eaters will indicate that there is
no issue with their over consumption with meat at the volume or level that they are eating this protein. These
are all very similar signs to drug addiction.
Overconsumption of protein can be difficult on the body because metabolism of protein requires the excretion
of the nitrogen group, which is the work of the kidneys. With consumption of a great deal of protein, the
kidneys must work much harder to remove the urea from the body. Therefore, while generally protein digestion,
absorption, and waste elimination are not an issue for the body, when large amounts of nitrogen must be
removed from the body, this puts a great stress on the kidneys to remove the urea.