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7
PROTEINS
CHAPTER OVERVIEW
As part of “The Energy-Yielding Nutrients” section, this chapter provides a thorough examination
of the role of protein in human nutrition. The chemical structure of amino acids is introduced, together with
an explanation of the terms dispensable, indispensable and conditionally indispensable. Next, physiological
aspects and dietary considerations of the essential and nonessential amino acids are introduced to the
learner. Protein synthesis, turnover, and organization is explored, and a discussion of protein digestion and
absorption follows. Functions of protein, including fluid balance, hormone and enzyme synthesis, and
immune function, among others, and protein balance are introduced to the learner. After this discussion of
the various vital roles placed by this critical energy nutrient, steps for the calculation of the RDA are
provided. The section on food sources of protein alerts the reader to the importance of plant proteins in the
diet and considers protein quality methods. Soybeans are examined in the “Expert Opinion” section of this
chapter. Finally, the chapter concludes with causes, symptoms, and treatment of protein-energy
malnutrition. The Nutrition Perspective highlights “Vegetarian Diets.”
CHAPTER OBJECTIVES
Upon completion of this chapter the student should be able to:
1.
Describe the chemical structure of amino acids and proteins.
2.
List the 20 amino acids and identify them as to their status as dietary essential or dietary
nonessential amino acids.
3.
Describe the special category of conditionally indispensable amino acids.
4.
Summarize the process of protein synthesis.
5.
Identify a transamination reaction and explain the processes.
6.
Explain the difference between high-quality and lower-quality proteins and the concept of a limiting
amino acid.
7.
Summarize protein digestion and absorption.
8.
Specify the physiological functions of protein.
9.
Calculate their recommended protein intake.
10.
Classify the various kinds of contemporary vegetarianism.
11.
Develop a daily eating plan for a vegetarian that follows Food Guide Pyramid guidelines.
LECTURE NOTES
I. Proteins –An Introduction
II. Proteins—Vital to Life
A. Proteins
1.
Major part of lean body tissue, 17% of body weight
2.
Body functions
a.
blood clotting
b.
fluid balance
c.
hormone and enzyme production
d.
visual processes
e.
cell repair
3. Provide energy – 4 kcal/g Protein
4.
Contain carbon, hydrogen, oxygen and nitrogen, sometimes sulfur
5.
Amino acid composition
a.
central carbon bonded to four groups
i.
amino group
ii. acid group
iii. hydrogen
iv. R
B.
Amino Acid Form Determines Function
1. Type name of amino acid determined by form of R portion
a.
R = hydrogen, the amino acid is glycine
b. R = methyl group, amino acid is alanine
2. Body needs 20 amino acids to function
a.
11 amino acids are non-essential
b. 9 amino acids are essential (must be obtained from food)
c. cysteine and tyrosine are classified as semiessential or conditionally dispensable
C.
Transamination and Deamination
1. Transamination is a common metabolic process for synthesizing nonessential amino acids
a.
requires vitamin B6
2. Deamination is process by which some amino acids lose their amino group without
transferring it to another carbon skeleton
a.
amino group as ammonia is incorporated into urea in liver
b.
transferred through blood to kidney where it is excreted
D. Essential and Nonessential Amino Acids in Perspective
1. Physiological Aspects
2.
a.
PKU illustrate concept of essential amino acid
b.
Untreated PKU can cause mental retardation (tested at birth)
Dietary Considerations
a.
animal proteins contain all essential amino acids
b.
high quality or complete proteins (egg, milk, meat, fish, and cheese)
c.
plant proteins lack 1 or more essential amino acids
d.
low quality or incomplete proteins (grains, legumes, nuts, seeds)
e.
either all essential amino acids are available or none can be used
f.
limiting amino acid illustrates the “all or none principle”
g.
when two or more proteins combine to compensate for deficiencies in each protein,
the proteins are called complementary proteins
h.
adult versus children requirement for essential amino acids
i.
infants and preschool children require 30% of energy from protein
III. Proteins-Amino Acids Joined Together
A.
Protein Synthesis
1. Part of DNA code (gene) is transcribed to mRNA in the nucleus
a.
DNA composed of 4 nucleotides
2.
mRNA leaves the nucleus and travels to cytosol
3.
Ribosomes in cytosol and rough endoplasmic reticulum read the mRNA code and
translate that into directions for a specific order of amino acids in a polypeptide chain
4.
To produce the polypeptide, tRNA brings the appropriate amino acid to the ribosomes as
dictated by the mRNA code.
5.
The amino acid is added to the existing amino acid chain, which begins with the amino
acid methionin
6.
When synthesis of polypeptide is complete, it is released from the ribosome
7.
Often the polypeptide will undergo further cell metabolism in order to function as a
specific body protein
B. Protein Turnover
1.
Cell proteins are constantly undergoing degradation and synthesis
2.
Insulin and growth hormone are the hormones that increase protein synthesis
3.
C.
Cortisol increases protein breakdown
Protein Organization
1.
Primary structure
a. sequential order of amino acids in a polypeptide chain
b. determines protein shape
Secondary structure = only correctly positioned amino acids can interact and fold properly to
form the intended shape for the protein and to allow for chemical attractions to form
between amino acids, e.g. hydrogen bonds
3. Tertiary structure
above results is a unique three dimensional form
dictates function of protein
D.
4.
Quaternary structure = results when two or more separate protein units form an even
larger form
5.
Sickle cell disease illustrates what happens when amino acids are out of order in the
primary structure of a particular protein
Denaturation of Proteins
1.
Alteration of a protein’s three-dimensional structure
2.
Due to treatment by acid, alkaline solution, heat, enzymes or agitation
IV. Protein Digestion and Absorption
A.
Protein Digestion
1.
Digestion of protein begins in the stomach
2. Proteins are denatured by stomach acid
3. Pepsin, an enzyme, breaks down polypeptide chains into shorter amino acid chains
a.
hydrolysis reaction
b.
controlled by hormone gastrin
4. Partially digested proteins move to duodenum
a.
CCK causes pancreas to release trypsin, chymotrypsin, and carboxypeptidase
b. these enzymes divide polypeptides into short peptides and amino acids
c. eventually all peptides are digested into amino acids
B. Amino Acid Absorption
1.
Small peptides and amino acids are actively absorbed in small intestine lumen into cells
of the small intestine
2.
Whole proteins are absorbed only up to 4 or 5 months of age
3. Absorbed small peptides are eventually broken down to amino acids inside the intestinal
cells
4.
Amino acids then travel to liver via portal vein
V. Function of Proteins
A.
B.
Producing Vital Body Constituents
1.
Every cell contains protein
2.
Measurements of the amounts of certain body protein are used as indicators of health or
disease
Maintaining Fluid Balance
1. Blood proteins, albumin and globulins, maintain body fluid balance
C.
2.
The presence of these proteins in the capillary beds attracts fluid back to the blood,
partially counteracting the force of blood pressure
3.
Edema
Contributing to Acid-Base Balance
1. Proteins help regulate the pH in the blood
2. Buffers are another means; hemoglobin important in maintaining blood pH
D.
E.
Forming Hormones and Enzymes
1.
Amino acids required for synthesis of many hormones
2.
Almost all enzymes and proteins or have a protein component
Contributing to Immune Function
Key parts of cells used by immune system and some antibodies
Immune incompetence is anergy
F.
Forming Glucose
1.
G.
Chronic conversion of amino acids into glucose wastes muscle tissue and can produce
adema
Providing Energy
During prolonged exercise
Costly source of energy
VI. Protein Needs
A.
Positive Protein Balance
B.
Negative Protein Balance
C.
Protein Equilibrium (calculations for Nitrogen balance are intake = output)
1. Nitrogen Balance = protein intake(24 hrs)/6.25 g – urinary urea(24 hrs) - (0.2 g x urinary
urea) – 2 g
D.
RDA is an allowance and not a requirement
1.
0.8 gram PRO/kg of DBW = PRO needs/d for adults
2.
1.0 gram PRO/kg of DBW = PRO needs/d for elderly
3.
1.5 grams PRO/kg of DBW = PRO needs /d for endurance athletes
4.
Special needs during illness or disease, tissue growth, after surgery
VII. Does Eating a High protein Diet Harm You?
A.
Benefits
1.
B.
Extra B-6, iron and zinc
Concerns
1.
Consumption of animal protein (if not well trimmed of fat) has been linked to heart
disease in humans
2.
Increased calcium loss in urine
3.
Excessive intake of red meat is linked to colon cancer in population studies
4.
Excess kidney function
5. Potential of amino acid imbalances and toxicities is too great to recommend amino acid
supplementation by an individual
C.
Recommendation: intake not more than 2 X RDA
VIII. Expert Opinion: A Close Look a Soybeans
A.
B.
Soy Nutrition
1.
Soy protein similar in quality to animal protein
2.
Source of omega-3 fatty acid and alpha-linoleic acid
3.
Calcium
Isoflavones
1.
Genistein
2.
C.
Daidzein
Soy and Heart Disease
1.
Cholesterol-lowering effects
2.
Increase arterial flexibility
3.
Inhibit LDL oxidation
4.
Inhibit smooth muscle cell proliferation
D. Soy Foods and Bone Health
E.
F.
1.
Effects observed at spine
2.
Long-term studies needed
Cancer Prevention and Treatment
1.
Role of Genistein
2.
Consumption early in life
Soy in Foods
IX. Protein in Foods
A.
Exchange System provides way to estimate protein
1.
Fruits and fats contain 0 gram protein
2.
Vegetable provides 2 grams protein
3.
Starch list provides 3 grams protein
4.
Meat list yields 7 grams protein
5.
Milk list yields 8 grams protein
B. Water-packed tuna is the most nutrient dense source of protein
C.
D.
The Value of Plant Protein
1.
Used less efficiently by body, but no cholesterol and little saturated fat
2.
Nuts are often overlooked as a source of plant protein
3.
Legumes are a plant family with pods that contain a single row of seeds
4.
Also excellent source of fiber in diet
Evaluation of Protein Quality
1.
Biological Value is a measure of how efficiently food protein can be turned into
body tissues once absorbed
a.
depends on how closely a food’s amino acid pattern reflects that of body tissue
b.
Egg white protein has BV of 100
c.
Milk and meat proteins also have high BV
d.
Plant proteins have low BV
2. Protein Efficiency Ratio
3.
a.
(PER) is another means of measuring food’s protein quality
b.
FDA uses this method to set standards for labeling of foods intended for infants
Chemical Score of Protein
a.
CS = actual mg of each essential amino acid per gram of protein/required mg
needs of that essential amino acid per gram of protein
b.
Scores vary from 0 to 1.0
4. Protein Digestibility Corrected Amino Acid Score (PDCAAS)
a.
most widely used measure of protein quality
b.
chemical score x digestibility of protein (generally, 0.9 to 1.0)
c.
a protein totally lacking any of the nine essential amino acids has a PDCAAS of
0, since its chemical score is 0
X. Protein-Energy Malnutrition
A.
B.
C.
Kwashiorkor
1.
Occurs primarily in young children who have an existing disease and consume a marginal
amount of energy and considerably insufficient protein in relation to needs
2.
The child suffers from infections and exhibits edema, poor growth, and increased
susceptibility to further illness
3.
Hallmark is presence of edema in child who has some subcutaneous fat still present
Marasmus
1.
Results from grossly consuming insufficient amount of protein and energy
2.
Victims have little or no fat stores, little muscle mass, and poor strength
3.
Death from infections is common
Kwashiorkor and Marasmus in the Hospital
1.
Hospitalized patients can develop either kwashiorkor or marasmus in the hospital
depending on a number of factors
2.
Measure albumin concentration in blood (kwashiorkor)
XI. Nutrition Perspective: Vegetarian Diets
A. Why Do People Practice Vegetarianism?
1.
Respect for animals
2.
Health
3.
Religion
4.
Cost
B. Food Planning for Vegetarians
C.
1.
Vegans = eat only plant foods
2.
Fruitarians = fruits, nuts, honey and vegetable oils
3.
Lactovegetarian = dairy products and plant foods
4.
Lactoovovegetarian = dairy products, eggs and plant foods
The Vegan
1.
Requires creative planning
a.
use grains and legumes (complementary proteins)
2.
Must include good sources of riboflavin, vitamins D and B 12, calcium, iron and zinc
3.
Special considerations during childhood
ACTIVITIES, EXERCISES, AND ASSIGNMENTS (Additional Activities in
Text)
1.
Using previous diet records, ask students to calculate their percent of kilocalories from protein and
compare their intake to the RDA.
2.
Suggest students determine the amount of dietary protein in their day's menu from plant and animal
sources.
3.
Request students calculate their own RDA based on desired body weight. Using previous diet
records, reduce or increase protein intake to meet RDA.
4.
Have students visit health food stores or consult athletic trainers to determine the kinds and amounts
of protein and amino acid supplements being promoted as muscle builders. Determine costs of
products and analyze nutrient content of products.
5.
Discuss with students the use of anabolic steroids and the long term health effects of their use.
6.
Ask students to examine popular magazines aimed at body builders and athletes for evidence of
misleading articles related to nutrition and physical fitness. Have students discuss how they might
convince others to follow sound eating practices and avoid spending money on worthless products.
7.
Have students visit feeding centers or food pantries for the homeless to determine the amounts and
types of legumes being served. What are the major sources of dietary proteins at these sites? What
recommendations would students make to improve the quality of diet for the recipients? Were the
recipients receiving adequate amounts of protein in the meal being served?
8.
Using the FoodWorks nutrient analysis printout from previous assignments analyze the protein and
amino acid content of the student's diet. Select the foods providing the greatest amount of dietary
protein and determine if they are being ingested in excess of the RDA.
9.
Use the following student assignment to practice planning menus for different categories of
vegetarians.
STUDENT ASSIGNMENT
Interview a vegetarian and ask the following questions:
1.
How is a vegetarian diet nutritionally balanced?
2.
What are the main concerns when menu planning?
3.
Do you need to take a nutritional supplement? Why? (If yes, what vitamins/minerals does it
contain?)
4.
What are the health advantages, as you see it, to being a vegetarian?
5.
What are some of the disadvantages to following a vegan diet?
6.
Try a food prepared with a soy product such as tofu, soy flour, or textured vegetable proteins or
milk substitutes. Describe how it tasted.
7.
What is your conclusion about being a vegetarian from this assignment?
ANNOTATED REFERENCES AND READINGS (Annotated References in
Text)
1.
Reeds PJ and Becklett PR (1996): Protein and Amino Acids. In Ziegler EE and Filer LJ (eds.),
Present Knowledge in Nutrition. International Life Sciences Institute, Washington, DC pp. 67-86.
2.
Fuller MF and Garlick PJ (1995): Human Amino Acid Requirements: Can the Controversy be
Resolved? Annu. Rev. Nutr. , 14:217-41.
3.
Should You Be Eating More Protein - Or Less? (1996). Annual Editions: Nutrition 98/99 (CookFuller, 10/e, 1998). pp. 47-49.
4.
Gopalan, C: The contribution of nutrition research to the control of undernutrition: The Indian
Experience. Protein-Energy malnutrition. pages 2-6. Annual Review of Nutrition. Vol 12, 1992.
5.
Belongia, EA and others: The Eosinophilia-Myalgia syndrome and tryptophan. Annual Review of
Nutrition 12,235, 1992.
6.
Constans, TY and others: Protein-energy malnutrition in elderly medical patients, Journal of
American Geriatrics Society 40:263, 1992.
7.
Fierman, AH and others: Growth delay in homeless children. Pediatrics 11:918, 1991.
8.
Kleiner, S M: performance-enhancing aids in sport: Health consequences and nutritional
alternatives, Journal of the American College of Nutrition 10:163, 1992.
MEDIA RESOURCES
Films:
1.
Vegetarian World. Bullfrog Films, Inc., Oley, PA 19547.
2.
Diet for a Small Planet. Bulldog Films, P.O. Box 114, Milford Square, PA 18935.
3.
Food or Famine. Shell Oil Co., 50 W. 50th St., New York, NY 10020.
Video:
1.
Protein I and Protein II. Audio-Visual Services, Pennsylvania State University, University Park, PA
16803.
2.
Protein and Meat Substitutes. Audio-Visual Services, Pennsylvania State University, University
Park, PA 16803.
3.
Nutrient Needs: Protein and Calories. Distribution Center, Health Sciences Consortium, 103 Laurel
Ave., Carrboro, NC 27510.
4.
Beans, Beans, Beans #7024. NCES, 1904 East 123rd Street, Olathe, KS 66061-5886.
Slides:
Nutrition: Energy and Protein. Milner-Fenwick, 2125 Greenspring Dr., Timonium, MD
21093.