Protein: Amino Acids
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
1. 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, glutamine, glycine, proline, serine, and tyrosine.
3. Essential amino acids, also called indispensable amino acids, must be supplied by
the foods people consume. Essential amino acids include histidine, isoleucine,
leucine, lysine, methionine, phenyalanine, threonine, tryptophan, and valine.
4. Conditionally essential amino acids refer to amino acids that are normally
nonessential but essential under certain conditions.
B. Proteins
1. Amino acid chains are linked by peptide bonds in condensation reactions.
a. Dipeptides have two amino acids bonded together.
b. Tripeptides have three amino acids bonded together.
c. Polypeptides have more than two amino acids bonded together.
2. Amino acid sequences are all different, which allows for a wide variety of possible
sequences.
3. Protein Shapes
a. Hydrophilic side groups are attracted to water.
b. Hydrophobic side groups repel water.
c. Coiled and twisted chains help to provide stability.
4. Protein Functions
a. Some carry and store materials.
b. Some provide strength.
c. Some require minerals for activation (example: hemoglobin and the mineral
iron).
5. 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.
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A. Protein Digestion
1. In the Stomach
a. Protein is denatured by hydrochloric acid.
b. Pepsinogen (a proenzyme) is converted into its active form pepsin in the
presence of hydrochloric acid.
c. Pepsin cleaves proteins into smaller polypeptides.
2. In the Small Intestine
a. Proteases hydrolyze protein into short peptide chains called oligopeptides, which
contain four to nine amino acids.
b. Peptidases split proteins into amino acids.
B. Protein Absorption
1. Used by intestinal cells for energy or synthesis of necessary compounds
2. Transported to the liver
3. Taking enzyme supplements or consuming predigested proteins is unnecessary
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
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
3. Transfer RNA lines up the amino acids and brings them to the messenger
4. Sequencing errors can cause altered proteins to be made. An example is sickle-cell
anemia where an incorrect amino acid sequence interferes with the cell’s ability to
carry oxygen.
5. Nutrients and Gene Expression - Cells regulate gene expression to make the type of
protein needed for that cell.
a. Epigenetics refers to a nutrient’s ability to activate or silence genes without
interfering with the genetic sequence.
B. Roles of Proteins
1. Building Materials for Growth and Maintenance
a. A matrix of collagen is filled with minerals to provide strength to bones and
teeth.
b. Replaces tissues including the skin, hair, nails, and GI tract lining
2. Enzymes are proteins that facilitate anabolic (building up) and catabolic (breaking
down) chemical reactions.
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3. Hormones regulate body processes and some hormones are proteins. An example is
insulin.
4. Regulators of Fluid Balance
a. Plasma proteins attract water
b. Maintain the volume of body fluids to prevent edema which is excessive fluid
c. Maintain the composition of body fluids
5. Acid-Base Regulators
a. Act as buffers by keeping solutions acidic or alkaline
b. Acids are compounds that release hydrogen ions in a solution.
c. Bases are compounds that accept hydrogen ions in a solution.
d. Acidosis is high levels of acid in the blood and body fluids.
e. Alkalosis is high levels of alkalinity in the blood and body fluids.
6. Transporters
a. Carry lipids, vitamins, minerals and oxygen in the body
b. Act as pumps in cell membranes, transferring compounds from one side of the
cell membrane to the other
7. Antibodies
a. Fight antigens, such as bacteria and viruses, that invade the body
b. Provide immunity to fight an antigen more quickly the second time exposure
occurs
8. Source of energy and glucose if needed
9. Other Roles
a. Blood clotting by producing fibrin which forms a solid clot
b. Vision by creating light-sensitive pigments in the retina
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.
b. Amino acid pool is the supply of amino acids that are available.
1. Amino acids from food are called exogenous.
2. Amino acids from within the body are called endogenous.
2. Nitrogen Balance
a. Zero nitrogen balance is nitrogen equilibrium, when input equals output.
b. Positive nitrogen balance means nitrogen consumed is greater than nitrogen
excreted.
c. Negative nitrogen balance means nitrogen excreted is greater than nitrogen
consumed.
3. Using Amino Acids to Make Proteins or Nonessential Amino Acids – Cells can
assemble amino acids into the protein needed.
4. Using Amino Acids to Make Other Compounds
a. Neurotransmitters are made from the amino acid tyrosine.
b. Tyrosine can be made into the melanin pigment or thyroxine.
c. Tryptophan makes niacin and serotonin.
5. Using Amino Acids for Energy and Glucose
a. There is no readily available storage form of protein.
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b. Breaks down tissue protein for energy if needed
6. Deaminating Amino Acids
a. Nitrogen-containing amino groups are removed.
b. Ammonia is released into the bloodstream.
c. Ammonia is converted into urea by the liver.
d. Kidneys filter urea out of the blood.
7. Using Amino Acids to Make Fat
a. Excess protein is deaminated and converted into fat.
b. Nitrogen is excreted.
IV. Protein in Foods
Eating foods of high-quality protein is the best assurance 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.
2. Plant proteins are 70-90% absorbed.
3. Soy and legumes are 90% 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.
b. Cells must dismantle to produce essential amino acids if they are not provided in
the diet.
c. Limiting amino acids are those essential amino acids that are supplied in less
than the amount needed to support protein synthesis.
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
a. Contains all the essential amino acids
b. Animal foods contain all the essential amino acids.
c. Plant foods are diverse in content and tend to be missing one or more essential
amino acids.
5. Complementary Proteins
a. Combining plant foods that together contain all the essential amino acids
b. Used by vegetarians
6. A Measure of Protein Quality - PDCAAS (protein digestibility-corrected amino
acid score)
a. Compares amino acid composition of a protein to human amino acid requirements
b. Adjusts for digestibility
B. Protein Regulation for Food Labels
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1. List protein quantity in grams
2. % Daily Values is not required but reflects quantity and quality of protein using
PDCAAS.
V. Health Effects and Recommended Intakes of Protein
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) – also called protein-kcalorie malnutrition
(PCM)
1. Classifying PEM
a. Chronic PEM and acute PEM
b. Maramus, kwashiorkor, or a combination of the two
2. Marasmus
a. Infancy, 6 to 18 months of age
b. Severe deprivation or impaired absorption of protein, energy, vitamins and
minerals
c. Develops slowly
d. Severe weight loss and muscle wasting, including the heart
e. < 60% weight-for-age
f. Anxiety and apathy
g. Good appetite is possible
h. Hair and skin problems
3. Kwashiorkor
a. Older infants and young children, 18 months to 2 years of age
b. Inadequate protein intake, infections
c. Rapid onset
d. Some muscle wasting, some fat retention
e. Growth is 60-80% weight-for-age
f. Edema and fatty liver
g. Apathy, misery, irritability and sadness
h. Loss of appetite
i. Hair and skin problems
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
b. Fever
c. Fluid imbalances and dysentery
d. Anemia
e. Heart failure and possible death
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6. Rehabilitation
a. Nutrition intervention must be cautious, slowly increasing protein.
b. Programs involving local people work better.
B. Health Effects of Protein
1. Heart Disease
a. Foods high in animal protein also tend to be high in saturated fat.
b. Homocysteine levels increase cardiac risks.
c. Arginine may protect against cardiac risks.
2. Cancer
a. A high intake of animal protein is associated with some cancers.
b. Is the problem high protein intake or high fat intake?
3. Adult Bone Loss (Osteoporosis)
a. High protein intake associated with increased calcium excretion.
b. Inadequate protein intake affects bone health also.
4. Weight Control
a. High-protein foods are often high-fat foods.
b. Protein at each meal provides satiety.
c. Adequate protein, moderate fat and sufficient carbohydrate better support weight
loss.
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
1. 10-35% energy intake
2. Protein RDA
a. 0.8 g/kg/day
b. Assumptions
1. People are healthy.
2. Protein is mixed quality.
3. The body will use protein efficiently.
3. Adequate Energy
a. Must consider energy intake
b. Must consider total grams of protein
4. Protein in abundance is common in the U.S. and Canada.
D. Protein and Amino Acid Supplements
1. Many reasons for supplements
2. Protein Powders have not been found to improve athletic performance.
a. Whey protein is a waste product of cheese manufacturing.
b. Purified protein preparations increase the work of the kidneys.
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.
b. Lysine appears safe in certain doses.
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c. Tryptophan has been used experimentally for sleep and pain, but may result in a
rare blood disorder.
b. Individualizing treatment
c. Individualizing medications
d. Increasing knowledge of nongenetic causes of disease
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.
E. Nutrition on the Net
1. www.wadsworth.com/nutrition
2. www.cdc.gov/genomics
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Study Questions
1. How does the chemical structure of proteins differ from the structures of carbohydrates
and fats?
2. Describe the structure of amino acids, and explain how their sequence in proteins
affects the proteins’ shapes. What are the essential amino acids?
3. Describe protein digestion and absorption.
4. Describe protein synthesis.
5. Describe some of the roles proteins play in the human body.
6. What are enzymes? What roles do they play in chemical reactions? Describe the
differences between enzymes and hormones.
7. How does the body use amino acids? What is deamination? Define nitrogen balance.
What conditions are associated with zero, positive, and negative balance?
8. What factors affect the quality of dietary protein? What is a complete protein?
.
9. How can vegetarians meet their protein needs without eating meat?
10. What are the health consequences of ingesting inadequate protein and energy? Describe
marasmus and kwashiorkor. How can the two conditions be distinguished, and in what
ways do they overlap?
11. How might protein excess, or the type of protein eaten, influence health?
12. What factors are considered in establishing recommended protein intakes?
13. What are the benefits and risks of taking protein and amino acid supplements?
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