CHAPTER 6 PROTEINS

Chapter 6
The Proteins and Amino
Acids
Sizer and Whitney, Nutrition: Concepts and Controversies, 15th
Edition. © 2020 Cengage. All Rights Reserved. May not be
scanned, copied or duplicated, or posted to a publicly accessible
website, in whole or in part.

Sizer and Whitney, Nutrition: Concepts and Controversies, 15th Edition. © 2020 Cengage. All Rights Reserved.
May not be scanned, copied or duplicated, or posted to a publicly accessible website, in whole or in part.
Learning Objectives (Slide 1 of 2)
• Describe the nature of proteins and amino acids
• Outline the processes of protein digestion and absorption
of amino acids
• Identify the roles of proteins and amino acids in the body
• List the factors that determine the daily protein needs of
an individual
• List the potential health problems that are caused by
eating patterns that are either too low or too high in
protein

Learning Objectives (Slide 2 of 2)
• Identify the benefits and drawbacks of protein-rich foods
in the diet
• Compare the advantages and disadvantages of a
vegetarian diet and a meat eater’s diet

The Structure of Proteins (Slide 1 of 2)
• Difference from carbohydrates and fats
• Contain nitrogen atoms in addition to the carbon, hydrogen, and
oxygen atoms
• Amino acids
• Carbon atom with amine group and acid group attached
• Have a distinctive chemical side chain attached to the center
carbon of the backbone

The Structure of Proteins (Slide 2 of 2)
• Essential amino acids
• Can be replenished only from foods
• Nonessential amino acid can become essential amino acid called
conditionally essential amino acid
• Recycling
• Body breaks protein and reuses their amino acids

Amino Acids
Single amino acids with
different side chains . . .
can bond to form . . . a strand of amino
acids, part of a protein.

Table 6.1: Amino Acids Important in Nutrition
Essential Amino Acids Nonessential Amino Acids
Histidine (HISS-tuh-deen) Alanine (AL-ah-neen)
Isoleucine (eye-so-LOO-seen) Arginine (ARJ-ih-neen)
Leucine (LOO-seen) Asparagine (ah-SPAR-ah-geen)
Lysine (LYE-seen) Aspartic acid (ah-SPAR-tic acid)
Methionine (meh-THIGH-oh-neen) Cysteine (SIS-tee-een)
Phenylalanine (fen-il-AL-ah-neen) Glutamic acid (glu-TAM-ic acid)
Threonine (THREE-oh-neen) Glutamine (GLU-tah-meen)
Tryptophan (TRIP-toe-fan, TRIP-toe-fane) Glycine (GLY-seen)
Valine (VAY-leen) Proline (PRO-leen)
N A Serine (SEER-een)
N A Tyrosine (TIE-roe-seen)

How Do Amino Acids Build Proteins?
(Slide 1 of 2)
• Peptide bonds
• Link amino acids
• A string of about 10 to 50 amino acids is known as a
polypeptide
• Strands of amino acids do not remain straight
• They coil or fold to form a globular structure

How Do Amino Acids Build Proteins?
(Slide 2 of 2)
• Side chain electrically charged are attracted to water
• Several strands may cluster together into a functioning
unit
• A metal ion, a vitamin, or a carbohydrate molecule may join

Figure 6.3: The Coiling and Folding of a Protein
Molecule
1 - The first shape of a strand of amino acids is
a chain, which can be very long. This shows just
a portion of the strand.
2 - Coiling the strand. The strand of amino acids
takes on a springlike shape as the side chains
variously attract and repel each other.
3 - Folding the coil. The coil then folds and flops
over on itself to take a functional shape.
4 - Once coiled and folded, the protein may be
functional as is, or it may need to join with other
proteins or to add a carbohydrate molecule or a
vitamin or mineral.

The Variety of Proteins
• Protein shapes enable performance of different tasks
• Globular shape of proteins in blood make it water soluble
• Some protein strands work alone, while others need
association
• Inherited amino acid sequences
• Hemoglobin in sickle-cell disease
• Nutrients do not alter genes but powerfully influence
genetic expression

Figure 6.4: The Structure of Hemoglobin

Figure 6.5: Normal Red Blood Cells and
Sickle Cells
Dr. Stanley Flegler/Visuals Unlimited

Figure 6.6: Protein Synthesis

Think Fitness: Can Eating Extra Protein
Make Muscles Grow Strong?
• Hard work is necessary to trigger the genes to build more
of muscle tissue
• Well-timed protein intakes can often further stimulate
muscle growth
• Protein intake cannot replace exercise
• Exercise generates cellular messages that stimulate the D N A to
begin synthesizing the muscle proteins needed to perform the
work

Denaturation of Proteins
(Slide 1 of 2)
• When a protein molecule
loses its shape, it can no
longer function as it was
designed to do
• Denaturing agents
• Heat, alcohol, acids, and so
on
• Digestion
• Inactivates proteins in
food
Fotokostic/Shutterstock.com

Denaturation of Proteins
(Slide 2 of 2)
• When a protein molecule loses its shape, it can no longer
function as it was designed to do
• Occurs in:
• Cooking
• Cooking eggs
• Salts of heavy metals
• Mercury and silver

Protein Digestion
• Stomach
• Hydrochloric acid denatures protein in food
• Uncoils protein’s strands
• Enzymes attack peptide bonds
• Small intestine
• Proteins enter as polypeptides
• Further break down to dipeptides and tripeptides
• Common misconceptions around protein digestion

Figure 6.8: A Dipeptide and Tripeptide

Figure 6.9: How Protein in Food Becomes Amino
Acids in the Body
1 - Stomach
When swallowed food arrives in the stomach,
hydrochloric acid denatures the protein strands,
and an enzyme cleaves amino acid strands into
polypeptides and a few amino acids.
2 - Small Intestine
Enzymes from the pancreas and the intestine
split polypeptide strands into tripeptides,
dipeptides, and abundant amino acids.
3 - Small Intestine
Enzymes on the surface of the small intestine’s
lining and within the absorptive cells split
tripeptides and dipeptides. The intestinal cells
absorb and transfer amino acids to the
bloodstream.
4 - Bloodstream
The bloodstream transports amino acids to the
liver.

Amino Acid Absorption
• Absorbed by cells of small intestine
• Larger molecules
• Act as hormones and play a role in food allergies
• Cells have separate sites for absorption
• Based on types of amino acids
• Once amino acids are circulating in the bloodstream, they
are carried to the liver
• Used or released into blood for use by other cells

The Importance of Protein
• Amino acids must be
continuously available
• Build proteins of new tissue
• Replace worn-out cells
• Protein turnover: Process
of breakdown, recovery,
and synthesis
Homydesign/Dreamstime

The Roles of Body Proteins (Slide 1 of 3)
• Regulation of gene expression
• Providing structure and movement
• Muscle tissue
• Other structural proteins
• Building enzymes
• Building hormones
• Other compounds
• Neurotransmitters

Figure 6.10: Enzyme Action
Enzyme plus two
compounds A and B
Enzyme complex with
A and B
Enzyme plus new
compound A B

Figure 6.11: Amino Acid Sequence of
Human Insulin

• Building antibodies
• Destroy one specific invader
• Transporting substances
• Hemoglobin and lipoproteins
• Maintaining fluid and
electrolyte balance
SPL/Science Source

Figure 6.13: Proteins Transport Substances into
and out of Cells
Molecule
enters protein
from inside cell.
Protein changes
shape; molecule
exits protein
outside the cell.
Molecule enters
protein from
outside cell.
Molecule exits
protein; proper
balance restored.

• Maintaining acid-base balance
• Buffers
• Acidosis
• Alkalosis
• Help in blood clotting
• Providing energy and glucose
• Under conditions of inadequate carbohydrates

Providing Energy and Glucose (Slide 1 of 2)
• Amino acids to glucose
• Can help maintain blood glucose level
• Incorporated by liver into urea
• Energy deficiency incurs wasting lean body tissues as
well as loss of fat
• No storage form of protein exists in the body

Providing Energy and Glucose (Slide 2 of 2)
• Oversupply of amino acids
• Body removes and excretes amine groups
• Uses the residues in three ways
• To meet immediate energy needs
• To make glucose for storage as glycogen
• To make fat for energy storage

Figure 6.14: Three Different Energy
Sources

The Fate of an Amino Acid
• Cellular use
• Build protein
• Make a needed compound
• Dismantle amino acid to build a different amino acid
• Wasting of amino acid occurs when body:
• Lacks energy from other sources
• Receives more proteins than needed or proteins of low quality
• Has too much of any single amino acid

Evaluating Protein and Amino Acid
Supplements
• Protein supplements
• Do not improve performance
• Not effective for weight loss
• Amino acid supplements
• No safe level of amino acid supplementation

People Most Likely to be Harmed by Amino
Acid Supplements
• Growth or altered metabolism makes these people
especially likely to be harmed by self-prescribed amino
acid supplements
• All women of childbearing age, especially those who are pregnant
or lactating
• Infants, children, and adolescents
• Elderly people
• People with inborn errors of metabolism that affect their bodies’
handling of amino acids
• Smokers
• People on low-protein diets
• People with chronic or acute mental or physical illnesses

Food Protein: Need and Quality
• Dietary Reference Intake (D R I)
• Depends on body size
• Higher for infants and growing children
• Recommended intake
• Athletes: 1.2 to 1.7 gram per kilogram per day
• Upper limit set
• Protein intake of no more than 35 percent of total calories
• Needed for enhanced immune functions
• Quality is crucial

Nitrogen Balance (Slide 1 of 2)
• Nitrogen excreted compared to nitrogen eaten
• Equilibrium in healthy adults
• Variation in nitrogen balance
• Positive balance: Nitrogen-in exceeds nitrogen-out
• Examples: Growing child, pregnant woman
• Negative balance: Nitrogen-out exceeds nitrogen-in
• Example: Muscle breakdown due to injury

Figure 6.15: Nitrogen Balance
Positive Nitrogen
Balance
These people—a
growing child, a person
building muscle, and a
pregnant woman—all
retain more nitrogen
than they excrete each
day.
Nitrogen Equilibrium
These people—a
healthy college student
and a young retiree—
are in nitrogen
equilibrium.
Negative Nitrogen
Balance
These people—an
astronaut and a surgery
patient—lose more
nitrogen than they take
in.

Nitrogen Balance (Slide 2 of 2)
• High-quality proteins
• Provide enough of all essential amino acids
• Influenced by:
• Amino acid composition
• Digestibility

Figure 6.17: Complementary Protein Combinations
John A. Rizzo/Getty Images © Polara Studios, Inc. Wiktory/Shutterstock.com

Protein Quality
• Limiting amino acids
helps limit the body’s
ability to build protein
• Protein-rich foods yield
complementary proteins
• Protein digestibility
measures the quality
• Protein quality can make
the difference between
health and disease

Protein Deficiency
• Consuming too little
protein
• Limiting amino acids
• Slows protein synthesis
• Increases breakdown of body
tissue
• Consequences
• Slow growth in children
• Impaired brain and kidney
functions
• Weakened immune defenses
• Impaired nutrient absorption
Malnutrition: too little food causes deficiencies of protein
and many other nutrients. See Chapter 15.
Wesley Bocxe/Getty Images

Excess Protein
• Weight loss through high protein diets
• Comes from calorie reduction, not from changing proportion of
energy nutrients
• Heart disease
• Animal-derived protein-rich foods
• Increases the kidneys’ workload
• Diet high in red meats and processed meats correlates
with cancer

Gluten-Free Diet
• Gluten triggers abnormal immune responses in people
with celiac disease
• Must eliminate all gluten-containing foods
• Non-celiac gluten sensitivity
• Patients suffer from digestive symptoms resembling those of celiac
disease but test negative for it
• Has no special power to spur weight loss

Getting Enough but Not Too Much Protein
• Protein-rich foods
• Protein is critical to nutrition
• Too much can displace other necessary foods
• Advantages of legumes
• Protein rich
• Vitamin and mineral source
• Example: Soybeans
• Tofu: Curd made from soybeans

Figure 6.22: Top Contributors of Protein to
the U.S. Diet
aThese foods supply about 70 percent of the protein in the U.S. diet. The remainder comes from foods that each
contributes less than 2 percent of the total.
bRounded values.

Figure 6.23: A Legume

Controversy 6:
Vegetarian or Meat-Containing Diets
• Affluent countries
• Disease risk
• Benefit from vegetarian diets
• Vegetarian lifestyle
• Vegetarians categorized by foods they choose to eat

Table C6.1: Terms Used to Describe Vegetarian
Diets (Slide 1 of 2)
Some of the following terms are in common usage, but others are useful only to
researchers
Fruitarian includes only raw or dried fruit, seeds, and nuts in the diet
Lacto-ovo vegetarian includes dairy products, eggs, vegetables, grains, legumes, fruit,
and nuts; excludes flesh and seafood
Lacto-vegetarian includes dairy products, vegetables, grains, legumes, fruit, and nuts;
excludes flesh, seafood, and eggs (lacto means “milk”)
Macrobiotic diet a vegan diet composed chiefly of whole grains, beans, and certain
vegetables; taken to extremes, macrobiotic diets can compromise nutrient status
Ovo-vegetarian includes eggs, vegetables, grains, legumes, fruit, and nuts, and
excludes flesh, seafood, and milk products (ovo means “egg”)
Partial vegetarian a term sometimes used to mean an eating style that includes
seafood, poultry, eggs, dairy products, vegetables, grains, legumes, fruit, and nuts;
excludes or strictly limits certain meats, such as red meats. Also called flexitarian

Table C6.1: Terms Used to Describe Vegetarian
Diets (Slide 2 of 2)
Some of the following terms are in common usage, but others are useful only to
researchers
Vegan includes only food from plant sources: vegetables, grains, legumes, fruit,
seeds, and nuts. Also called strict vegetarian
Vegetarian includes plant-based foods and eliminates some or all animal-derived
foods

Positive Health Aspects of Vegetarian Diets
• Provides defense against:
• Obesity
• Heart and artery disease
• High blood pressure
• Cancer
• Other health benefits
• May help prevent cataracts,
diabetes, diverticular disease,
gallstones, and osteoporosis
denio109/Shutterstock.com

Positive Health Aspects of the Meat Eater’s
Diet
• Iron and zinc are less
readily absorbed from
vegan sources
• Beneficial during critical
times in life
• Pregnancy and infancy
• Childhood
• Adolescence
• Aging and illness
Stock.com/Fcafotodigital

Planning a Vegetarian Diet
• Choosing within the food groups
• Emphasize sources of calcium and iron
• Dried fruits
• Fortified foods or supplements
• Convenience foods

Figure C6.2: Filling the Vegetarian MyPlate

Table C6.3: Vegetarian Sources of Key Nutrients
(Slide 1 of 5)
Nutrien
ts
Grains Vegetable
s
Fruit Protein Foods Milk Oils
Protein Whole
grains
N A N A Legumes,
seeds, nuts,
soy products
(tempeh, tofu,
veggie
burgers) Eggs
(for ovo-
vegetarians)
Milk,
cheese
,
yogurt
(for
lacto-
vegeta
rians)
N A
Iron Fortified
cereals,
enriched
and
whole
grains
Dark green
leafy
vegetables
(spinach,
turnip
greens)
Dried
fruit
(apricots,
prunes,
raisins)
Legumes
(black-eyed
peas, kidney
beans, lentils)
N A N A

(Slide 2 of 5)
Nutrient
s
Grains Vegetable
s
Fruit Protein
Foods
Milk Oils
Zinc Fortified
cereals,
whole
grains
N A N A Legumes
(garbanzo
beans,
kidney
beans, navy
beans), nuts,
seeds
(pumpkin
seeds)
Milk,
cheese,
yogurt (for
lacto-
vegetarian
s)
N A

(Slide 3 of 5)
Nutrient
s
Grains Vegetables Fruit Protein
Foods
Milk Oils
Calcium Fortified
cereals
Dark-green
leafy
vegetables
(bok choy,
broccoli,
collard
greens,
kale,
mustard
greens,
turnip
greens,
watercress)
Fortified
juices,
figs
Fortified
soy
products,
nuts
(almonds
), seeds
(sesame
seeds)
Milk,
cheese,
yogurt
(for lacto-
vegetaria
ns)
Fortified
soy or
pea milk
N A

(Slide 4 of 5)
Nutrient
s
Grains Vegetabl
es
Fruit Protein
Foods
Milk Oils
Vitamin
B12
Fortifie
d
cereals
N A N A Eggs (for
ovo-
vegetarians)
Fortified soy
products
Milk, cheese,
yogurt (for
lacto-
vegetarians)
Fortified soy or
pea milk
N A

(Slide 5 of 5)
Nutrients Grains Vegetables Fruit Protein
Foods
Milk Oils
Vitamin
D
N A High-vitamin D
mushrooms
(wild types
grown
in sunlight or
commercial
types treated
with ultraviolet
light; see
Chapter 7 for
details)
N A N A Milk,
cheese,
yogurt (for
lacto-
vegetarian
s)
Fortified
soy or
pea milk
N A
Omega-3
fatty
acids
N A N A N A Flaxseed,
walnuts,
soybeans
N A Flaxseed
oil,
walnut oil,
soybean
oil

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