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Proteins
Chapter 4
The Nature of Proteins
Basic Building Material
All proteins are made
up of amino acids.
Amino acids are
joined in unique, chain
sequences to form
specific proteins.
Each amino acid is
joined by a peptide bond.
Amino acids are then reassembled in
the body in a specific order to form a
variety of proteins needed by the
body.
Because proteins are
relatively large, complex
molecules, they are often
subject to mutations, or
malformations in structure.
Example: protein-folding mistakes are
involved in Alzheimer’s disease.
When protein is eaten, the protein in
in foods is broken down into amino
acids in the digestive process.
Major Functions of Protein
Dietary Importance
The word amino refers to
compounds containing
nitrogen.
Proteins have a
basic structure of:
carbon,
hydrogen,
oxygen and
nitrogen.
Protein is about 16% nitrogen.
As such protein is the primary
source of nitrogen in the diet.
Chemical
composition of
protein:
C – carbon
H – hydrogen
O – oxygen
N - nitrogen
Classes of Amino Acids
There are 20 common amino
acids, which are all vital to
human life and health.
These amino acids
are classified as:
indispensable, (9)
dispensable, (5) or
conditionally
indispensable (6 ) in
the
diet according to
whether
the body
can make them.
Indispensible Amino Acids
(amino acids necessary in the
diet and cannot be left out)
Nine (9) amino acids are
classified as indispensable
amino acids because the body
cannot manufacture them in
sufficient quantity, or at all.
Refer to Box 4-1 page 48.
Classes of Amino Acids – Cont’d
Dispensable Amino Acids
Refers to five (5)
amino acids that
the body can
synthesize from
other amino acids.
These amino acids are
needed by the body for
healthy life, but are
dispensable in the diet.
The remaining six (6) amino
acids are classified as
conditionally indispensable.
Like other amino
acids they must be
consumed in the diet.
Balance
Relative intake and output
of substances in the body
to maintain equilibrium
necessary for health in
various circumstances
throughout life.
Protein Balance:
The body’s tissue
proteins are
constantly being
broken down into
amino acids, a
process called
catabolism, and
they resynthesized
into tissue proteins
as needed, a process
called anabolism.
Protein Balance – Cont’d
To maintain nitrogen balance, the
part of the amino acid that
contains nitrogen may be
removed by deamination, then
converted to ammonia (NH³) and
excreted as urea in the urine.
The remaining nonnitrogen residue can be
used to make
carbohydrate or fat or
reattached to make
another amino acid, if
necessary.
Tissue turnover is a continuous
process of:
 reshaping,
 rebuilding, and
 adjusting as necessary to
maintain overall protein
balance within the body.
The body also
maintains a balance
between:
 tissue proteins and
plasma protein, which
are then further balanced
with dietary protein
intake.
Nitrogen Balance
The body’s nitrogen balance
indicates how well its tissues are
being maintained.
The intake and use of
dietary protein is
measured by the amount
of nitrogen intake in food
protein and the amount
of nitrogen excreted in
the urine.
Positive Nitrogen Balance exists
when the body takes in more
nitrogen than it excretes, thus
storing more nitrogen by building
more tissue than it is losing.
This situation occurs
normally during
periods of rapid growth.
It can also occur with
individuals who have
been ill or malnourished
and are being “built back
up” with increased
nourishment.
Negative Nitrogen Balance
When body takes in less
nitrogen than it excretes.
This means that the
body has an
inadequate protein
intake and is losing
nitrogen by breaking
down more tissue
than it is building up.
This condition can be seen
in situations of malnutrition
and illness.
Failure to maintain nitrogen
balance may not become
apparent for some time but
eventually causes:
 loss of muscle tissue,
 impairment of body
organs and functions,
and
 increased susceptibility
to infection.
In children it causes
growth retardation.
Functions of Protein
Primary Function: Tissue Building
The primary functions of protein are:
Protein is the fundamental
structural material of every cell in the body.
repair worn-out wasted, or
The largest portion of the body is made
up of protein.
damaged tissue and
Protein not only makes up the bulk of:
the muscles,
internal organs,
brain,
nerves,
skin,
hair, and
nails,
but is also a vital part of regulatory substances such as:

enzymes,

hormones, and

blood plasma.
build up new tissue
Thus protein meets growth needs and maintains tissue
health during adult years.
Additional Body Functions
Protein has other body
functions relating to:
 energy,
 water balance,
 metabolism, and
 the body’s defense system
Energy System:
In times of need,
protein may furnish
additional body fuel
to sustain body heat
and energy, but this is
a less efficient, backup source for use
only when there is an
insufficient supply of
carbohydrate and fat.
The available fuel factor of
protein is 4 kcal/g.
Water Balance
Plasma protein, especially albumin
helps control water balance
throughout the body.
Metabolism: Protein aids metabolic functions
through enzymes, transport agents, and
hormones.
Controlling metabolic
processes are the digestive and cell
enzymes.
 Enzymes necessary for the
digestion of carbohydrates,
fats, and proteins.
Proteins also act as the
vehicle in which nutrients are
carried throughout the body.
Lipoproteins are necessary to transport fats in
the water-soluble blood supply. (chylomicrons)
Other proteins:
 hemoglobin (oxygen carrier),
transferrin (iron transport),
hormones:  insulin and
glucagon function in the
metabolism of glucose
Body Defense System
Protein is used to build
 (lymphocytes)
and
antibodies as
part of the
body’s immune
system to help
defend against
disease and
infection.
Food Sources Of Protein
In a mixed diet, animal and plant foods provide a
wide variety of many nutrients and proteins that
supplement each other.
The key to a balanced diet is
variety.
Food proteins are classified as
complete or incomplete proteins,
depending on their amino acid
composition.
Complete Proteins:
Protein foods that contain all nine
indispensable amino acids in
sufficient quantity and ratio to meet
the body’s needs are called complete
proteins.
These proteins are primarily of animal origin.
Soybeans and soy products are
exceptions.
Soy products are the only plant sources of
complete proteins.
It
Gelatin is not a complete protein.
lacks three essential amino acids.
The tasteless, odorless, brittle
mixture of proteins, extracted by
boiling pig skin and calf bones.
Gelatin dissolves in hot water,
forming a jellylike substance when
cool and is used in preparation of
various foods, medicine capsules,
photographic film, etc.
Incomplete Proteins
Protein foods that are deficient in
one or more of the nine
indispensable amino acids are called
incomplete proteins.
These proteins
are of plant origin
(grains, legumes,
nuts, seeds,
vegetables, and
fruits).
Vegetarian Diets
A mixture of plant proteins can
provide adequate amount of amino
acids when the basic use of various
grains is expanded to include soy
protein and other dried legume
proteins such as beans and peas.
This is the art of
combining plant
protein foods so that
they “complement”
one another and supply
all nine indispensable
amino acids.
The underlying requirement
for vegetarians, as for all
people, is to eat a sufficient
amount of varied foods to
meet normal nutrient and
energy needs.
Review: Cultural
Considerations
Indispensable Amino Acids
and Their Complementary
Food Proteins
Types of Vegetarian Diets
1. Lacto-ovo-vegetarians
2. Lacto-vegetarians
3. Ovo-vegetarians
4. Vegans
Lacto-ovo-vegetarians:
allows dairy products and
eggs.
Their mixed diet
of plant and animal
food sources,
excluding only meat
and fish, poses no
nutritional problems.
Types of Vegetarian Diets
Lacto-vegetarians: accept Vegans: Follow a strict
only dairy products from
vegetarian diet and use no
animal sources to
animal foods.
complement their basic
The use of soybeans,
soy milk, soybean curd
diet of plant foods.
(tofu) and processed
Ovo-vegetarians: The only
soy protein products
enhances the nutritional
animal foods included in
value of the diet.
this diet is eggs.
Health Benefits and Risk
The preventive mechanism at
work in the vegetarian diet is
the rich supply of:
monounsaturated and
polyunsaturated fatty
acids,
fiber,
complex carbohydrates,
antioxidants, and a
restriction in saturated fat.
Digestion of Protein:
– The protein must be
changed into amino
acids by a process of
mechanical and chemical
digestion.
– The mechanical
breaking down of
protein foods occurs by
chewing in the mouth.
Digestion of Proteins
Stomach: A series of enzymes
are necessary to break
proteins into amino acids for
absorption.
All enzymes involved
in protein digestion
(proteases) are stored
as inactive proenzymes
called zymogens.
Zymogens are then activated
upon as needed.
Chemical digestion of
protein begins in the
stomach.
The stomach’s chief
digestive function overall
is the first stage in the
enzymatic breakdown of
protein.
Digestion Of Protein – Cont’d
Pepsin: It is the main gastric
enzyme, specific to proteins.
It is first produced as
an inactive proenzyme,
pepsinogen by the
single layer of cells in
the stomach wall.
The hydrochloric acid within
gastric juices then changes
pepsinogen to the enzyme
pepsin.
The active pepsin begins
splitting the links between the
proteins’ amino acids, which
changes the large amino acid
chains that make up the
protein into smaller short
chains called peptides.
Pepsin only
completes the first
stage of breakdown
due to normal gastric
emptying time.
Digestion of Proteins
Hydrochloric AcidProvides the acid
medium necessary
to convert
pepsinogen to
active pepsin.
Begins the
unfolding and
denaturing of the
complex protein
chains.
Rennin- (gastric enzyme)
present only in infancy and
childhood, especially
important in the infant’s
digestion of milk
Small Intestine
Protein digestion begins
in the acidic medium of
the stomach and is
completed in the alkaline
medium of the small
intestine.
Digestive
enzymes from the
pancreas and
intestines take
part.
Small Intestine
Pancreatic SecretionsThe following three
enzymes from the
pancreas continue to
break down protein.
1. Trypsin : secreted as inactive
trypsinogen is activated by the
enzyme enterokinase.
Enterokinase is secreted from
the intestinal cells upon contact
with food entering the
duodenum, the first section of
the small intestine.
2. Chymotrypsin:
secreted first as the
inactive
chymotrypsinogen, is
activated by trypsin
already present
3. Carboxypeptidase: attacks
the acid (carboxyl) end of the
peptide chains, producing
small peptides and some free
amino acids
Small Intestine
Intestinal Secretions:
To complete the
breakdown and free
the remaining
amino acids the
intestinal wall
produces two more
protein splitting
enzymes:
Aminopeptidase
Dipeptidase
Intestinal Secretions
1. Aminopeptidase
attacks the nitrogencontaining (amino) end of
the peptide chains and
releases amino acids one
at a time.
Review Figure 45 Summary of
Protein Digestion
2. Dipeptides completes
the large task by breaking
the remaining dipeptides
into two free amino acids.
The free amino
acids are now ready
to be absorbed
directly into the
portal blood
circulation for use in
building body tissues.
Body Needs For Protein
Protein requirementsThe following three factors
influence our requirement for
protein:
– 1. tissue growth needs
– 2. quality of the dietary
protein
– 3. additional needs
owing to illness or
disease
Tissue Growth: During rapid growth
periods, more protein is necessary to
build new tissue and maintain
present tissue.
Human growth is most rapid during:
 fetal growth
 mother’s pregnancy,
 infant growth
 first years of life
 lactation needs of a breastfeeding
mother, and
 adolescent growth and
development into adulthood
Illness or Disease
An illness or disease
especially with fever and
increased tissue
breakdown (catabolism)
increases the body’s need
for protein and kcalories
for rebuilding tissue and
to meet the demands of
increased metabolic rate.
After surgery, extra
protein is needed for
wound healing and
restoring losses.
Massive burns
requires a large
protein increase for
the healing and
grafting processes.
Dietary Deficiency or Excess
Protein-Energy
Malnutrition- (PEM)
The most severe
cases are found in
less industrialized
countries where not
only protein-rich
foods are in short
supply, but all foods
are.
Children are at the highest risk
for developing PEM because of
their elevated needs during
rapid growth and
development.
Dietary Deficiency or Excess
Persons with poor nutrient
intake, such as the elderly or
patients w/eating disorders,
may suffer from PEM as well.
Without the amino
acids building blocks,
the body cannot
synthesize needed
structural (muscle) or
functional (enzymes,
antibodies, hormones,
etc.) proteins.
Two severe forms of PEM
are:
Kwashiorkor
Marasmus
Protein-Energy Malnutrition
Kwashiorkor: result from an
acute deficiency of protein.
It is common in
children between 18 –
24 months who had
been breastfeeding and
then are rapidly
weaned.
They are switched from a
nutritionally balanced breast
milk to a diluted diet of mostly
carbohydrates and little
protein.
They lack enough
kcalories from protein
sources.
Characteristics include:
edema in the feet and
legs,
 a bloated
abdomen, both from lack
of protein in the blood
to maintain fluid balance
and transport fat away
from the liver.
Protein-Energy Malnutrition
Marasmus: It is more
likely to affect individuals
of all ages, suffering from
inadequate food sources.
This is a chronic form of
energy and protein
deficiency which is basic
starvation.
Characteristics include:
very emaciated
appearance with
little or no body fat
stunted growth
and development
are even more
severe with this
form of PEM.
Excess Dietary Intake
The body has a finite need for
protein.
Once a person has
met the dietary
protein needs,
additional protein is
deaminated (the
nitrogen is
removed) and stored
as fat or used as
energy.
Eating excess protein does not
build muscle.
The main problems with diets high in
protein are:
usually high in saturated
fats- a risk for
cardiovascular disease
little room for fruits,
vegetables, and other whole
grains
kidneys have to work
overtime to rid the body of
excess nitrogen
Excess protein increases
calcium losses in the bone
when dietary calcium is
inadequate
Dietary Guides
Children and adults should get 10%
to 35% of their total caloric intake
from protein.
The RDA for both
men and woman is
set at 0.8 g of highquality protein per
kg of desirable body
weight per day.
Example: The grams of protein
required for an individual who
weighs 150 lb ideal body weight
would be?
Answer: 54.4 g
150/2.2=68 kg
68 kg X 0.8 = 54.4 g
The RDA is set to meet the
nutritional requirements
of most healthy people.
Severe physical
stress, such as
illness, disease, and
surgery can increase
one’s needs for
protein.
Dietary Guidelines For Americans
There are no known benefits
in the consumption of a diet
with a high animal protein
content, which also carries
added fat.
The potential risks are related
to:
 certain cancers,
CAD,
urinary calcium loss
and kidney stones
 CRF
The Dietary Guidelines for
Americans recommend
choosing foods from each of
the five food groups based on
the Food Guide Pyramid,
consuming moderate amounts
of foods from the meat and
bean group.
 Moderate amounts
equal two to three
servings per day.
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