Proteins
Presented by
Janice Hermann, PhD, RD/LD
OCES Adult and Older Adult Nutrition Specialist
Chemistry of Proteins
 Chemically, proteins contain the same
elements as carbohydrates and lipids - carbon
(C), hydrogen (H), and oxygen (O)
 But proteins also contain nitrogen (N).
 Some also contain sulfur.
Amino Acids
 All amino acids have a central carbon (C)
atom; attached to a hydrogen (H), an amino
group (NH2), and an acid group (COOH).
 Carbons need four bounds, the fourth bond
is what distinguished one amino acid from
another, this is called the side group.
Amino Acids
 There are 20 different amino acids, each
with a different side group
 11 amino acids are nonessential because the
body can make them
 9 amino acids are essential because the body
can’t make them. Therefore they must be
provided by the diet.
Essential Amino
Acids
Histidine
Isoleucine
Nonessential Amino
Acids
Alanine
Arginine
Leucine
Lysine
Methionine
Asparagine
Aspartic acid
Cysteine
Phenyalalnine
Threonine
Tryptophan
Valine
Glutamic acid
Glutamine
Glycine
Proline
Serine
Tyrosine
Proteins
 Amino acids are linked end-to-end with a
peptide bond in different sequences to form
proteins.
 Two amino acids is a dipeptide
 Three amino acids is a tripeptide
 More than three amino acids is a polypeptide
Proteins
 Most proteins are a few dozen to several
hundred amino acids long.
 The body contains 10,000 to 50,000 different
kinds of proteins.
Proteins
 As amino acids are bonded together to make
a polypeptide, the chain folds and twists so
that hydrophilic side groups are on the
outside and hydrophobic groups are tucked
inside away from water.
 The intricate, coiled shape the polypeptide
finally assumes gives it maximum stability.
Protein
 The unique shapes of proteins enable them to
perform their various functions in the body.
 Some are hollow balls that carry materials
 Some much longer than they are wide
 Some function just they are;
others need other polypeptides
to form larger complexes.
 Some proteins need
minerals to activate them.
Proteins
 When proteins are exposed to heat, acid, or
other conditions that disturbs their stability
and they denature.
 When proteins are denatured they uncoil,
lose their shape and consequently their
ability to function.
 An example of denaturation is milk curdling when
acid is added
Digestion
 In the stomach hydrochloric acid uncoils
(denatures) each protein so that digestive
enzymes can attack the peptide bonds.
 Hydrochloric acid also converts the inactive
form of the enzyme pepsinogen into the
active form pepsin. Pepsin breaks proteins,
large polypeptides, into smaller polypeptides
and some amino acids.
Digestion
 In the small intestine, pancreatic and
intestinal proteases further breakdown
polypeptides into short peptide chains,
tripeptides, dipeptides and amino acids.
 Peptidase enzymes on the intestinal cell
membrane surfaces split most of the
dipeptides and tripeptides into single amino
acids.
 Only a few peptides escape digestion and
enter the blood intact.
Misconceptions
 A common misconception is that enzymes
are absorbed intact.
 Enzymes are proteins and the digestive enzymes
will breakdown the enzyme into single amino
acids just like any other protein.
 Another misconception is it is better to
consume amino acid supplements.
 The digestive system handles whole proteins
better than individual amino acids.
Protein Synthesis in the Body
 The body contains 10,000 to 50,000
different kinds of protein
 Each protein has a specific function
determined by its amino acid sequence
 The process of making proteins involves
DNA, messenger RNA, ribosomes, and
transfer RNA, amino acids and enzymes
Protein Synthesis in the Body
 DNA gene carries the protein code
 Messenger RNA copies the protein code and
leaves the nucleus
 Messenger RNA attached to the ribosome
 Transfer RNA brings amino acids to the
ribosome and line them up in the order of the
protein code
 Enzymes bind the amino acids together
Protein Synthesis
Protein Synthesis
Functions
 Building Materials
 Body cells and tissues
 Muscles
 Blood
 Skin
 Hair and fingernails
 Collagen (bone and tooth matrix, ligaments,
tendons, wound healing)
Functions
 Enzymes are protein
 Smaller proteins that catalyze (speed up) chemical
reactions. Enzymes are needed for most metabolic
reactions in body cells
 Enzymes breakdown substances, build substances
and transform one into another
 Half the protein eaten daily is used to make new
enzymes
Functions
 Some hormones are protein
 Hormones are messengers that turning on and off
metabolic processes.
 Some examples of protein hormones include:
Insulin, thyroid hormones, histimine, adrenaline,
serotonin.
Functions
 Proteins help maintain fluid and electrolyte
balance.
 Body fluids can flow freely and are contained
inside and outside cells and between cells and
within blood vessels.
 Proteins are larger and cannot cross freely
 Proteins attract water
Functions
 Acid-base balance
 Metabolic processes produce acids and bases
which are released into the blood for excretion
 By accepting and releasing hydrogen, proteins act
like buffers to help maintain the body’s acid-base
balance.
Functions
 Proteins act as carriers or transporters for
nutrients and other molecules
 Hemoglobin carries oxygen
 Lipoproteins carry lipids
 Some proteins nutrients across cell walls, other
carry nutrients in body fluids
Functions
 Antibodies are proteins
 When the body detects an in invading antigen, it
makes antibodies, giant protein molecules
designed to combat them.
 Without sufficient protein, the body cannot
maintain its antibodies to resist disease.
 Each antibody is designed to destroy one specific
antigen.
Functions
 Energy
 If glucose is limited the body can metabolize
amino acids for energy.
 4 calories per gram protein.
 The body will break down body tissues in order to
provide amino acids for energy, which can lead to
lean body tissue wasting.
Other Functions
 Blood clotting
 An injury starts a chain reaction leading to
production of fibrin, a stringy protein that forms
a clot. Protein collagen forms a scar to replace
the clot and heals the wound.
 Vision
 Opsin is a light sensitive protein pigment in the
retina. It responds to light by changing its shape,
initiating the nerve impulses conveying the
sense of sight to the brain.
Metabolism
 Using Amino Acids to Make Proteins or
Nonessential Amino Acids
 If a nonessential amino acid is missing, body cells
can make if from another amino acid.
 If an essential amino acid is missing, the body may
break down some of its own proteins to obtain it.
Metabolism
 Using Amino Acids to Make Other Compounds
 Tyrosine can be made into the pigment melanin
 Tyrosine can also be made into the hormone
thyroxin
 Tryptophane serves as a precursor for the vitamin
niacin and for serotonin (a neurotransmitter)
Metabolism
 Using Amino Acids for Energy and Glucose
 When glucose or fatty acids are limited, cells are
forced to use amino acids for energy and glucose.
 The body does not make a specialized storage
form of protein, so the body dismantles its tissue
proteins and uses them for energy.
 This can lead to lean body tissue wasting.
Metabolism
 Deaminating Amino Acids
 When amino acids are broken down they are first
deaminated, the nitrogen group is removed,
which is convered into ammonia and then urea
and excreted
 The remaining carbon fragments of the
deaminated amino acids can be used for energy
or for the production of glucose, ketones,
cholesterol or fat.
Metabolism
 Amino acids can be used to make fat
 When calorie and protein intakes exceed needs
and carbohydrate intake is adequate, amino acids
are deaminated, the nitrogen is excreted, and the
remaining carbon fragments are converted to fat
and stored for later use.
Protein Quality
 High-quality proteins provide enough of all
the essential amino acids needed to support
the body’s work and low-quality proteins
don’t.
 Two factors affect protein quality, the
protein’s digestibility and its amino acid
composition.
Protein Digestibility
 Protein digestibility depends on such factors
as the protein’s source and the other foods
eaten with it.
 Digestibility of most animal proteins is high
(90 to 99 percent); plant proteins are less
digestible (70 to 90 percent for most, but
over 90 percent for soy and legumes).
Amino Acid Composition
 All amino acids are needed simultaneously
to make protein.
 Nonessential amino acids can be made if in
short supply but if essential amino acids are
limited protein must be dismantled.
 To prevent protein breakdown, dietary
protein must supply at least the nine
essential amino acids.
Amino Acid Composition
 Animal foods (meat, fish, poultry, cheese,
eggs, yogurt, and milk) provide high-quality
proteins.
 Plant proteins (vegetables, nuts, seeds, grains,
and legumes) have more diverse amino acid
patterns and tend to be limiting in one or
more essential amino acids. A few such as soy
protein are high quality.
Complementary Proteins
 The quality of plant proteins can be improved
by combining plant protein foods that have
different but complementary amino acid
patterns.
 This strategy produces complementary
proteins that together contain all the
essential amino acids
Complementary Proteins
 Complement a plant protein by combining
one plant protein low in one amino acid,
with another plant protein high in that
amino acid.
 Another way is to combine a plant protein
with a small amount of animal protein.
 When complementary proteins are eaten
together or within the course of the day all
essential amino acids are provided.
Complementary Proteins
 Plant proteins low in certain amino acids
 Wheat and rice low in lysine
 Corn low in lysine and tryptophane
 Beans low in methionine
 Examples of complementary proteins
 Rice and beans
 Peanut butter and bread
 Corn bread and beans
 Corn tortillas and beans
 Macaroni and cheese
Protein Requirement
 The RDA for adults for protein is 0.8 grams
protein/kg body weight.
 The RDA covers the needs for replacing
worn-out tissue, so it increases for larger
people; it also covers the needs for building
new tissue during growth, so it increases
for infants, children and pregnant women.
 The protein RDA is the same for athletes as
for others.
USDA Daily Food Plan
 For a reference 2,000 calorie intake the USDA
Daily Food Plan can provide adequate protein
 18 g protein from 6 oz grain
 10 g protein from 2 ½ cups vegetables
 24 g protein from 3 cups dairy
 38 g protein from 5 ½ protein foods
 90 g protein total, which would be adequate protein
for a 250 lb adult
Lowering Fat and Cholesterol
 Many people limit meat and dairy foods to
lower fat and cholesterol.
 Lowering fat doesn’t mean omitting meat and
dairy products. These foods are part of a well
balanced diet.
 Choose lower fat, use lower fat preparation
methods and watch portions
Protein and Amino Acid Supplements
 Protein and amino acid supplements don’t
perform miracles and they can cause harm.
 Using muscles builds muscles, not supplements.
 Protein is not found as single amino acids in food
and taking single amino acids is not benefit.
Too Much Protein
 If more protein is consumed than the body
needs to build or repair body tissues or to
provide energy the excess is converted into
fatty acids and stored as fat for later use.
 In this process nitrogen groups are
removed from the amino acids which are
converted into urea and excreted in the
urine.