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Proteins
One of the most Versatile
Organic Molecules
What do you Already Know
About Protein Structure?
• What are the building blocks (subunits) that
make proteins?
• How many of these subunits are there?
• How are the subunits held together?
• What are the levels of structure of proteins?
• Excess dietary protein cannot be stored in body.
• Proteins contain nitrogen (N) as well as C, H and O,
• Some contain sulfur (S)
Proteins
• Proteins are essential components of every cell
• Proteins-contains C, H, N, O essential for cell
functions
• Amino Acids are the building blocks (subunits) of
proteins.
• Amino Acids contain an amine group on one end
and a carboxyl group on the other.
Amino Acids
• Composed of central C bonded to 4 groups
– Amino group: -NH2
– Carboxyl group: -COOH
– Hydrogen: -H
– R group: is the variable in structure
• Essential AA’s are those that cannot be
synthesized from other AAs in humans, must be
consumed
• Non-Essential AAs- can be synthesized from other
AAs in the human body
Dehydration Synthesis and Hydrolysis Rxns
Proteins are Classified by # AA’s in chain
o Dipeptides: 2 amino acids
o Tripeptides: 3 amino acids
o Polypeptides: More than 10 amino acids
o Peptides: Fewer than 50 amino acids
o Proteins: More than 50 amino acids
Typically 100 to 10,000 AAs linked together
The 20 Amino Acids
The Organization and Shape of Proteins Affect
Their Function
• 4 Levels of Structure
– Primary structure: Amino acids are linked
together to form a simple linear chain.
– Secondary structure: Geometric shape of the
protein that is folded and twisted
– Tertiary structure: Three-dimensional globular
shape of the protein
– Quaternary structure: Two or more polypeptide
chains bond together.
4 Levels of Protein Structure
α-Helix
β-Pleated Sheet
(H bonds)
3-D Shape
R group
Interactions
Not all
Proteins
Denaturation Shape of Proteins
• Denaturation (unfolding) of proteins occurs in
the presence of:
– Heat
– Acids
– Bases
– Salts
– Mechanical agitation
• Altering the shape of the protein alters its
function.
• Primary structure is unchanged by denaturing.
Denaturing a Protein
Protein Synthesis
Transcription is the process of making an RNA copy of
a gene sequence, called a messenger RNA (mRNA).
This occurs in the nucleus of the cell.
Translation is the process of translating the sequence
of mRNA to a sequence of amino acids that will result
in a protein. This occurs in the cytoplasm of the cell
with ribosomes and tRNA. Ribosomal RNA (rRNA) is
also involved in translation.
*Sequencing errors can cause alterations in proteins to
be made; an example is sickle-cell anemia.
Stages of Translation
• Initiation- bring together all of the materials
required for protein synthesis
• Elongation- sequential addition of amino
acids in the order specified by the mRNA
• Termination- release of completed protein
from the ribosome
Codon-Anticodon base pairing occurs
for assembly of amino acid into protein
Basic Steps in Protein Synthesis
1. DNA unwinds in nucleus allowing a copy of a gene to be
made messenger RNA (mRNA) = Transcription
2. The mRNA moves the cytoplasm and becomes associated
with ribosomes.
3. Transfer RNA (tRNA) brings in the specific amino acid
called for by the mRNA = Translation
4. Translation continues, as incoming amino acids form a
growing peptide and protein chain = Elongation
5. Protein synthesis is terminated and the completed
protein is release from ribosomes = Termination
Digesting and Absorbing Proteins
• Chewing Proteins in Mouth – Mechanical Digestion
• Bolus goes down esophagus & enters stomach.
• Chemical Digestion of Protein begins in Stomach.
• Gastrin stimulates the release of HCl.
• HCl from parietal cells in stomach trigger activation
of pepsinogen to pepsin.
Pepsinogen
(inactive)
HCl
Pepsin
(active)
• Pepsin breaks polypeptides into shorter chains.
Gastrin - peptide hormone made by stomach, duodenum, pancreas; aids in gastric motility
Enzymes involved in Protein Digestion
Digestion continues in Small Intestine
Polypeptides broken down into tri- and dipeptides.
• Cholecystokinin (CCK) stimulates the release of
proteases by the pancreas.
• Proteases break peptides to tripeptides & dipeptides.
• Dipeptidases and tripeptidases break the dipeptides
and tripeptides into amino acids.
CCK - peptide hormone of small intestine stimulates digestion of protein and fat;
acts as a hunger suppressant.
Absorbing Proteins
• Amino acids absorbed in Small Intestine
• Amino acids transported to
liver from the intestines via
the Hepatic Portal Vein.
– In the liver, amino acids are:
• Used to synthesize new proteins
• Converted to E, glucose, or fat
• Released to bloodstream and
transported to cells in entire body
• Occasionally proteins are absorbed intact.
Amino Acid Metabolism
Liver metabolizes AA’s
depending on bodily needs
muscle
skin
collagen
– Most AA’s sent into circulation for use by cells
membranes
enzymes
transporters
– If ↓ carbs, AA’s converted into glucose*
Gluconeogenesis
Metabolic Fate of Amino Acids
Amino Acid Pool
AA Pool = supply for body's ongoing needs
Protein Turnover – Tap into AA’s Pool
• Body constantly makes and breaks down proteins
• Body degrades ~300g pro-/day: we take in ~65-90g
 Most AA’s recycled to build new proteins
 Some are eliminated in feces or urine
 Some proteins broken down for energy
• Hormones can alter protein synthesis
• Insulin and Growth Hormone increase protein syn.
• Glucagon and Cortisol increases protein breakdown.
Deamination
= Removal of the Amine Group from Amino Acids
(AA loses an amino group without transferring it to another molecule)
• When this occurs Ammonia (NH3) is formed.
• NH3 converted to Urea by Liver, urea then excreted in urine.
• The C remnants are:
– Converted to glucose…
glucogenic amino acids
– Converted to fatty acids (→triglycerides in adipose tissue)
ketogenic amino acids
When AA pool reaches capacity, the AAs broken down for other uses.
Using Amino Acids as Fuel
Transamination
= Forming Nonessential Amino Acids by transferring the N from
one amino acid to a keto acid to form a new nonessential
amino acid.
Functions of Protein in the Body
• Provide Structural Support – e.g., collagen in skin
• Enable Body Movement - e.g. muscle
• Enzymes = Biological Catalyst
Speed up Chem Rxns
• Act as a chemical messenger
Hormones regulate cell actions, e.g. Insulin, glucagon.
• Regulate Fluid Balance – create ‘colloid’ fluid of
plasma (plasma proteins)
An Enzyme
in Action
Substrates
Becomes
different
Products
Functions of Proteins
• Transportation in body – e.g., lipoprotiens,
cell mem transporters… (ions, glucose)
• Contribute to a healthy immune system
– Antibodies bind and neutralize pathogens.
• Help maintain acid-base balance (H and OH)
• Provide Eergy - 4 kcals/gram
• Improve satiety and appetite control
Proteins as Transporters
The Many Roles of Proteins
Amino Acid Metabolism
1. Protein Turnover continual making and breaking down of
proteins in the body. Forms the amino acid pool:
Exogenous amino acids from food
Endogenous amino acids from within the body
2. Nitrogen Balance = Protein utilization in the body.
Zero N balance in equilibrium.
Positive N balance - N consumed is greater than excreted.
Negative N balance - N excreted is greater than consumed.
3. Amino Acids to make Proteins or Non-Essential AAs
4. Amino Acids to Make Other Compounds
Neurotransmitters (e.g., tyrosine, glutamate, glycine…).
Tyrosine made into melanin or thyroxine.
Tryptophan needed to make niacin and serotonin.
5. Using Amino Acids for Energy and Glucose
No readily available storage form of protein
Break down of protein in tissue for energy if needed
6. Deamination of Amino Acids - N containing amino groups
removed, 2 products result ammonia and keto acids.
7. Using Amino Acids to make Proteins or Nonessential
Amino Acids (Transamination)
8. Converting Ammonia to Urea. Deamination of AAs yields
toxic ammonia NH3. Liver converts NH3 to Urea carbon
dioxide (CO2).
9. Excreting Urea. Urea into bloodstream goes to the
kidneys and is excreted in urine. ↑ H2O necessary with highprotein diet to flush excess urea from body.
Excess protein is de-aminated and converted into fat.
Phenylketonuria (PKU)
Caused by very low or absent levels of enzyme
Phenylalanine Hydroxylase (PAH)
– Results in decreased ability to metabolize Phenylalanine (Phe)
– PAH enzyme converts Phe  Tyrosine (Tyr)
– If cannot, then both Phe and Tyr must be obtained from diet
– Person with PKU must carefully regulate intake of Phe
– If Phe and its products (phenylpyrivic acid) build up, it can
cause mental retardation, seizures & nervous system damage.
Proteins in Food
• Good sources of proteins include
– Red meat
– Tuna
– Poultry
– Milk
– Fish
– Beans and other legumes
– Nuts
Nitrogen Balance and Imbalance
Protein Needs
• Most Americans consume enough protein in their diet
• Those growing or ill require larger amounts of protein.
• Calculation of N balance is used to determine protein
equilibrium.
• Excess protein cannot be stored and is converted into
glucose or fat for later use.
• Athletes who are highly trained for endurance activities
may need to exceed the RDA for protein.
Proteins in Foods
• In North America 70% of our protein comes
from animal sources.
• Vegetable sources of protein contain more
fiber and minerals than animal sources
• Dried foods such as nuts and beans can be
good sources of protein, fiber and minerals
– Animal protein has all 9 essential AA, plants do not
– High-quality proteins- animal proteins contain all
proteins we need in sufficient amounts
– Lower-quality proteins- plant proteins lack one or
more AA we need
5 Brain Nutrients Found Only in Meat, Fish and Eggs (NOT Plants)
1. Vitamin B12 - body can’t produce it! Water-soluble vitamin involved in function of every cell;
blood and brain function. Deficiency: anemia, impaired brain function, symptoms of mental
disorders and a smaller brain.
2. Vitamin D3 – from cholesterol in your skin and ultraviolet rays from the sun. Vit D2
(ergocalciferol) from plants but studies show D3 (cholecalciferol) from animals, is much more
effective. Cod fish liver oil best source (also fatty fish). Deficiency: linked to depression, impaired
immune system, cardiovascular disease and cancer, autoimmune disease, multiple sclerosis
and cognitive impairment.
3. Creatine - in our skeletal muscle and brain cells, forms an energy reserve to quickly recycle
ATP, not essential nutrient Liver can produce, but inefficiently. Deficiency adversely affects
muscle and brain function
4. Docosahexaenoic Acid (DHA) - essential Omega-3 fatty acid - if we don’t eat them, we
get sick. DHA most abundant Omega-3 fatty acid in the brain, critical for normal brain
development (ALA needs to be converted to DHA for it to work). Fatty fish best source of DHA;
also grass-fed and pastured animal products.
5. Carnosine - a dipeptide (histidine and beta-alanine) in both muscle tissue and brain, very
protective against degenerative processes in body. A potent antioxidant, inhibits glycation
caused by elevated blood sugars and may prevent cross-linking of proteins. Levels are
significantly lower in patients with Parkinson’s and Alzheimer’s
Soy Protein – Best to Avoid all
Non-Fermented Soy products
List some important reasons why….
Types of Protein-Energy
Malnutrition (PEM): Kwashiorkor
• Severe Protein Deficiency
– Generally the result of a diet high in
grains and deficient in protein
• Symptoms include:
– Edema in legs, feet, and stomach
– Diminished muscle tone and strength
– Brittle hair that is easy to pull out
– Lethargic
– ↑infection, rapid heart rate, excess
fluid in lungs, pneumonia, septicemia,
and water and electrolyte imbalances
Types of PEM: Marasmus
• Results from a severe deficiency
in calories (energy)
– Frail, emaciated appearance
– Weakened and lethargic
– Often cannot stand without
support
– Appears old beyond years
– Hair thin, dry, and lacks sheen
– Low Tb and blood pressure
– Prone to dehydration,
infections, and blood clotting
Types of PEM: Marasmic Kwashiorkor
• Chronic deficiency in Calories and Protein
– Edema in legs and arms
– A "skin and bones" appearance
– With treatment, the edema subsides and
appearance becomes more like someone with
marasmus.
Treatment for PEM
• Medical and Nutritional treatment can
dramatically reduce the mortality rate.
• Implemented carefully and slowly
– Step 1: Address life-threatening factors.
• Severe dehydration
• Fluid and nutrient imbalances
– Step 2: Restore depleted tissue.
• Gradually provide nutritionally dense calories and highquality protein.
– Step 3: Transition to foods and more physical
activity.
Evaluation of Protein Quality
• Measure of ability of protein to support body
growth and maintenance
• Protein quality only valid under conditions
where amount consumed meets or exceeds
requirements for essential amino acids
• Non-essential amino acids are used as energy
sources or are degraded
Biological Value (BV)
The Biological Value (BV) is a measure of how
efficiently food protein, once absorbed from GI tract,
can be turned into body tissue.
• BV depends on how closely its amino acid pattern
reflects the amino acid pattern in body tissue.
• The better the match the more completely food
protein is turned into body protein.
• Ex. Egg white protein: BV 100, highest BV of any
single food protein.
Protein Efficiency Ratio (PER)
• Means of measuring a food’s protein quality
• Used by the FDA to set standards when labeling food
for infants
• Determine PER by comparing weight gain against
protein consumed
• PER = g weight gain/g protein consumed
• PER generally reflects Biological Value, since both
measure protein retention by body tissues
Chemical Score of Protein
• Used to calculate quality of protein
• Determined by dividing the amount of
essential AA per g of protein by required
needs of that essential AA per g of protein
• Lowest AA ratio calculated for any essential
amino acid is the chemical score
• Scores vary from 0.0 to 1.0
Protein Digestibility-Corrected Amino Acid
Score (PDCAAS)
• Most widely used measure of protein quality
• Used in place of PER evaluations for foods intended
for children over 1 year of age and non-pregnant
adults.
• To calculate PDCAAS of protein, its chemical score is
determined.
• Proteins totally lacking any essential amino acids has
a PDCAAS of 0, since its chemical score is 0.