WHAT IS PROTEIN?
• Proteins are a sequence of amino acids
• Of the 20 amino acids that exist, 9 are
essential amino acids, and 11 are nonessential
• There are also 4 amino acids that can be
considered conditionally essential:
arginine, tyrosine, glutamine, and cysteine
AMINO ACIDS: Structure
• Consist of a central carbon atom bonded to:
a hydrogen, a carboxylic acid, an amino
group, and an additional side group that is
unique to each amino acid
AMINO ACIDS: Structure
• The side group creates unique
characteristics for each amino acid so they
differ in: shape, size, composition, electrical
charge, and pH.
AMINO ACID: Sequence
• Amino acids link in specific sequences to
form strands of protein
• One amino acids is joined to the next by a
PEPTIDE bond
AMINO ACID: Sequence
•
•
•
•
•
Dipeptide – 2 amino acids
Tripeptide – 3 amino acids
Oligopeptides – 4-10 amino acids
Polypeptide – more than 10 amino acids
Proteins in the body and diet are long
polypeptides (100s of amino acids)
DENATURING of PROTEINS
• Acid, alkaline, heat, alcohol, and agitation
can disrupt the chemical forces that stabilize
proteins and can cause them to lose their
shape (denature)
• Denaturing of proteins happens during food
preparation (cooking, whipping, adding
acids) or digestion (in the stomach with
hydrochloric acid)
PROTEINS: Function
Structural Functions:
• Collagen – is the most abundant
protein in mammals, and gives bone
and skin their strength
• Keratin – provides structure to hair and
nails
PROTEIN: Functions
ENZYMES
• Enzymes are proteins that catalyze chemical
reactions without being used up or destroyed in
the process
• Used in – digestion, releasing of energy from
nutrients for fuel, triggering reactions that build
muscle and tissue
PROTEIN: Functions
HORMONES
• Hormones are chemical messengers that are
made on one part of the body, but act on
cells in other parts of the body
• Insulin, Glucagon
• Antidiuretic Hormone (ADH)
PROTEIN: Functions
IMMUNE FUNCTION
• The Immune Response is a series of steps
your body takes to mount an attack against
invaders
• Antibodies are blood proteins that attack
and inactivate bacteria and viruses
• Once an antibody has been made for a
certain invader, your body can more quickly
respond (Immunization)
PROTEIN: Functions
FLUID BALANCE
• Fluids in the body are intracellular or
extracellular (interstitial and intravascular)
and must remain balanced
PROTEIN: Functions
FLUID BALANCE
• Blood proteins like albumin and globulin
help to regulate this balance by remaining in
the capillaries and attracting fluid
• Edema is the result of fluid imbalance
PROTEIN: Functions
ACID-BASE BALANCE
• Proteins help to maintain a stable pH level
in our body fluid by picking up extra
hydrogen ions when conditions are acidic,
and donating hydrogen ions when
conditions are alkaline
• Otherwise, the resulting conditions of
acidosis or alkalosis could lead to coma or
death
PROTEIN: Functions
TRANSPORT
• Lipoproteins (chylomicrons, LDL, HDL)
• Albumin transports a variety of nutrients such
as calcium, zinc, and Vitamin B6
• Transferrin transports iron (hemoglobin – a
protein, contains iron, but it transports oxygen)
• Proteins may also acts as channels or pumps
across the cell membrane
PROTEIN: Functions
ENERGY SOURCE
• If the diet does not provide enough energy, the
body must begin to break down its own protein
• The proteins are broken down into individual
amino acids, then deaminated, and the remaining
carbon, hydrogen, and oxygen compounds are
used to make energy or glucose
• If the diet contains too much protein, the excess
will be converted to glucose, or stored as fat
DIGESTION
• No digestion of protein takes place in the
mouth, it begins in the stomach
• Hydrochloric acid denatures protein and
also converts pepsinogen to pepsin
• Pepsin breaks the protein down into
peptides of various lengths and some amino
acids
• Pepsin completes ~ 10-20% of digestion
DIGESTION
• Pancreas makes trypsinogen and
chymotrypsinogen (proenzymes) in
response to protein in the small intestine
• They will be activated to trypsin and
chymotrypsin (now called proteases)
• Proteases break down polypeptides into
smaller peptides (very few peptides have
been broken down to amino acids at this
stage)
DIGESTION and ABSORPTION
• The intestinal wall produces peptidases
which continue to split the remaining
polypeptides into tripeptides, dipeptides,
and some amino acids
• These smaller units are transported into the
enterocytes
ABSORPTION
• In the enterocyte, other peptidases
immediately digest everything into single
amino acids which are absorbed into the
bloodstream
• Some amino acids share the same transport
system, so if you take in a large amount of
one particular amino acid, you may be
inhibiting the absorption of others
ABSORPTION
• Most protein absorption takes place in the
duodenum and jejunum
• Most amino acids are absorbed into the
bloodstream, but some remain in the
enterocytes and are used to synthesize
enzymes and new cells
• >99% of protein enters the bloodstream as
amino acids
• Absorption of whole protein can cause a
severe allergic reaction
PROTEINS in the BODY
• Amino Acid Pool – amino acids that are
available throughout the body (tissues and
fluids) for use when needed
• Protein Turnover – of the ~ 300 grams of
protein synthesized by the body each day,
200 grams are made from recycled amino
acids
NITROGEN EXCRETION
• Amino acid breakdown yields an amino group
(containing nitrogen)
• This molecule is unstable and is converted to
ammonia
• Ammonia is toxic, so it is excreted from the cells
and sent to the liver, where it is converted to urea
and water
• The urea is transported to the kidney, where it is
filtered from the blood and finally sent to the
bladder for excretion in the urine (page 227)
• Nitrogen is also lost through hair, skin, GI cells
mucus, nails, and body fluids like sweat
How Much Protein Do We Need?
Adults:
0.8 grams of protein per kilogram of body weight
per day
Endurance Athletes:
1.2 to 1.4 g/kg/day
Heavy Weight Trainers:
1.7 to 1.8 g/kg/day
Protein Sources
Almonds (1 cup)
24 grams
Pinto Beans (1 cup)
15 grams
Cheese (1 oz.)
7 grams
Ham (3 oz.)
18 grams
1 Egg
6 grams
2% Milk (1 cup)
8 grams
Clams (3 oz.)
60 grams
Whole Wheat Bread
3 grams
Lean Hamburger
30 grams
Peanut Butter (1 T)
4 grams
Salmon (3 oz.)
20 grams
Tofu (4 oz.)
9 grams
Yogurt (8 oz.)
10 grams
White rice (1 cup)
4 grams
PROTEIN QUALITY
• Complete Proteins – proteins that provide
all the essential amino acids (most animal
proteins)
• Incomplete Proteins – proteins that are
missing one or more essential amino acids
(most plant proteins except soy protein)
• Incomplete proteins can be served with a
complementary protein to make it complete
PROTEIN: Health Effects
INSUFFICIENT DIETARY PROTEIN
• Protein-Energy Malnutrition (PEM) can occur
anywhere in the world, but is most common in
developing countries
• Kwashiorkor
• Marasmus
• In industrialized nations, PEM may exist in the
elderly population, in the poor, and those with
anorexia, cancer, AIDS, or malabsorption
syndromes
PROTEIN: Health Effects
EXCESS DIETARY PROTEIN
• May strain the kidneys
• May cause mineral losses (especially calcium)*
• May increase risk of obesity*
• May increase risk of heart disease*
• May increase risk of cancer*
*only with animal protein
VEGETARIANISM
• What are the PROS and CONS of
vegetarianism?
ATKIN’S DIET
• How does it work?
• What are some of the possible negative
consequences?