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
Nitrogen is also lost through hair, skin, 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 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