Protein
By Carly Magill, Tommy Turner, Michael Lister, and Victoria Brown
Structure
• Proteins are polymers
of amino acids
covalently linked
through peptide bonds
to form a chain.
• There are 20 amino
acids that essentially
make up all the
proteins on this earth!
Amino Acids
• Each amino acid has a
fundamental design composed
of a central carbon group, a
hydrogen, a carboxyl group, an
amino group, and a unique
side chain or R-group.
• The R-group is what
distinguishes one amino acid
from another. Each amino acid
has unique chemical
properties. (hydrophobic vs
hydrophilic or positivelycharged vs negatively charged)
Peptides
• Amino acids are covalently bonded together in chains by
peptide bonds.
• If the chain length is short (less than 30 aa) it is called a
peptide. If longer, it is polypeptides or proteins.
• Peptide bonds are formed between the carboxyl group
of one amino acid and the amino group of the next
amino acid. These bonds are formed using dehydration
synthesis.
Peptide structure cont.
• Amino acids have a head to tail arrangement, meaning
there is an amino group to one end (the aminoterminus or N-terminus) and a carboxyl group on the
other end (carboxyl-terminus or C-terminus).
• The carboxyl-terminal corresponds to the last one
added to the chain during translation of the mRNA.
Levels of Protein Structure
• Structural features of
proteins are described
at four levels of
complexity:
• 1. Primary structure
• 2. Secondary structure
• 3. Tertiary structure
• 4. Quaternary structure
Function
• Proteins have a large range of abilities:
• Antibodies (bind to foreign particles and add
protection) Ex. Immunoglobulin G
• Enzyme (Carry our vast amount of chemical reactions in
cells) Ex. Lactase
• Messenger (transmit signals to coordinate biological
processes between different cells, tissues, and organs)
Ex. Growth hormone
• Structural Component (structure and support for cells)
Ex. Actin
• Transport/storage (bind to and carry atoms and small
molecules within cells and throughout body) Ex. Ferritin
Amino acids, vitamins, minerals and trace
elements play a significant role in weight loss.
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Whether we gradually put on weight or stay slim
generally depends on our hormones. And herein
lays the key to weight loss: the systematic
supplementation of certain amino acids allows
us to stimulate the body to produce enough fatburning hormones – in a natural manner and in
harmony with the body's needs.
One important fat-burning hormone is the
growth hormone (somatotropin, STH). We
produce this hormone while we sleep. It
stimulates protein synthesis and boosts fat
oxidation. Overweight patients generally have
lower STH concentrations, which often hinders
weight reduction.1 Unfortunately, the growth
hormone is very expensive (approximately GBP
400–650 for a monthly ration) and must be
injected under close and competent medical
supervision. It is thus safer to simulate our
bodies to secrete this hormone naturally.
Certain amino acids have been shown to do this
in many cases if sufficient quantities are taken
on an empty stomach at night.
Digestion or Proteolysis
• Entails breaking the
complex molecule first into
peptides and then into
individual amino acids. This
break down of proteins is
called proteolysis.
• The physical act of protein
digestion begins in the
mouth during mastication.
The chemical process of
protein digestion begins in
the stomach.
Esophagus
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The role of the esophagus is
simply to convey boluses of
food from the pharynx to the
stomach.
It’s stratified squamous
epithelium secretes mucus
which lubricates the inner
lining and allows for passage.
Peristaltic contractions are
responsible for propelling
food through esophagus.
• The upper and lower
sphincters allow for entry
and exit of boluses.
Digestion in Stomach
• The stomach secretes
hydrochloric acid and
pepsinogen. When pepsinogen
and HCl react, they produce
the protein-digesting enzyme
pepsin.
• Pepsin separates proteins
through hydrolysis creating
peptone and proteose
(intermediate products of
digestion)
• After about 4 hours in
stomach, a thick, soupy liquid
called chyme remains which
travels through the pyloric
sphincter into the duodenum.
Digestion in Duodenum
• Highly acidic chyme enters the
duodenum and is
counteracted by an alkaline
solution secreted by the
duodenum.
• The pancreas secretes
pancreatic juice which includes
the enzyme, trypsinogen,
which is activated by
enterokinase to form trypsin.
• Trypsin finishes the break
down of complex proteins into
amino acids through
hydrolysis.
Absorption
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When all protein is broken down, the
amino acids move to the small
intestine.
Here, the villi contain a large network
of blood capillaries and lymph
vessels.
The amino acids are absorbed into
these capillaries and carried by the
blood through the hepatic portal vein
which carries them to liver.
They are then transported
throughout the body to various
organs that need protein
replenishment.
If too much protein, it will not be
absorbed and will continue to large
intestine and kidneys = excreted in
urine
Absorption of Amino Acids
The mechanism by which amino acids are
absorbed is conceptually identical to that of
monosaccharides. The lumenal plasma
membrane of the absorptive cell bears at least
four sodium-dependent amino acid transporters one each for acidic, basic, neutral and amino
acids. These transporters bind amino acids only
after binding sodium. The fully loaded
transporter then undergoes a conformational
change that dumps sodium and the amino acid
into the cytoplasm, followed by its reorientation
back to the original form.
Thus, absorption of amino acids is also absolutely
dependent on the electrochemical gradient of
sodium across the epithelium. Further,
absorption of amino acids, like that of
monosaccharides, contributes to generating the
osmotic gradient that drives water absorption.
Absorption of Peptides
• There is virtually no absorption of peptides longer than four
amino acids. However, there is abundant absorption of di- and
tripeptides in the small intestine. These small peptides are
absorbed into the small intestinal epithelial cell by co
transport with H+ ions via a transporter called PepT1.
• Once inside the enterocyte, the vast bulk of absorbed di- and
tripeptides are digested into amino acids by cytoplasmic
peptidases and exported from the cell into blood. Only a very
small number of these small peptides enter blood intact.
Brush border
• The microvillus border of
intestinal epithelial cells
– Microvilli form a digestive
absorptive surface to
transfer food products
across the enterocyte from
the intestinal lumen to the
interstitial fluid and blood
– Looks like a brush under a
light microscope
• The microvilli contain
enzymes responsible for
the last stages of
digestion
Brush border enzymes
• Amino peptidase—digests proteins from the
amino terminal end
• Dipeptidase + Mg dependent—digests the
peptide bond in dipeptides
Absorption of Intact Proteins
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As emphasized, absorption of intact proteins occurs only in a few circumstances. In the first place, very few
proteins get through the gauntlet of soluble and membrane-bound proteases intact. Second, "normal"
enterocytes do not have transporters to carry proteins across the plasma membrane and they certainly
cannot permeate tight junctions.
One important exception to these general statements is that for a very few days after birth, neonates have
the ability to absorb intact proteins. This ability, which is rapidly lost, is of immense importance because it
allows the newborn animal to acquire passive immunity by absorbing immunoglobulins in colostral milk.
large quantities of immunoglobulins and acquire a temporary immune system that provides protection
until they generate their own immune responses.
Deamination
Deamination is the removal of an amine group from a molecule.
In the body, deamination takes place primarily in the liver, however glutamate
is also deaminated in the kidneys. Deamination is the process by which amino
acids are broken down if there is an excess of protein intake. . The oxidation
pathway starts with the removal of the amino group by a transaminase, The
amino group is removed from the amino acid producing ammonia.
Urea
• The rest of the amino acid
is made up of mostly
carbon and hydrogen, and
is recycled, stored, or
oxidized for energy.
• Ammonia is toxic to the
human system, and
enzymes convert it to urea
or uric acid by addition of
carbon dioxide molecules
(which is not considered a
deamination process) in
the urea cycle, which also
takes place in the liver.
Urea and uric acid can
safely diffuse into the
blood and then be
excreted in urine.
Urea Cycle
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Ammonia (NH3) is a common
byproduct of the metabolism
of nitrogenous compounds.
Ammonia is smaller, more
volatile and more mobile than
urea. If allowed to
accumulate, ammonia would
raise the pH in cells to toxic
levels. Therefore many
organisms convert ammonia
to urea, even though this
synthesis has a net energy
cost. Being practically neutral
and highly soluble in water,
urea is a safe vehicle for the
body to transport and excrete
excess nitrogen.
In water, the amine groups
undergo slow displacement by
water molecules, producing
ammonia and carbonate
anion. For this reason, old,
stale urine has a stronger odor
than fresh urine.
Keto -Acids
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Amino acids, when deaminated, produce alpha-keto acids that, That will feed
into the Krebs Cycle.
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Pyruvic acid and the Krebs cycle acids are keto acids
Amino acids are grouped into 2 classes, based on whether or not their carbon
skeletons can be converted to glucose:
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glucogenic
Ketogenic -producing Acetyl CoA or an alternate that feeds directly into Krebs
• Carbon skeletons of ketogenic amino acids can be catabolized for energy in Krebs
Cycle, or converted to ketone bodies or fatty acids.
• They cannot be directly converted to glucose.
Amino acids for ATP production
• Proteins are broken down into individual amino acids
• Deamination to remove the amino group (—NH2)
• The remaining amino acid carbon chains are then used
at various stages in the Kreb’s cycle to generate ATP
– The amount of ATP produced varies with the type of amino acid
• Depends on where it enters the Kreb’s cycle
Amino acids for ATP production
• Catabolism of amino acids is not a practical
source of quick energy and is typically only
used in starvation situations (when no other
energy source is available)
– Proteins are harder to break apart than
carbohydrates or lipids
– Generates toxic waste products
– They are the structural and functional parts of
every cell
Amino acids into fat
• Fat cells can take up amino
acids, which have been
absorbed into the
bloodstream after a meal, and
convert them into fat
molecules
– The conversion of protein into
fat is 10 times less efficient
than simply storing fat in a fat
cell
• Lipogenesisthe process of
synthesizing fatty acids by
linking the 2 carbon acetylCoA molecules into a long
chain
Amino acids into glucose
• Gluconeogenesisthe process of synthesizing glucose from noncarbohydrate sources
– Occurs mainly in the liver with a small amount also occurring in the
cortex of the kidney
– Catabolism of muscle proteins to amino acids contributes the major
source of carbon for maintenance of blood glucose levels during
starvation
• The starting point of gluconeogenesis is pyruvate, although
oxaloacetate and dihydroxyacetone phosphate also provide entry
points
• Deamination of amino acids facilitates entering of their carbon
skeleton into the cycle directly (as pyruvate or oxaloacetate), or
indirectly via the Kreb’s cycle
• All of the amino acids can be used for gluconeogenesis except
leucine and lysine
Gluconeogenesis
• The reverse of glycolysis with 2 additional
reactions at the beginning:
• 2 enzymes
– Pyruvate carboxylase
• Pyruvateoxaloacetate
– Phosphoenolpyruvate carboxykinase
• Oxaloacetatephosphoenolpyruvate (PEP)
• The rest of the reactions are the reversible
reactions of glycolysis
Consumes 6 ATP/glucose
Transamination
• A chemical reaction between an amino acid, which contains an amine (NH2)
group and a keto acid, which contains a keto (=O) group.
• the NH2 group on one molecule is exchanged with the =O group on the other
molecule. The amino acid becomes a keto acid, and the keto acid becomes an
amino acid.
• enzymes called transaminases or aminotransferases facilitate these reactions
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They require B6 as a coenzyme
• With this pathway, the body can synthesis non-essential amino acids
Non-Essential Amino Acids
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can be either manufactured directly by the body or obtained by conversion from
another amino acid.
Of 20 Amino Acids, 11 are non-essential:
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Alanine
Arginine
Asparagine
Aspartate
Cysteine
Glutamate
Glutamine
Glycine
Proline
Serine
Tyrosine
Essential Amino Acids
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An adequate amount of all 20 amino acids is required to build proteins for growth and replace proteins that
are turned over, however only 8 in Adults and 9 in children cannot be produced by the body and must be
obtained in the diet.
The nine essential amino acids are: histidine, isoleucine, leucine, lysine, methionine, phenylalanine,
threonine, tryptophan, and valine.
Amino acids: the building blocks of life.
Eat your amino acids, keep egging on!