2. 2
PROTEINS
The protein molecule contains carbon,
hydrogen, oxygen, and about 16% nitrogen
and some may also contain sulfur and
phosphorus
It is the presence of nitrogen that distinguishes
a protein molecule from that of a carbohydrate
or fat
Excess nitrogen is changed into a waste
product, urea, in the liver, and then excreted in
the urine
3. 3
This process places an extra load on both the
liver and the kidneys
Amino Acids are commonly referred to as "the
building blocks of proteins," just as
monosaccharides are the building blocks of
sugars, and fatty acids and glycerol are the
building units of fats
When a protein food is eaten, the body must
break down protein molecules into useable
amino acids, rebuilding them into the
thousands of different kinds of proteins it
needs
4. 4
More than 50,000 protein combinations are
possible for the 100,000 or so cells that require
them
The construction (anabolism) and destruction
(catabolism) of proteins is a continual, non-
stop body function
The amino acids that are not used for the
building and repair of tissues are broken down
into carbon dioxide, ammonia, and water to
produce energy or waste
5. 5
Another destination for excess amino acids is
the Amino Acid Pool
When the number of amino acids within the
body is high, the liver absorbs them and stores
them for future use
Cells also have the capacity to store amino
acids for short periods, when the amino acid
level in the bloodstream falls, the liver deposits
some of the stored amino acids back into
circulation
6. 6
If the amino acid content in the bloodstream
falls or if other cells require certain amino
acids, the cells are able to release their stored
supply back into circulation
Since most of the cells synthesize more
proteins than they can use, the cells are able
to reconvert their proteins into amino acids and
make deposits into the amino acid pool
7. 7
Importance of proteins and amino acids
It is protein that provides the structure for all
living things
Every living organism, from the largest animal
to the tinniest microbe, is composed of protein
and in its various forms, protein participates in
the vital chemical processes that sustain life
Proteins are a necessary part of every living
cell in the body
Next to water, protein makes up the greatest
portion of our body weight
8. 8
In the human body, protein substances make
up the muscles, ligaments, tendons, organs,
glands, nails, hair, and many vital body fluids,
and are essential for the growth, repair and
healing of bones, tissues and cells
The enzymes and hormones that catalyze and
regulate all bodily processes are proteins
Proteins help regulate the body's water balance
and maintain the proper internal pH
9. 9
They assist in the exchange of nutrients
between the intracellular fluids and the tissues,
blood, and lymph
A deficiency of protein can upset the body's
fluid balance, causing edema (water retention)
Proteins form the structural basis of
chromosomes, through which genetic
information is passed from parents to offspring
The genetic "code" contained in each cell's
DNA is actually information for how to make
that cell's protein
10. 10
Each individual type of protein is composed of
a specific group of amino acids in a specific
chemical arrangement
It is the particular amino acids present and the
way in which they are linked together in
sequence that gives the proteins that make up
the various tissues their unique functions and
characters
Each protein in the body is tailored for a
specific need, proteins are not interchangeable
The body cannot directly use proteins found in
food
11. 11
The proteins that make the human body are
not obtained directly from the diet
Rather, dietary protein is broken down into its
constituent amino acids, which the body then
uses to build the different specific proteins it
needs
Thus, it is the amino acids rather than protein
that are the essential nutrients
In addition to combining to form the body's
proteins, some amino acids act as
neurotransmitters or as precursors of
neurotransmitters, the chemicals that carry
information from one nerve cell to another
12. 12
Certain amino acids are thus necessary for the
brain to receive and send messages
Unlike many other substances,
neurotransmitters are able to pass though the
blood-brain barrier
Because certain amino acids can pass through
this barrier, they can be used the brain to
communicate with nerve cells elsewhere in the
body
Amino acids also enable vitamins and minerals
to perform their jobs properly
13. 13
Even if vitamins and minerals are absorbed and
assimilated the body, they can not be effective
unless the necessary amino acids are present
e.g. low levels of the amino acid tyrosine may
lead to iron deficiency
These are amino acids that can be used to
provide energy directly to muscle tissue
High doses of branched-chain amino acids are
used in hospitals to treat people suffering from
trauma and infection
14. 14
Chemical nature of amino acids
All peptides and polypeptides are polymers of
alpha-amino acids
There are 20 a-amino acids that are relevant to
the make-up of mammalian proteins (see below)
The a-amino acids in peptides and proteins
(excluding proline) consist of a carboxylic acid (-
COOH) and an amino (-NH2) functional group
attached to the same tetrahedral carbon atom
Each of the 20 a-amino acids found in proteins
can be distinguished by the R-group substitution
on the a-carbon atom
16. 16
There are two broad classes of amino acids
based upon whether the R-group is
hydrophobic or hydrophilic
The hydrophobic amino acids tend to repel the
aqueous environment and, therefore, reside
predominantly in the interior of proteins
This class of amino acids does not ionize nor
participate in the formation of H-bonds
The hydrophilic amino acids tend to interact
with the aqueous environment, are often
involved in the formation of H-bonds and are
predominantly found on the exterior surfaces
proteins or in the reactive centers of enzymes
17. 17
Acid-Base Properties of the Amino Acids
The a-COOH and a-NH2 groups in amino acids
are capable of ionizing (as are the acidic and
basic R-groups of the amino acids)
As a result of their ionizability the following
ionic equilibrium reactions may be written:
R-COOH <--------> R-COO- + H+
R-NH3
+ <---------> R-NH2 + H+
The equilibrium reactions, as written,
demonstrate that amino acids contain at least
two weakly acidic groups
18. 18
However, the carboxyl group is a far stronger
acid than the amino group
At physiological pH (around 7.4) the carboxyl
group will be unprotonated and the amino
group will be protonated
An amino acid with no ionizable R-group would
be electrically neutral at this pH and this
species is termed a zwitterions
19. 19
The net charge (the algebraic sum of all the
charged groups present) of any amino acid,
peptide or protein, will depend upon the pH of
the surrounding aqueous environment
As the pH of a solution of an amino acid or
protein changes so too does the net charge
This phenomenon can be observed during the
titration of any amino acid or protein
When the net charge of an amino acid or
protein is zero the pH will be equivalent to the
isoelectric point (pI)
20. 20
Protein Structure
Proteins are made up of such elements as
carbon, hydrogen and oxygen
Proteins are about 50% of the dry weight of
most cells, and are the most structurally
complex macromolecules known
Polymers are any kind of large molecules made
of repeating identical or similar subunits called
monomers
The starch and cellulose are polymers of
glucose and proteins are polymers of amino
acids (the monomer)
21. 21
Protein contain amino acid chains, made up from 20
different L-α-amino acids, also referred to as residues,
that fold into unique three-dimensional protein
structures
All amino acids have a similar chemical structure—
each contains an amino group (NH2), an acid group
(COOH), a hydrogen atom (H), and a distinctive side
group that makes proteins more complex than either
carbohydrates or lipids. All amino acids are attached to
a central carbon atom (C)
22. 22
Primary structure
A protein's primary structure is simply the
order of its amino acids
Order of amino acids in a protein molecule is
genetically determined
This primary sequence of amino acids must
contain all the information required for the
protein to assume its correct three-dimensional
structure
24. 24
The primary structure is composed of amino
acids linked together in what are termed
peptide bonds
At first glance these appear to contain only
single bonds and free rotation between all such
atoms would be expected
25. 25
Secondary Structure
Secondary structures are short regions of
ordered structure arising from folding of the
protein chain
The structures are stabilized by hydrogen
bonding within the backbone
The two major types of secondary structure
are the alpha-helix and the beta-pleated sheet
26. 26
Alpha Helix
The alpha helix is a helical structure held
together by hydrogen bonds between the
backbone N-H and C=O groups
The hydrogen bonds are formed within the
backbone and the side chains do not
participate
An alpha-helix is a tight helix formed out of the
polypeptide chain
The polypeptide main chain makes up the
central structure, and the side chains extend
out and away from the helix
27. 27
The CO group of one amino acid (n) is
hydrogen bonded to the NH group of the
amino acid four residues away (n +4)
In this way every CO and NH group of the
backbone is hydrogen bonded
A-helices are most commonly made up of
hydrophobic amino acids, because hydrogen
bonds are generally the strongest attraction
possible between such amino acids
28. 28
a-helices are found in almost all proteins to
various extents
The structure of an alpha helix maximizes
interactions between amino acid residues. The
helical structure also requires that there be
favorable interactions between at least three
consecutive amino acid residues
this means that a helix doesn't begin to form
easily, but once it is formed it is a fairly stable
structure
29. 29
The hydrogen bonds between atoms within the
helix are generally not exposed to full contact
with the solvent
This lack of solvent interaction causes these
hydrogen bonds to be more stable than those
observed in small compounds that are more
fully hydrated
30. 30
Beta-Structure
The beta-pleated sheet (or beta sheet) is
similar to the alpha-helix in that it is held
together by hydrogen bonding between groups
in the
The backbone loops around several times to
form the beta-pleated sheet and the strands
run anti-parallel (in opposite directions) to each
other
There is a special type of molecular model
used to highlight protein secondary structure
31. 31
If a chain of amino acids is drawn in a linear
extended conformation, the R groups will fall
alternately above and below the plain of the
peptide bonds
If another chain of extended amino acids is
brought near the first chain, it is a simple
matter to line up the chain so as to maximize
hydrogen bonding
This can be done whether the chains have the
same N to C sense (parallel) or not (anti
parallel)
32. 32
Tertiary Structure
The overall three-dimensional arrangement of
the secondary structures within a single
peptide chain of a protein is called tertiary
structure
This level of structure defines the location of
each amino acid of the protein in three-
dimensional space
Tertiary structure may be considered as being
the same as the conformation of the protein
With few exceptions proteins are not long
extended structures, but have dimensions that
are not too different from spheres or ovoids
33. 33
This suggest that once secondary structures
have formed the molecules fold into relatively
compact structures.
The specific tertiary structure assumed by a
protein can have considerable impact on the
properties of the molecule
The protein folds in such a way as to remove
as many hydrophobic groups as is possible
from contact with the aqueous phase
The final conformation should also attempt to
maximize favorable interactions between
different portions of the molecule
34. 34
This usually results in a molecule having a very
compact interior
The hydrophobic groups are associated away
from the water and are able to interact due to
London forces
the interior of the molecule is usually devoid of
water molecules or of charged amino acid
residues
The energy required to over come the
interactions of charged groups with water that
would be necessary for their insertion into the
protein interior is generally not available
35. 35
When a protein does find the necessary energy
to bury a charged group or even a dipole, the
buried group usually can be shown to perform
some specific function necessary to the
functionality of the protein
Changes in temperature, ionic strength,
dielectric constant, pH, etc. would be expected
to have affects on the structure of the protein
These changes may be very subtle or of great
consequence to the structure and function of
the molecule
36. 36
The structures previously described as beta
sheets are technically tertiary structures
The secondary structure is the extended chain
of amino acids
The interaction of two or more chains that
leads to the formation of beta structures are
more properly classified as tertiary
As long as it is clear that the extended
structure is the real secondary structure there
is no problem with this type of treatment
37. 37
There are several ways in which structures
within a protein may be held together, such as:
Disulfide bonds - the thiol groups on the amino
acid cysteine may be oxidized to form a
covalent disulfide bond between different
portions of the protein chain
Hydrogen bonding - hydrogen bonding may
occur between polar groups on the sidechains
of the protein structure
38. 38
Salt bridges - an acidic side chain and a basic
side chain may react to form a salt, forming a
strong ionic interaction between the side
chains
Hydrophobic interactions - in aqueous solution,
proteins will tend to keep their non-polar
(hydrophobic) side chains within the interior of
the protein, away from water
39. 39
Quaternary Structures
Quaternary structure is the arrangement of the
chains in a protein that contains more than one
peptide chains
The types of interactions that hold quaternary
structure are the same as for tertiary structure,
but they occur between two different chains
contains one iron heme unit which is
responsible for binding the oxygen
Many protein molecules tend to associate in
well-defined structures and such associations
are termed quaternary structures
40. 40
These structures are often caused by the
addition of small molecules or by slight
changes in the structure of the individual
molecules
Many enzymes, for instance, can be
polymerized or depolymerized by the action of
phosphatases or kinases
The addition or removal of a phosphate group
from a protein molecule can greatly change its
tendency to form associated structures
41. 41
In many cases an organism can rapidly change
the activity of an enzyme by such modification
of its quaternary structure
These modifications can be accomplished very
rapidly and are readily reversible
This allows for rapid control of enzyme activity
Many proteins of importance in food systems
exhibit quaternary structure. Soy globulins,
casein and actomyosin are just a few examples
of such proteins
