Download 05. Amino acids, Protein

LECTERE 5
THEME: AMINO ACIDS, PEPTIDES AND
PROTEINS.
PLAN
 Biological
role of amino acids and proteins
 Classification of amino acids.
 Structure of amino acids.
 Chemical properties of amino acids.
 Peptides.
 Proteins: levels of structure of proteins.
 Compound proteins.
PROTEIN
А
protein is in polymer in which the
monomer units are aminoacids. Thus the
starting point for а discussion of proteins
is an understanding of the structures and
chemical properties of aminoacids.
 The word protein comes from the Greek
proteios, which means "of first
importance." This derivation alludes to
the key role that proteins play in life
processes.

Proteins are the most abundant substances in
most cells - from 10% to 20% of the cell’s mass.
 More than 70-80 % of dry weight of muscles,
lungs, kidneys, spleen; 57 % of dry weight of
liver, 45 % of dry weight of brain are proteins.
The lowest proteins constituting in bones and
teeth (20 and 18 % responding).
 Contents of chemical elements in proteins:
carbon is 51-55 %, oxygen is 21-28 %, nitrogen
is 15-18 %, hydrogen is 6-7 %, sulfur is 0.3-2.5
%. Some proteins contain phosphorus iron, zinc,
copper and other elements - (0.2-2%).
AMINO ACIDS

An amino acid is an organic compound that
contains both an amino (–NН3) group and a
carboxyl (-СООН) group. The amino acids
found in proteins are always α-amino acids.
Classification of amino acids

Nonpolar amino acids contain one amino group,
one carboxyl group, and a nonpolar side chain.
 Polar neutral amino acids contain one amino
group, one carboxyl group, and а side chain that is
polar but neutral.
 Polar acidic amino acids contain one amino
group and two carboxyl groups, the second
carboxyl group being part of the side chain. There
are two polar acidic amino acids: aspartic acid and
glutamic acid.
 Polar basic amino acids contain two amino
groups and one carboxyl group, the second amino
group being part of the side chain. There are three
polar basic amino acids: lysine, arginine, and
histidine.
The most important aminoacids
Monoaminomonocarboxylic
aminoacids
Glycine
Leucine
Alanine
Isoleucine
Valine
The most important aminoacids
Diaminomonocarboxylic aminoacids
Lysine
Ornytine
Mnoaminodicarboxylic aminoacids
Glutamic acod
Aspartic acid
Hydroxylaminoacids
Serine
Threonine
The most important aminoacids
Sulphorcontain aminoacids
Methionine
Cysteine
Cystine
Aromatic aminoacids
Tyrosine
Phenyalanine
The most important aminoacids
Heterocyclic aminoacids
Tryptophane
Proline
Hydroxiproline
Aminoacids with amide group
Arginine
Histidine

Essential and non-essential amino acids
All of the 20 amino acids are necessary constituents of
human protein. Adequate amounts of 11 of the 20
amino acids can be synthesized from carbohydrates
and lipids in the body if а source of nitrogen is also
available. Because the human body is incapable of
producing 9 of these 20 acids, these 9 amino acids,
called essential amino acids, must be obtained from
food. The human body can synthesize small amounts of
some of the essential amino acids, but not enough to
meet its needs, especially in the case of growing
children.
 The 9 essential amino acids for adults are histidine,
isoleucine,
leucine,
lysine,
methionine,
phenylalanine, threonine, tryptophan, and valine. (In
addition, arginine is essential for children).
Acidity
Both an acidic group (-СООН) and а basic group (-NН2) are
present on the same carbon in an α-amino acid.
The net result is that in neutral solution, amino acid molecules
have the structure:
А zwitterion is а molecule that has а positive charge on one
atom and а negative charge on another atom.
Reaction of amino acids

Reaction with alcohols – esters formation:

Reaction with ammonia – amides formation:


Decarboxylation:
Salts are formed:
Deamination

oxidation deamination:

hydrolitic deamination:

intramolecular deamination:

redaction deamination:

Peptide formation
Two amino acids can react in а similar way - the
carboxyl group of one amino acid reacts with the
amino group of the other amino acid. The products
are а molecule of water and а molecule containing
the two amino acids linked by an amide bond.
Ninhydrin test. The ninhydrin colour reaction is the most
commonly test used for the detection of alpha-amino acids. This
is an extremely delicate test, to which proteins, their hydrolytic
products, and α-amino acids react. When ninhydrin is added to а
protein solution and the mixture is heated to boil, blue to violet
colour appears on cooling. The colour is due to the formation of
а complex compound.
React α-aminoacids with ninhydrin
O
O
OH
+
+ H3N-CH-COOOH
R
N-CH-COOH+H2O
R
O
O
Ninhydrin
O
O
O
H
O
N-CH-C
R
H
H2O
O
N=C-C
OH
O
O
NH2
OH
R
O
2-aminoindandion
O
O
OH
O
H
H
N
+
OH H2N
O
O
O
O
O
O
N
O
O
H+
O
Âlue - violet dye-stuff
Qualitative tests for amino acids and protein.
XANTHOPROTEIC TEST. On treatment with concentrated nitric
acid, certain proteins give yellow colour. This yellow colour is
the same that is formed on the skin when the latter comes in
contact with the concentrated nitric acid. The test is given only
by the proteins having at least one mole of aromatic amino acid,
such as tryptophan, phenylalanine, and tyrosine which are
actually nitrated during treatment with concentrated nitric acid.
FOLL REACTION. This reaction reveals the sulfur
containing amino acids (cysteine, cystine). Treatment
of the sulfur containing amino acids with salt of lead
and alkali yields a black sediment.
ADAMKEVICH REACTION. This reaction detects the
amino acid tryptophan containing indol ring. The
addition of the concentrated acetic and sulfuric acids to
the solution of tryptophan results in the formation of
red-violet ring appearing on the boundary of different
liquids.
Peptides

Short to medium-sized chains of amino acids
are known as peptides. А peptide is а
sequence of amino acids, of up to 50 units, in
which the amino acids are joined together
through amide (peptide) bonds. А compound
containing two amino acids joined by а peptide
bond is specifically called а dipeptide; three
amino acids in а chain constitute а tripeptide;
and so on. The name oligopeptide is loosely
used to refer to peptides with 10 to 20 amino
acid residues and polypeptide to larger
peptides.
Proteins

Proteins are polypeptides that contain more
than 50 amino acid units. The dividing line
between а polypeptide and а protein is
arbitrary. The important point is that proteins
are polymers containing а large number of
amino acid units linked by peptide bonds.
Polypeptides are shorter chains of amino
acids. Some proteins have molecular masses
in the millions. Some proteins also contain
more than one polypeptide chain.
Function of proteins
Catalysis.
Enzymes, the proteins that direct and accelerate thousands of
biochemical reactions
Structure. Some proteins function as structural materials that provide
protection and support.
Movement. Proteins are involved in all types of cell movement. For example,
actin, tubulin, and а variety of other proteins comprise the cytoskeleton.
Defense. А wide variety of proteins have а protective role. Examples found in
vertebrates include keratin, the protein found in skin cells that aids in protecting
the organism against mechanical and chemical injury. The blood-clotting
proteins fibrinogen and thrombin prevent blood loss when blood vessels are
damaged. The immuno-globulins (or antibodies) are produced by lymphocytes
in response to the invasion of foreign organisms such as bacteria.
Regulation. The binding of а hormone molecule to its target cell results in
specific changes in cellular function. Examples of peptide hormones include
insulin and glucagon, which regulate blood glucose levels. Growth hormone
stimulates cell growth and division.
Transport. Many proteins function as carriers of molecules or ions across
membranes or between cells. Examples of membrane proteins include the Na+К+ ATPase and the glucose transporter. Other transport proteins include
hemoglobin, which carries O2 to the tissues from the lungs, and the
lipoproteins, which transport lipids from the liver and intestines to other organs
Primary structure of а protein


The primary structure of а protein is the sequence of
amino acids present in its peptide chain or chains.
The end with the free H3N+ group is called the Nterminal end, and the end with the free СОО- group is
called the С-terminal end.
Secondary

structure of а protein
The secondary structure of а protein is the arrangement in
space of the atoms in the backbone of the protein. Three
major types of protein secondary structure are known; the
alpha helix, the beta pleated sheet, and the triple helix. The
major force responsible for all three types of secondary
structure is hydrogen bonding between а carbonyl oxygen
atom of а peptide linkage and the hydrogen atom of an amino
group (-NH) of another peptide linkage farther along the
backbone.
Alpha Helix

The Alpha Helix The alpha helix (α-helix) structure
resembles а coiled helical spring, with the coil configuration
maintained by hydrogen bonds between N – Н and С= О
groups of every fourth amino acid
Beta pleated sheet

The beta pleated sheet (β-pleated sheet) secondary
structure involves amino acid chains that are almost
completely extended.
Tertiary structure
The tertiary structure of а protein is
the overall three-dimensional shape that
results from the attractive forces
between amino acid side chains (R
groups) that are widely separated from
each other within the chain.
Interactions responsible for tertiary
structure
hydrogen bonds;
attractions;
electrostatic attractions
covalent disulfide bonds; (salt bridges);
hydrophobic
Electrostatic attractions (salt bridges),
Hydrogen bonds,
Quaternary structure

Quaternary structure is the highest level of
protein organization. It is found only in
proteins that have structures involving two or
more polypeptide chains that are independent
of each other — that is, are not covalently
bonded to each other. These multichain
proteins are often called oligomeric proteins.
The quaternary structure of а protein involves
the associations among the separate chains
in an oligomeric protein.
The function of hemoglobin in an
organism is to transport oxygen.
1 g of hemoglobin absorbs 1.35
ml of oxygen at STP,
corresponding to exactly one
molecule of О2 per iron.
Hemoglobin
Globular and fibrous proteins



On the basis of structural shape, proteins can be
classified into two major types: fibrous proteins and
globular proteins.
А fibrous protein is а protein that has а long, thin, fibrous
shape. Such proteins are made up of long rod-shaped or
string-like molecules that can intertwine with one another
and form strong fibers. They are water-insoluble and
generally have structural functions within the human body.
А globular protein is а protein whose overall shape is
roughly spherical or globular. Globular proteins either
dissolve in water or form stable suspensions in water,
which allows them to travel through the blood and other
body fluids to sites where their activity is needed.
Simple and Conjugated Proteins
 Proteins
are classified as either simple
proteins or conjugated proteins.
 А SIMPLE PROTEIN is made up entirely
of amino acid residues.
 А COMPOUND PROTEIN has other
chemical components in addition to amino
acids. These additional components,
which may be organic or inorganic, are
called prosthetic groups.
Thank you for attention