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LECTERE 5
AMINO ACIDS, PEPTIDES
AND PROTEINS
Lecturer: Dmukhalska Ye. B.
PLAN
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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.
Conjugated proteins.
• 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.
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).
Optical properties
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:
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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.
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
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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 N-terminal
end, and the end with the free СОО- group is called the Сterminal 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;
electrostatic attractions
(salt bridges);
covalent disulfide bonds;
hydrophobic attractions;
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.
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 stringlike 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