STRUCTURE OF PROTEINS:
Definition:
Proteins
are polymers of amino acids, the compounds containing carbon, nitrogen, oxygen
and hydrogen.
The number of amino acids varies from a few to 3000 or even more in different proteins.
Abundance:
Proteins
are the most abundant organic compounds in cells. They are over 50% of total
dry weight of the cell.
Amino
Acids:
About 170
types of amino acids are present in cells and tissues.
Of these,
about 25 types are involved in the formation of proteins. However, most of the proteins
are made of 20 types of amino acids.
Structure
of Amino Acid:
All the amino acids have an amino group (-NH₂) and a carboxyl group (-COOH) attached to the same carbon (alpha carbon). The general formula of a amino acid is a follows:
Formation
of Protein Molecule by Peptide Linkage:
Amino acids
are linked to form polypeptide. The polypeptides may assemble to form proteins.
The amino
group of one amino acid reacts with the carboxyl group of another amino acid
and a water molecule is released. Here OH is released from carboxyl group of one amino acid while H is released from amino group of next amino acid.
The linkage between C of carboxyl group of one amino acid and N of amino group of next amino acid is called peptide bond
Example:
Glycine and alanine combine to form glycylalanine.
Glycylalanine has two amino acids and is called dipeptide.
A dipeptide has an amino group at one end and a carboxyl group at the other end.
So both reactive sites are available for the formation of further peptide bonds. As a result tripeptides, tetrapeptides, pentapeptides and polypeptides etc can be produced.
Structure of Proteins:
Each protein has specific properties which are due to:
(i) Number of amino acids,
(ii) Kinds of amino acids.
(iii) Specific sequence of amino acids and.
(iv) The shape of protein molecule.
As a result protein structure is formed which is organized at four levels.
(1) Primary Structure of Protein:
It includes the number, kind and sequence of amino acids in a protein molecule.
Size of protein Molecule:
The size of a protein molecule is determined by the type of amino acids and the number of amino acids in that protein molecule.
Sequence of Amino Acids:
There are over 10,000 proteins in the human body. These are formed by the specific arrangements of 20 types of amino acids.
The sequence of amino acids is determined by the order of nucleotides in the DNA.
(2) Secondary Structure:
It is the coiling of primary polypeptide chains.
The chains in a protein molecule are not flat. They usually coil into α helix, or into β-pleated sheet.
(i) α-Helix:
It is most common structure in which the basic polypeptide chain is spirally arranged.
It is very uniform geometric structure with 3.6 amino acids in each turn of the helix.
The helical structure is kept by the formation of hydrogen bonds among amino acid molecules in spiral turns.
(ii) β-Pleated Sheets:
These are formed by the fold backs of the polypeptide (e.g. silk).
(3) Tertiary Structure:
In the tertiary structure, a polypeptide chain bends and folds upon itself forming a globular shape.
Structural Maintenance:
It is maintained by three types of bonds:
(a) Ionic bonds, (b) Hydrogen bonds and (c) Disulfide (-S-S-) bonds
Stable Configuration:
In aqueous environment (aqueous solution) the most stable tertiary structure is that in which hydrophobic amino acids are hidden inside while the hydrophilic amino acids are on the surface of the molecule.
(4) Quaternary Structure:
Quaternary proteins are polymers of several tertiary structures.
In quaternary structure, the highly complex polypeptide tertiary chains are aggregated and held together by:
(i) Hydrophobic interactions.
(ii) Hydrogen bonds and,
(iii) Ionic bonds.
Haemoglobin, the oxygen carrying protein of red blood cells show quaternary structure.
Example:
(i) Structure of Insulin:
F. Sanger was the first scientist who determined the sequence of amino acids in a protein molecule.
After ten years of work, he concluded that insulin is composed of 51 amino acids in two chains. One with 21 amino acids and the other with 30 amino acids. Both the chains are linked by disulphide bonds.
(ii) Structure of Haemoglobin:
Haemoglobin is composed of four chains, two alpha and two beta chains.
Each alpha chain contains 141 amino acids, while each beta chain contains 146 amino acids.
Abnormal Sequence of Amino Acids:
The arrangement of amino acids in a protein molecule determines its function.
If any amino acid is not at its normal place, the protein fails to carry on its normal function.
Example:
The best example is the sickle cell haemoglobin of human beings.
In this case only one amino acid (Glutamic acid) in each beta chain out of the 574 amino acids does not occupy the normal place in the proteins. Actually glutamic acid is replaced by valine, Therefore hemoglobin fails to carry sufficient oxygen. The result is the death of the patient.
Reviewed by SaQLaiN HaShMi
on
12:13 AM
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