Proteins are compounds, which are formed by the combination of amino acids. Amino acids are the compounds having both Amino and Carboxylic (COOH) groups attach to the same carbon atom. Proteins are considered as complex organic compounds.
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When amino acids are attached together, a “polypeptide” chain is formed. When many polypeptide chains are linked together a protein is produced.
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The polymers of amino acids containing carbon, nitrogen, oxygen and hydrogen are called proteins. The number of amino acids is different in different proteins. It may be from a few to 3000 or more.
Each protein has specific properties. These properties are determined by numbers and specific sequences of amino acids in protein molecules. These properties also depend on the shape of proteins. It gives final shape to the proteins. These are four levels of organization in proteins:
The structure of protein which shows number and sequence of amino acids is called primary structure of proteins.
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When amino acids are arranged in straight manner in a polypeptide chain, it is called primary structure of protein.
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This protein also contains disulphide (S-S) bond. Insulin and Haemoglobin are examples of primary structure of protein.
Haemoglobin is discovered by Ingram (scientist). Haemoglobin is composed of 574 amino acids. These amino acids form four chains of amino acids. These are two alpha and two beta chains. Each alpha chain has 141 amino acids and each beta chain has 146 amino acids.
The size of protein depends on the type of and number of amino acids present in a protein.
There are 1000 proteins in the human body. Each protein is composed of unique and specific arrangement of 20 types of amino acids. A sequence of nucleotides is present in the DNA. This sequence of nucleotides determines the sequence of amino acids in a protein molecule.
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The arrangement of amino acids is highly specific for the proper functioning of protein. If a single amino acid is not in its normal place, the protein fails to perform its normal functions. The best example is sickle cell Haemoglobin in human beings. In this case, one molecule of amino acid is replaced by some other amino acid. This Haemoglobin fails to carry sufficient oxygen. It may cause death.
The structure of protein formed by coiling and folding of polypeptide chains is called secondary structure of protein. The protein molecules usually do not remain flat. They coil in many ways:
The spiral (like spring) coiling of the polypeptide chain is called 𝜶-helix. It is one of the most common secondary structures. The α-helix forms a uniform geometric structure. Each turn of the helix has 3.6 amino acids. Hydrogen bonds are formed among the amino acids of successive turn of the spiral. It keeps the helix intact.
The folding back of the polypeptide is called 𝛃-pleated sheet. It forms a folded sheet like structure.
The globular structure formed by the bending and folding of polypeptide chain upon itself is called tertiary structure. It forms tertiary conformation (shape) of proteins.
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Tertiary conformation of protein is maintained by three types of bonds: ionic, hydrogen and disulphide (S-S). Most stable tertiary conformation is made in an aqueous environment. The hydrophobic amino acids are buried inside the aqueous medium. While the hydrophilic (water loving) amino acids are present on the surface of the protein molecule.
The structure formed by the aggregation of tertiary polypeptide chains is called quaternary structures. These polypeptide chains are held together by hydrophobic interactions, hydrogen and ionic bonds. Haemoglobin carries oxygen of red blood cells.
Proteins have complex structures. Different proteins perform different functions. So, there is no single method of classification of protein. However, proteins are divided into following types on the basis of their structures:
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Examples: Silk fibers (from silk worm and spider’s web), Myosin (in muscle cells), Fibrin (of blood clot), and Keratin (of nail and hair).
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Examples: Enzymes, Antibodies, Hormones and Haemoglobin.
Proteins are most abundant compound present in the cells. Proteins form over 50% of dry weight of the bodies of the organisms. They are present in all types of cells and in all parts of the cell. Proteins perform following functions:
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The compounds with an amino group and a carboxylic group (COOH), attached to the same carbon atom, known as Alpha Carbon are called “Amino Acids”. 170 types of amino acids are present in cells and tissues. Only 25 amino acids form proteins. However, most of the proteins contain 20 types of amino acids. They have general formula as:
R may be a hydrogen atom as in glycine amino acid. Or it may be HCHH as in alanine amino acid. Or it may be any other group. Thus amino acids mainly differ due to the type or nature of R group.
Amino acids are linked to form polypeptide chain. The amino group of one amino acid reacts with the carboxyl group of another amino acid and C-N bond is formed. This C-N bond is called peptide bond. Water is released during peptide bond formation.
For example, glycine and alanine may combine to form glycylalanine. It has two amino acid units. Thus it is called dipeptide. A dipeptide has again an amino group at one end and a carboxyl group at the other end. So, both reactive parts are again available for further peptide bond formation. In this way, tripeptide, tetrapeptide and pentapeptide etc. are formed. They ultimately form a polypeptide chains.
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