Protein is the polymer of amino acids. They are complex compound found in animal and plants which are responsible for growth and maintenance of living cells. They have the wide variety of biological function. They are essential for growth and maintenance of life. They are high molecular weight and contains nitrogen agent as an essential element. On hydrolysis, they give amino acid.
The name proteins are derived from Greek word ‘proteios’ meaning of first importance. They are constituted of cells and hence are present in all living bodies. They contain the element carbon, hydrogen, oxygen, and sometimes sulphur and phosphorous.
Hence protein is defined as the nitrogenous compound of very high molecular weight which is essential for the growth and maintenance of life and gives amino acid on complete hydrolysis.
Some of the important protein and their function are given in table given below.
The plant built up their proteins from carbon dioxide , water and minerals salts and nitrates in the presence of sunlight. Animals derive proteins from plants. The plant protein is hydrolyzed in the presence of enzymes to amino acids in the animal in the body. These amino acids are absorbed by blood and are transported to various parts of the body. From these amino acids, the animal proteins are synthesised.
Peptide bond or peptide linkage.
During polymerization of amino acids to form proteins (polypeptides), there is the formation of a bond between amino acids which is called peptide bonds. Hence the bond formation between the amino group of one amino acid and carboxylic group of another amino acid to give COONH type of bond is called peptide bond or peptide linkage.
Hydrolysis of the dipeptide.
Dipeptide id formed by the combination of two amino acids. In such compounds, the two amino acids are linked together by the bond, called peptide bond or linkage [Co-NH], which id formed by bonding the amino group of one acid with the carboxylic group of another amino group.
Dipeptide undergoes hydrolysis to give constituent amino acids. A dipeptide gives two amino acids.
Denaturation of protein.
The process by which the biological functions of proteins is damaged by the application of heat, radiation, or chemicals like acids, alkalies, alcohols etc id called denaturation of the protein. In other words, the process by which the physical and biological property of a protein are damaged (disturbed) however, the chemical composition is similar is called denaturation of a protein.
Denaturation of protein takes place only in the presence of heat, acid, base or salts of acid and base.
When protein is denatured, its secondary and tertiary structure are destroyed due to the cleavage of non-covalent weak bonds like H-Bond, ionic interaction, van der wall interaction. However, the primary structure is unaffected due to the peptide bond. In addition , the water-soluble proteins now become water insoluble. For example. Coagulation of white portion of egg on boiling.
Conversion of milk to cheese by treating with an acid.
Types of protein.
Protein is broadly classified into two types.
1. Simple protein.
The proteins which on hydrolysis gives only alpha amino acid are called simple protein. For example, albumins, globulins, histones etc.
2. Conjugated protein.
The protein which, in addition to alpha amino acids, yield non-protein material on hydrolysis is called conjugated proteins. For example, phosphoprotein, lipoproteins, glycoproteins etc.
The function of protein.
Proteins are the basis of protoplasm and hence, they are present in all living organism. Some of the important function of proteins are as follows.
The structure of protein.
Proteins are made up of peptide chains of amino acid residue joined together by amino acid residue joined together by the amine linkage. Protein differs from polypeptide chain poly in their molecular weight. The protein has the molecular weight higher than 1000 and they have more complex structure. They have the definite three-dimensional structure which can be studied in the following three levels.
Proteins are divided into three structure. They are as follows.
1. The primary structure of protein.
The primary structure of the protein refers to the number, nature, sequence and sequence of amino acid residue linked to a protein. The difference in chemical and biological properties of various proteins are due to a difference in the primary structure. The primary structure of the protein is most important and the change in the amino acid in the sequence causes to change in the biological activities. For example. Haemoglobin has 574 amino acid unit and change in just one amino acid results from the defective haemoglobin found in protein suffering from sickle cell anaemia.
Sickel cell anaemia.
2. Secondary structure.
The secondary structure of the protein refers to the shape of peptide chain i,e-linear, cyclic, branched or arrangement in the helical form. A helical structure consists of peptide chain coiled as the spiral.
The important type of secondary structure which have been proposed are described below.
1 α Helix structure.
The formation of hydrogen bonding between amide group within the same chain, causes the peptide chains to coil up into a spiral structure called the α helix.
Fibrous structural proteins such as present in wool and hair have α helix structure. These proteins are elastic. On stretching the weak hydrogen bonds are broken and this increases the length of helix like a spring. On releasing the tension, the hydrogen bonds are reformed, giving back the original helical shape.
2. β or pleated sheet structure.
A different type of secondary structure is possible when peptide chains are fully extended to form flat zig zags as shown below. These chains lie side by side to form a flat sheet, each chain being held by hydrogen bonds (N-H….O=C) to the two neighbouring chains.
Silk fibre has this type of structure. If is not elastic since stretching leads to pulling the peptide covalent bonds. On the other hand, it can be bent easily, during this process , the protein sheets slide over each other.
3. The tertiary structure of proteins.
Protein such as myoglobin in muscle and haemoglobin in blood have more complex structure. These proteins also consist of helices but they are folded up in a complex way to give a compact structure which is more or less spherical in shape. The overall shape of the protein determines by all the bends, kinks and selections of α helical structure are called tertiary structure of the protein.