Proteins are compounds, which are formed by the combination of amino acids. Amino acids are compounds having both Amino and Carboxylic (COOH) groups attached to the same carbon atom. Proteins are considered complex organic compounds.
When amino acids are attached together, a “polypeptide” chain is formed. When many polypeptide chains are linked together a protein is produced.
Structure of Proteins
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 the final shape to the proteins. These are four levels of organization in proteins:
1. Primary Structure of Proteins
The structure of the protein which shows the number and sequence of amino acids is called the primary structure of proteins.
When amino acids are arranged in a straight manner in a polypeptide chain, it is called the primary structure of the protein.
This protein also contains a disulfide (S-S) bond. Insulin and Haemoglobin are examples of the primary structure of the protein.
a) Sequence of Amino Acids in Proteins
A Sequence of Insulin molecule
- Sanger determines the sequence of the first protein molecule. Sanger worked on insulin for ten years. He found that insulin is composed of 51 amino acids. It has two chains of amino acids. One chain has 21 amino acids. The other chain has 30 amino acids. Both these chains are held together by disulfide bridges (bonds).
A sequence of Haemoglobin molecule
Hemoglobin is discovered by Ingram (scientist). Hemoglobin 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.
b) Size of Protein molecule
The size of the protein depends on the type and the number of amino acids present in a protein.
c) Specific arrangement of Amino Acids
There are 1000 proteins in the human body. Each protein is composed of a 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.
The arrangement of amino acids is highly specific for the proper functioning of the 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.
2. Secondary Structure of Protein
The structure of protein formed by the coiling and folding of polypeptide chains is called the secondary structure of the protein. The protein molecules usually do not remain flat. The coil in many ways:
The spiral (like spring) coiling of the polypeptide chain is called ?-a 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 turns of the spiral. It keeps the helix intact.
The folding back of the polypeptide is called ?-a pleated sheet. It forms a folded sheet-like structure.
3. Tertiary Structure of Protein
The globular structure formed by the bending and folding of the polypeptide chain upon itself is called tertiary structure. It forms the tertiary conformation (shape) of proteins.
The tertiary conformation of the protein is maintained by three types of bonds: ionic, hydrogen, and disulfide (S-S). Most stable tertiary confirmation 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.
4. Quaternary Structure of Protein
The structure formed by the aggregation of tertiary polypeptide chains is called a quaternary structure. These polypeptide chains are held together by hydrophobic interactions, and hydrogen, and ionic bonds. Hemoglobin carries oxygen from red blood cells.
Classification of Protein
Proteins have complex structures. Different proteins perform different functions. So, there is no single method of classification of protein. However, proteins are divided into the following types on the basis of their structures:
1) Fibrous proteins
- Their molecules are composed of one or more polypeptide chains. These chains are present in the form of fibrils.
- Secondary structure is most important in them.
- These are insoluble in aqueous media.
- They are non-crystalline and elastic in nature.
- These proteins perform structural roles in cells and organisms.
Examples: Silk fibers (from silk worms and spider’s web), Myosin (in muscle cells), Fibrin (of blood clots), and Keratin (of nails and hair).
2) Globular Proteins
- They have multiple folding of polypeptides. So, their chains are spherical and ellipsoidal.
- Tertiary structure is the most important in them.
- These proteins are soluble in aqueous media such as salt solution, solution of acids or bases, or aqueous alcohol.
- They can be crystallized.
- They disorganize with the change in the physical and physiological environment.
Examples: Enzymes, Antibodies, Hormones, and Haemoglobin.
Importance of Proteins
Proteins are the most abundant compound present in the cells. Proteins form over 50% of the 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 the following functions:
- They build many structures of the cells like the cell membranes.
- All Enzymes are proteins in nature. These enzymes control the metabolism of cells.
- Most Hormones are also proteins in nature. Hormones regulate metabolic processes.
- Some proteins (e.g. Haemoglobin) work as carriers. They transport specific substances such as oxygen, lipids, metal ions, etc.
- Some proteins are called Antibodies. Antibodies defend the body against pathogens.
- Blood clotting proteins prevent the loss of blood from the body after an injury.
- The proteins cause the movement of organisms. They also cause the movement of chromosomes during anaphase of cell division.
- They form the plasma membrane of the cell.
- Proteins from seeds in plants.
- On oxidation, it provides energy.
- Protein act as membrane receptors and transporters, allowing the passage of important substances.
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 proteins contain 20 types of amino acids. They have the general formula:
R may be a hydrogen atom as in glycine amino acid. Or it may be HCHH as in the alanine amino acid. Or it may be any other group. Thus amino acids mainly differ due to the type or nature of the R group.
Formation of Protein from Amino Acids
Amino acids are linked to form a polypeptide chain. The amino group of one amino acid reacts with the carboxyl group of another amino acid and the C-N bond is formed. This C-N bond is called a 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 a 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 polypeptide chains.