Protein Polymerisation, Structure and Stability

 Protein is a monomers of amino acids. So we know about amino acids, and these are the monomers that will form proteins, which are also known as polypeptides. Proteins are polymers of amino acids, and they are the most diverse type of biomolecule in your body. 

  Different kinds of proteins include enzymes that catalyze chemical reactions, receptors that control signaling in your body, hemoglobin, which carries oxygen throughout the bloodstream, muscle and organ tissue, which gives your body structure and mobility, and so many other things. 

So how do amino acids polymerize?

 This happens when amino acids form peptide bonds with one another, such as the peptide bond between two amino acids. Peptide bond formation is an example of a dehydration reaction because the two hydrogens and the oxygen are lost, and two hydrogens plus one oxygen equals a water molecule, so as a water molecule is lost these two amino acids come together to form a peptide bond, which results in an amide

 An amide is a functional group with a nitrogen atom next to a carbonyl and this is the functional group that will connect each amino acid during polymerization.

 If two amino acids combine we get a dipeptide. If between three and ten come together, we would call that an oligo peptide, since oligo means just a few. And if more than ten come together we will call that a polypeptide, since poly means many and proteins are large polypeptides of around three hundred to a thousand amino acids that are folded in such a way that they have some biological activity. When we look at any peptide, we must notice that there is an N-terminus, meaning the side of the chain that ends with the amino group, and a C-terminus, the side that ends with the carboxyl group. 

 Proteins are very large compared to simple molecules. They contain hundreds of amino acid residues and they have very specific shapes from which their function is derived

Structure of Protein :
 A polypeptide chain can fold up to form specific shape (conformation). This conformation is a 3D arrangement of atoms and is determined by the sequence of amino acids, Four levels of structures are observed in proteins. They are 
1. Primary structure, 
2. Secondary structure, 
3. Tertiary structure and 
4. Quaternary structure
1. Primary structure 
    A linear sequence of amino acids, joined by peptide bond represents primary structure. Hence, the proteins with primary structure are under influence of only one type of chemical bond i.e. covalent bond.

2. Secondary structure 
    The protein molecules, under influence of two types of chemical sources form secondary structure. These are the forces of covalent bonding (represented by peptide bond) and force of hydrogen bonding. Partially charged secondary groups of amino acids are responsible for the formation of hydrogen bonds. 
 The secondary structure represents regular folding of regions of poly peptide chains. The two most common types of protein folds are α-helix and β-sheet structures 

 These structures are found in fibrous, linear or rod shaped proteins, where the peptide chain shows a regular helical conformation. The structure is formed by hydrogen bonding between carbonyl oxygen of each peptide bond and amino group of fourth amino acid away. The hydrogen bonds run nearly parallel to the axis of helix. The side chains of amino acids are positioned along the outside of the cylindrical helix. e.g. myoglobin. 

  In this case, hydrogen bonds are formed between the peptide bonds in different peptide chains are in different regions of the same peptide chain. These bonds form sheet or plate like structures. The polypeptide chains, oriented in these structures may be parallel or anti parallel. Multiple ?- pleated sheets provide strength and rigidity to structural proteins. 

3. Tertiary structure 
    Protein molecules, under influence of additional chemical forces, other than that of covalent boniding and hydrogen bonding show tertiary structure. Proteins with tertiary structure have a three dimensional structure. Proteins with tertiary structure exhibit folding of their polypeptide chain in aqueous medium (water), where hydrophobic non polar groups of the amino acids are buried interior and hydrophilic polar group remain on the surface. The chemical forces involved in such folding of protein molecule include electrostatic forces, hydrogen bonding and disulfide bonds. e.g. globular protein of myoglobin in water.

4. Quaternary structure
    Proteins having more than one polypeptide chains exhibit fourth level of protein structure, called quaternary structure. These proteins have two or more polypeptide subunits joined by covalent links (disulfide bonding) or non-covalent forces such as ionic forces, hydrogen bonding or hydrophobic interactions.

Protein stability :
  The native three dimensional conformation of proteins maintained by influence of different types of covalent and non-covalent bonds. They include electrostatic forces, hydrogen bonding, hydrophobic forces, disulfide covalent bonds.

Electrostatic forces 
  These include interactions between two ionic groups of opposite charge such as that between positive charge of ammonium ion of lysine and negative charge of carboxyl ion of aspartic acid. 

Hydrogen bonding
  These bonds involve electrostatic attractions between weakly acidic donor group and acceptor group, that has alone pair of electrons.

Hydrophobic forces
  Non polar groups are hydrophobic and attempt to remain away from water or to minimize their contact with water. These forces are hydrophobic forces. 

Disulphite bonds
  These are covalent bonds that form between cysteine residues and are formed under oxidizing conditions.these bonds are mainly use to making high heat stable proteins