Peptides

Peptides are polymers of amino acids. Peptides are made from amino acids and therefore always contain the elements of carbon, hydrogen, oxygen and nitrogen sometimes Sulphur. Amino acids can be joined covalently through peptide bonds to form peptides and proteins which are macro molecules of high molecular weight. Cells generally contain thousands of different proteins, each with a different biological activity. 

 Proteins are one of the four major biomolecules that involve in metabolic process and maintenance of living organisms which is a class of indispensable biomolecules. Proteins are polymers of amino acids arranged as polypeptide chains. Thousands of different proteins with different biological activities are found in cells.

Amino Acids

There 20 amino acids commonly found as residues in proteins. These amino acids consist of an α-carboxyl group, an α-amino group, hydrogen atom, and a distinctive R group substituted on the α-carbon atom. Amino acids are classified into five types on the basis of the polarity and charge (at pH 7) of their R groups.


Peptides

A chain of amino acid monomers which are covalently joined through a peptide bond called as peptides. Peptide bonds are the substituted amide linkage which is formed by removal of the elements of water (dehydration) from the carboxyl group of one amino acid and the α-amino group of another. 
 
Two amino acids- dipeptide 
Two amino acid molecules can be covalently joined through a substituted amide linkage, termed a peptide bond, to yield a dipeptide. 
Three amino acids -tripeptide 
Four amino acids- tetrapeptides 
 Five amino acids- pentapeptides 
 Few amino acids- oligopeptide 
 Many amino acids- polypeptide


Arrangement of peptides 


There are 4 levels of organization of the structure protein molecule: primary, secondary, tertiary and quaternary structures. 
 
Primary structure: Polypeptide chains of various combinations of amino acids 

 Secondary structure: Attractive and repulsive forces among amino acids determine the secondary structure of proteins.
 i. alpha helix structure- results from intra-chain weak hydrogen bonds 
 ii. Beta sheet (pleated sheet) structure- results from inter-chain weak hydrogen bond 

 Tertiary structure: Describes the packing of alpha-helices, beta-sheets and random coils of one whole polypeptide chain due to various bonding interactions such as ionic, hydrogen, or covalent bonds. This may cause a number of bends, folds, and loops in the protein chain. 
 · Most of biologically active proteins (enzymes) come under this category 
 · Globular proteins are of this type. 
 · More compact than primary/secondary structures. 

 Quaternary structure: Non-covalent association of two or more polypeptide chains Mostly inter-chain interactions.
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