Proteins contain Carbon, Hydrogen, Oxygen and Nitrogen as the major components while Sulfur and Phosphorus are minor constituents. Nitrogen is characteristic of proteins. The variety of protein structures is made possible by the order and arrangement of the amino acid building blocks. All amino acids have the same basic structure.
Proteins are polymers: similar molecules (called monomers) are repeated many times to form a chain. Chains that are less than 40-50 amino acids or residues are often referred to as polypeptide chains since they are too small to form a functional domain. Larger than this size, they are called proteins.
Each polypeptide chain has a unique amino acid sequence decided by the genes. The primary structure is maintained by the covalent bonds of the peptide linkages.
Amino acids (except glycine) contain a central chiral carbon, i.e., a carbon atom covalently linked to four different groups of atoms, often called carbon α (Cα). This central chiral atom is linked to an amino group and a carboxylic acid group, thus the term amino acid.
The sequence of amino acid side chains gives proteins unique properties. They not only determine the fold of the protein, but they also determine surface properties that are important for selective interactions with other molecules and catalysis of chemical reactions.
Generally, the polypeptide chains are linear. However branching points in the chains may be produced by interchain disulphide bridges. The covalent disulphide bonds between different polypeptide chains in the same protein (interchain) or portions of the same polypeptide chain (intrachain) are also part of the primary structure.
Structure of protein
