text: what is a protien?   a protein is any of a large group of nitrogenous organic compounds that are essential constituents of living cells; consist of long strings of amino acids.  they are the polymers, with the condensation polymerisation of their subunits(amino acids), joined together by the amide linkage, and the bond is called a peptide bond.  proteins have four types of structures: primary structure secondary structure tertiary structure quartenary structure  due to the peptide bonding, proteins are also referred as polypeptides and are synthesised from the dna.