Protein

A protein is a biological molecule which is made up of amino acids.  The amino acids join together with peptide bonds to form a polypeptide chain. A protein can be made up of a single polypeptide chain or multiple polypeptides linked together. 

Examples of proteins include enzymes, receptors and hormones.  They are found in evey form of life from viruses to bacteria, yeasts to humans. 

A protein has several 'layers' of structure.  The primary structure is the sequence of amino acids.  This is determined by the DNA sequence that encodes for that particular protein, called the gene.  Secondary structure is the first level of protein folding. A protein can fold in two different ways or not at all.  It can either fold as an alpha-helix or a beta-sheet depending on the sequence of amino acids.  Tertiary structure relates to the protein function.  If the tertiary structure is wrong then the protein is unlikely to function properly.  Tertiary structure is held together by either hydrogen bonds or disulphide bridges depending o the amio acids present.  Finally, if there are more than one peptide chains linked together to form a protein then you get a quarternary structure.