Tertiary structure  

The  3rd  level  of structure  found  in proteins,  tertiary  structure,  which refers  to the spatial arrangement  of amino acids which are far apart in linear sequence  as well as those residues which are adjacent. Again, it is sequence of amino acids which speci?es this  ?nal  3-dimensional structure.  In  water- soluble  globular  proteins  like myoglobin  , the main  driving force behind folding of the polypeptide chain is the energetic requirement  to bury nonpolar amino acids in hydrophobic interior away from surrounding  aqueous,  hydrophilic  medium.  The polypeptide  chain spontaneously  folds so that the majority  of its hydrophobic  side-chains  are buried in interior,  and  majority  of its polar,  charged  side-chains  are on surface. Once  folded,  the 3-dimensional biologically-active (native)  conformation of protein  is maintained  not only by hydrophobic  interactions,  but by electrostatic  forces also,  hydrogen  bonding  and,  if  is present,  covalent  disul?de bonds. The electrostatic  forces include salt bridges between oppositely  charged groups  and multiple  weak  vander Waals  interactions  between  tightly packed aliphatic side-chains in interior of the protein.