The first structure is called the primary structure. The primary structure of a protein is a linear sequence of amino acids in its molecule (It is the specific sequence of amino acids in a polypeptide). This structure is found in all proteins and it involves peptide bonds.
The Secondary structure of proteins involves polypeptide chains folding or twisting in various ways. The most common ways are to coil to form a helix (alpha-helix) or to fold into sheets (beta pleated sheet). The secondary structure of proteins occurs due to the formation of hydrogen bond in the polypeptide. It is the hydrogen bond which keeps the particular shape of the polypeptide chain. These hydrogen bonds are present in the backbone of every polypeptide. The backbone of every polypeptide is exactly the same and therefore many polypeptides can take up same shapes.
In alpha-helix, hydrogen bonds are formed between the CO of one amino acid with the NH of and amino acid four further along the chain. This twists the chain into a spiral form which is maintained by the hydrogen bonds. There are 3.6 amino acids in 1 complete turn.
The beta-sheets occurs as a flat zig-zag chain. They occur if polypeptide chains are formed in opposite directions to each other. H- bonds holds the –CO and the –NH group of adjacent molecules. These bonds make the beta-sheets stronger but less elastic than the alpha-helix.
In both, alpha-helix and beta-sheets, the ‘R’ groups are not involved.
The Tertiary structure is formed by the further folding of secondary structures of proteins molecules to produce a complex globular shape. Tertiary structures are single polypeptides whose shape is determined by the various bonds formed between parts of the R-groups. Every polypeptide has a different order of R-group so bonds form in different places and therefore each protein has a different shape.
There are 3 type of bonds which may be formed between the R-groups. The hydrogen bonds are formed between the R-group of a variety of different amino acids. They form when electropositive H atom of the –OH or –NH of the R-group attract the electronegative O of a CO group or another R-group. These bonds are easily broken but very numerous. Ionic bonds form between amino and carboxyl parts present on some R-groups. They are stronger than hydrogen bonds. The ionic and hydrogen bonds are affected by changes in pH. The disulphide bond is a covalent bond which is formed between the R-groups of the amino acids such as cysteine which have –SH groups. This is the strongest bond of the 3.
All these bonds and interactions cause proteins to have an irregular compact globular shape. The shape is also affected by the presence of hydrophobic R-groups.
Some proteins are made up from more than 1 polypeptide chain. This is also known as the quaternary structure. An example is haemoglobin which consists of 4 separate polypeptide chains. This structure can be used to describe the structure of conjugated proteins. These proteins contain amino acid and some other chemical molecule. Finally they are specific in globular proteins.