proteins

PRIMARY STRUCTURE OF PROTEINS

Sequence of amino acids in a polypeptide chain. 

DNA of a cell determines the primary structure of a protein by instructing the cell to add certain amino acids in specific quantities in a certain sequence. 

This affects the shape and therefore the function of the protein

The primary structure is specific for each protein (one alteration in the sequence of amino acids can affect the function of the protein)

SECONDARY STRUCTURE OF PROTEINS

Hydrogen bonds form between amino acids close together (weak negatively charged nitrogen and oxygen atoms interact with the weak positively charged hydrogen atoms between carboxyl group and amine group). This causes the polypeptide chain to be coiled into an alpha helix or folded into a beta pleated sheet. 

TERTIARY STRUCTURE OF PROTEINS

Further conformational change of the secondary structure, coiled or folded further, leads to additional bonds forming between the R groups (side chains). The additional bonds are:

Hydrogen bonds (these are between R groups)

Disulphide bridges (only occurs between cysteine amino acids)

Ionic bonds (occurs between charged R groups). 

Hydrophobic interactions (between non-polar R groups)

QUATERNARY STRUCTURE OF PROTEINS

The way polypeptide chains are assembled. Stabilised with hydrogen bonds, ionic bonds and disulfide bridges. 

Final 3D structure of proteins with more than one polypeptide chain. 

EXPLAIN HOW A CHANGE IN DNA BASE SEQUENCE CAN AFFECT AN ENZYME REACTION 

1. Base sequence determines sequence of amino acids in polypeptide chain/primary structure 

2. 3 bases code for one amino acid 

3. Primary structure determines position of bonds between R groups in the tertiary structure

4. Hydrogen, ionic bonds and disulphide bonds

5. A change in tertiary structure/ bonds changes shape of active site of the enzyme

6. Meaning the substrate can no longer bind to form an enzyme substrate complex

TEST FOR PROTEINS

METHOD 

Biuret test

POSITIVE RESULT 

Blue →purple