The Structure Of Proteins

All proteins share three superimposed levels of structure, known as primary, secondary, and tertiary structure

Primary structure:

• This is the protein s sequence of amino acids. If an amino acid is misplaced/wrongly encoded, then the entire protein can be rendered useless.
• The primary structure in turn dictates secondary and tertiary structure, due to the chemical nature of the backbone and the side chains (R groups) of the amino acids along the polypeptide.
• The precise primary structure of a protein is determined not by the random linking of amino acids, but by inherited genetic information.

Secondary structure:

• Most proteins have segments of their polypeptide chains repeatedly coiled and folded in patterns that contribute to the protein s overall shape. These coils/shapes are known as the secondary structure.
• They are the result of the hydrogen bonds between the repeating constituents of the polypeptide backbone (not the amino acid side chains).
• Within the backbone, the oxygen atoms of the carbonyl group have a partial negative charge, and the hydrogen atoms attached to the nitrogen have a partial positive charge this forms hydrogen bonds.

Structures include:

• helix delicate coil held together by hydrogen bonding between every fourth amino acid.
• pleated sheet two or more segments of polypeptide lye side by side ( strands) are connected by hydrogen bonds between parts of the two parallel segments of the polypeptide backbone.

Tertiary structure:

• Tertiary structure is the overall shape of the polypeptide resulting from interactions between the side chains (R groups) of the amino acid.
• One type of interaction is a hydrophobic interaction. As a polypeptide folds into the functional shape, amino acids with hydrophobic side chains usually end up in clusters at the core of the protein, out of contact with water.
• Once non polar amino acids are close together, van der Waals interactions hold them together.
• Hydrogen bonds between polar side chains and ionic bonds between positively and negatively charged side chains also help stabilise the tertiary structure.
• Covalent bonds called disulphide bridges/bonds may further reinforce the shape of a protein (where two cysteine monomers which have sulfhydryl groups SH on their side chains are brought together by the folding of the protein).

Quaternary structure:

• Some proteins consist of two or more polypeptide chains aggregated into one functional macromolecule. This overall protein structure resulting from this aggregation is the quaternary structure.
• Conjugated proteins can also form.
• Examples include haemoglobin, collagen and transthyretin.