Question

what has the quarternary structure of hemoglobin got to do with it?

Answer 1

Quaternary structure is the fourth level of protein structure, although quaternary structure is not found in all proteins, but only exists where two or more tertiary units (Each then called a sub-unit) combine to form one functioning protein. Haemoglobin on the other hand consists of four polypeptide chains as follows; two identical α-chains consisting of 141 amino acid residues and two identical β-chains consisting of 146 amino acid residues. Each chain is called a sub-unit and together they form the quaternary structure of the protein Haemoglobin.

In summary quaternary structure describes the arrangement of sub-units in a protein consisting of more than one polypeptide chain, where the sub-units may be identical or different. The sub-units in a quaternary structure are held together by non-covalent interactions where the ‘contact regions’ between the sub-units resemble the interior of tertiary structure proteins as being hydrophobic. These structures cannot have mirror image super-positioning resulting in symmetrical distribution of the sub-units in the quaternary structure.

Reasons for quaternary structure

By combining more than one sub-unit into one functioning protein, enhanced biological activity and control can be achieved. The following summarises the main points to consider for the reasons of having associated sub-units;

·        Enhanced stability (Reducing surface area vs. protein volume)

·        Genetic economy & efficiency (limited genetic information needed)

·        Coordinated regulation of proteins

·        Defects easily repaired by replacing flawed sub-units

·        Cooperativity (Allosteric behaviour)