Question 1

What are the four levels of protein structure?

Accepted Answer

Primary structure is the linear amino-acid sequence. Secondary structure includes local arrangements such as alpha-helices and beta-sheets. Tertiary structure is the complete 3-D conformation of one polypeptide chain, while quaternary structure is the association of two or more polypeptide subunits.

Question 2

What stabilises the tertiary structure of a protein?

Accepted Answer

Tertiary structure can be stabilised by hydrophobic packing, hydrogen bonds, electrostatic interactions, van der Waals interactions and, in some proteins, covalent disulfide bonds between cysteine residues. Hydrophobic side chains often lie inside the protein, while polar groups are more often exposed to water.

Question 3

Why is haemoglobin a quaternary protein?

Accepted Answer

Human haemoglobin is made of four polypeptide subunits: two identical alpha subunits and two identical beta subunits. Each subunit contains one haem group with one Fe2+ ion, so the complete tetramer can bind four O2 molecules when fully oxygenated.

Question 4

What happens to a protein during denaturation?

Accepted Answer

Denaturation disrupts the secondary, tertiary and, where present, quaternary structures, usually causing loss of biological activity. The peptide-bonded primary sequence usually remains intact because peptide bonds are not normally broken by ordinary denaturation.

Protein Folding — Primary to Quaternary in 3D

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