This is pq of 2022/2023 1) Explain the non-covalent interactions that maintain the 3-dimensional structure of proteins. 2) State the steps involved in determining the amino acid sequence of a peptide. 3) Write short notes on the following separation methods: i. Isoelectric pH ii. α-helix and β-pleated sheet iii. Coenzyme iv. Heteromultimeric proteins 2021/2022 1) Describe the 1°, 2°, and 3° levels of protein structure. Write short notes on:

Here's the explanation for the first question from the 2022/2023 paper:

1. The 3-dimensional structure of proteins is maintained by several non-covalent interactions, along with covalent disulfide bonds (though the question specifically asks for non-covalent). These interactions occur between the side chains (R-groups) of amino acids and between backbone atoms:

• Hydrogen Bonds: These form between a hydrogen atom covalently bonded to an electronegative atom (like oxygen or nitrogen) and another electronegative atom. In proteins, they occur between the backbone amide and carbonyl groups, and between polar side chains. They are crucial for stabilizing secondary structures like $\alpha$-helices and $\beta$-pleated sheets, and also contribute to tertiary structure.
• Ionic Interactions (Salt Bridges): These are electrostatic attractions between oppositely charged R-groups. For example, the positively charged amino group of lysine can interact with the negatively charged carboxyl group of aspartate or glutamate. These interactions are strong and contribute significantly to protein stability.
• Hydrophobic Interactions: Nonpolar amino acid side chains tend to cluster together in the interior of a protein, away from the aqueous environment. This "burying" of hydrophobic residues minimizes their contact with water, which is a major driving force for protein folding and stabilization of the tertiary structure.
• Van der Waals Forces: These are weak, transient attractive forces that arise from temporary fluctuations in electron distribution around atoms. They occur between all atoms in a protein, but are most significant when atoms are very close together. The cumulative effect of many such interactions contributes to the overall stability of the protein's folded structure.