Levels of protein structure:

• Linear sequence of amino acids linked by peptide bonds represents the Primary structure $\rightarrow$ (I)
• Regular local conformations such as $\alpha$-helix and $\beta$-pleated sheet stabilised by hydrogen bonding represent Secondary structure $\rightarrow$ (II)
• Overall three-dimensional folding of polypeptide chain, classifying proteins into fibrous and globular shapes, represents Tertiary structure $\rightarrow$ (IV)
• Spatial arrangement of two or more polypeptide subunits with respect to each other represents Quaternary structure $\rightarrow$ (III)

Correct match: (A)-(I), (B)-(II), (C)-(IV), (D)-(III).

Functions and clinical associations of biomolecules:

• Vitamin A (Retinol) deficiency leads to hardening of the cornea called Xerophthalmia and night blindness $\rightarrow$ (II)
• Thiamine (Vitamin $\text{B}_1$) deficiency causes Beri-Beri $\rightarrow$ (III)
• Glucocorticoids regulate carbohydrate metabolism; hyposecretion of adrenal cortex hormones causes Addison's disease $\rightarrow$ (IV)
• Estradiol is the primary female sex hormone that controls the development of secondary sexual characteristics and regulates the menstrual cycle $\rightarrow$ (I)

Correct match: (A)-(II), (B)-(III), (C)-(IV), (D)-(I).

Key chemical properties of D-glucose:

• Conc. $\text{HNO}_3$ oxidizes glucose to saccharic acid (a dicarboxylic acid).
• Acetic anhydride reacts with the five hydroxyl groups of glucose to form glucose pentaacetate.
• Hydroxylamine ($\text{NH}_2\text{OH}$) reacts with the carbonyl group to form glucose oxime.
• Despite having an aldehyde group, glucose does not react with sodium bisulphite ($\text{NaHSO}_3$) to form a bisulphite addition product, nor does it give 2,4-DNP test or Schiff's test. This provided evidence that the aldehyde group is not free, but involved in forming a cyclic hemiacetal ring structure.

Thyroxine is an iodinated amino acid derivative produced by the thyroid gland. It regulates the body's basal metabolic rate (BMR) by stimulating the oxygen consumption and metabolism (burning) of carbohydrates, lipids, and proteins, releasing energy for cellular activities.