Chapter 9 Solutions — Biomolecules

Macromolecules (biomacromolecules) are large molecular weight compounds found in living organisms. They are generally found in the acid-insoluble fraction when living tissue is ground in trichloroacetic acid.

Key features:

1. Molecular weight is typically in the range of ten thousand daltons (10,000 Da) and above.

2. They are polymeric in nature — made up of repeating monomer units (with the exception of lipids).

3. They form the structural and functional backbone of cells.

The four classes of biomacromolecules are:

- Proteins: polymers of amino acids (e.g., haemoglobin, insulin, collagen, enzymes)

- Polysaccharides (complex carbohydrates): polymers of monosaccharides (e.g., starch, glycogen, cellulose, chitin)

- Nucleic acids: polynucleotides (e.g., DNA and RNA)

- Lipids: though not strictly macromolecules (individual molecules are <800 Da), they are grouped here because membrane fragments appear in the acid-insoluble fraction

Proteins can be described at four structural levels — primary, secondary, tertiary, and quaternary.

Tertiary structure refers to the three-dimensional folding of the entire polypeptide chain upon itself.

Key points:

1. After the polypeptide adopts secondary structures (alpha-helices and beta-pleated sheets in specific regions), the whole chain folds further into a compact, globular or fibrous 3D shape.

2. This folding is stabilised by various interactions: hydrogen bonds, disulphide bonds (between cysteine residues), hydrophobic interactions, ionic interactions, and van der Waals forces.

3. The tertiary structure creates specific three-dimensional pockets or crevices, one of which is the 'active site' in enzyme proteins.

4. Tertiary structure is absolutely necessary for the biological activity of a protein. If it is disrupted (denaturation), the protein loses its function.

Example: The tertiary structure of an enzyme like lysozyme determines the exact shape of its active site, enabling it to bind and cleave its specific substrate.