Practice Questions

The covalent bond linking the nitrogenous base (purine or pyrimidine) to the 1' carbon of the pentose sugar (ribose or deoxyribose) is an N-glycosidic bond. The phosphate group is linked to the 5' carbon.

Pepsin is a gastric enzyme that has adapted to function in the highly acidic environment of the stomach, where HCl is present. Therefore, its optimum pH is strongly acidic (around 1.5-2.0), unlike enzymes like trypsin which function in the alkaline small intestine (pH ~8.0).

The lock-and-key model proposes a rigid active site that is perfectly complementary only to a specific substrate, ensuring high specificity. The induced fit model expands on this, adding flexibility, but the lock-and-key concept directly explains absolute specificity.

Sucrose is dextrorotatory, but upon hydrolysis, the resulting mixture of glucose (dextrorotatory) and fructose (strongly levorotatory) makes the overall solution levorotatory. This change in optical rotation is called inversion, and the product is called invert sugar.

Terpenoids (or terpenes), including steroids, carotenoids, and natural rubber, are a large class of lipids built from multiple isoprene units (C5H8). This distinguishes their biosynthetic origin from acylglycerols, which are fatty acid esters.

The specific base pairing (A-T with 2 H-bonds, G-C with 3 H-bonds) between a purine and a pyrimidine ensures the two DNA strands are equidistant apart, creating a uniform diameter. This complementarity is also the molecular logic for semi-conservative replication.

A non-competitive inhibitor binds to a site different from the active site (an allosteric site). This binding alters the three-dimensional shape of the enzyme, including the active site, so the substrate can no longer bind effectively, regardless of substrate concentration.

Changing one amino acid (primary structure) can disrupt the local folding (secondary), which in turn alters the overall 3D shape (tertiary) and its ability to bind with other subunits (quaternary). Thus, all higher levels of structure are ultimately dependent on the primary sequence.

Tertiary structure is the overall 3D conformation of a single polypeptide chain, driven by interactions between the R-groups. This includes hydrophobic interactions, ionic bonds, hydrogen bonds, and disulfide bridges. The backbone H-bonding defines secondary structure.

Both α-helices and β-pleated sheets are secondary structures stabilized by regular hydrogen bonding between the backbone atoms (the C=O of one amino acid and the N-H of another). R-group interactions define the tertiary structure. Disulfide bridges are covalent, not hydrogen, bonds.