B1.2 Proteins

The question asks for the optimum pH range of the bacterial protease — the pH at which enzyme activity is highest.

The highest rate of hydrolysis is observed at pH 5.5, with activity declining significantly below pH 3 and above pH 8. This pattern indicates the enzyme performs best near pH 5.5.

pH 5.5 falls within the range between 5 and 6. At this pH, the R-group ionisation states allow optimal ionic bond formation and active site geometry for catalysis.

The optimum pH range is between 5 and 6, as this bracket contains pH 5.5 where maximum activity was recorded.

We need to identify which pH range best describes the optimum for this protease, given that peak activity occurs at pH 5.5.

Activity declines sharply at pH 3, meaning the region between 2 and 3 is far from the optimum. This option is eliminated.

Activity also declines sharply at pH 8, so the range 7–8 is on the descending portion of the activity curve, not the peak. This option is eliminated.

pH values above 8 are even further from the observed peak at 5.5, and highly alkaline conditions would severely alter R-group ionisation states. This option is eliminated.

Between 5 and 6 is the only range that contains the observed peak activity at pH 5.5. This is the correct answer.

The question asks about the molecular mechanism by which extreme pH disrupts enzyme (protein) function — specifically how pH 1.0 causes loss of activity.

At pH 1.0, excess $H^{+}$ ions protonate R-groups: acidic R-groups (–COO⁻) gain protons and lose their negative charge, while basic R-groups (–NH₂) become –NH₃⁺. These charge changes alter the ionic interactions between R-groups.

Ionic bonds (salt bridges) between oppositely charged R-groups are disrupted when those charges are removed or altered. Hydrogen bonding patterns also change. The tertiary structure — and therefore the active site geometry — is lost.

The correct answer is that pH alters R-group protonation states, disrupting ionic bonds and hydrogen bonds that stabilise tertiary structure, causing denaturation and loss of function.

We need to find the option that correctly describes how pH (not temperature) disrupts protein structure at the molecular level.

Peptide bonds remain intact during denaturation — pH-induced denaturation disrupts non-covalent interactions and the 3D structure, not the primary sequence. This option is incorrect.

Increased kinetic energy is the mechanism for temperature-induced denaturation, not pH. Furthermore, disulfide bonds are covalent and the most resistant to disruption — they are not typically broken by pH changes. This option confuses two separate mechanisms.

While hydrophobic interactions can be disrupted during denaturation, this is primarily a consequence of unfolding, not the primary molecular mechanism triggered by pH change. The causal explanation is incorrect here.

Changes in pH alter the protonation state of R-groups, disrupting ionic bonds and hydrogen bonds is the correct mechanistic explanation for pH-induced denaturation.