Define the active site of an enzyme.

Explain how the active site promotes enzyme–substrate specificity.

Outline possible effects of acids on enzyme activity.

region/site where a substrate binds

a. shape of active site matches that of the substrate;
b. chemical properties/charges of active site attract the substrate;
c. active site can change to induce fit of substrate;

a. changes the charge/ionization of amino acids/R-groups;
b. changes 3-D structure (of active site)/tertiary structure / denatures enzyme;
c. substrate no longer binds/fits so decreases activity;
d. could increase activity if optimum pH of enzyme is acidic;

Most students knew the correct definition of active site.

Better prepared candidates got full marks discussing induced fit. Chemical compatibility was discussed more rarely. Students had surprising difficulty describing the relationship between the shape of the substrate and the shape of the active site.

This question differentiated performance well. Many stated that acids denature enzymes. Better prepared candidates mentioned increased activity for enzymes with an optimum pH that was acidic. Better prepared candidates also referenced the altered shape of the active site.