Q1. Assume that an enzyme requires an active site Cys thiol group to be in the unprotonated form (thiolate anion, R-S-) to act as a nucleophile in its chemical mechanism. If the pKa of that specific thiol is 8.3, what percent of all the enzyme molecules will have their Cys thiol in the active form at pH 8.6?

Q2. Survival depends on the capability of an organism to respond to changes in its environment. Some plants flower in response to changes in day length. Some mammals might run or fight when frightened. For both of these examples, illustrate the physiological mechanisms involved in the answer.