Enzymes and their Regulation

Practice: In a solution with 10 mM of enzyme and 100 mM of substrate, a particular reaction proceeds at a rate of 10 mM/sec. If the concentration of substrate is increased to 300 mM, the reaction continues to proceed at a rate of 10 mM/sec. Which of the following explains this phenomenon?

Practice: Pepsin is an enzyme in the stomach that digests protein. It works best at a pH of 2. When pepsin gets into the small intestine, where the pH is about 5-6, pepsin stops working. Which of the following explains the decrease in the activity of pepsin in the duodenum?