@turkay1-- Actually, most enzymes are activated at a neutral pH, so neither alkaline nor acidic. Of course, there are exceptions to this such as the enzymes in the stomach that are activated at an acidic pH.
But think of it this way, enzymes are made to be activated at different pH levels for a specific reason. The body doesn't want these enzymes to be activated just anywhere. It needs to be activated in the right place. So the difference in pH levels and its affects on different enzymes is actually what makes the system work.
Of course if there are underlying conditions that affect pH levels in different parts of the body, it will be problematic for enzyme function leading to certain health issues. But for the most part, our body knows what to do and how to do it when it comes to enzyme activity and pH.