Usually enzymatic reactions require well buffered environment for optimal enzyme work, so this will not allow for efficient monitoring of pH change. Besides I would not expect that during your reaction massive amount of ammonia will be released which can cause pH change. And finally, usually product of the reaction inhibits the enzyme, so it is much better to remove it from reaction mixture to avoid this effect.

If you want to monitor the enzymatic reaction easier than by HPLC I could suggest monitoring:

(1) p-coumaric acid by fluorescence ex. 330nm; em. 430-440nm

(2) ammonia by spectrophotometry after Nessler colour reaction

Both targets should be specific enough for your purpose.

I am not sure if you could do it through the equation Handerson Hasselbach, (you don't have to use for this purpose) what I don't know if through an optimum you can see when it is denatured. The optimal and then the lack of function.

Thanks for the answers.

I will use another method rather than calculations from pH measurements.