We ran our reactions at four pH levels: 4.0 (acidic), 5.5 (slightly acidic), 7.0 (neutral), and 8.5 (basic). The cytoplasm of most cells has a pH of about 7.0, so we might expect our enzyme to work best there. However, while our enzyme did work faster at a pH of 7.0 than at pH 5.5, based on the best fit line, a slightly acidic pH is probably optimal for our turnip peroxidase. It may be that turnip cells are normally slightly acidic. It is also possible that the enzyme does work better at an acidic pH even if that's not the typical turnip cell pH -- the structure of the enzyme may prevent from it being simultaneously optimized for all of the many cellular environmental factors which can vary. Finally, since we didn't check the pH of our final reaction mixtures, it may be that their final pH values didn't exactly match those of the buffers we started with.
We see the expected results for our other pH values: as pH diverges from the optimum, the reaction rate drops. Our enzyme seems particularly sensitive to basic pHs, as it stopped working at a pH of 8.5. The low pH probably denatured the enzyme.