Which is the optimum for this enzyme

A type of phosphatase can be assayed using p-nitrophenyl phosphate but the assay is performed at pH 7.8, which is the optimum for this enzyme. The initial velocity measured at the highest substrate concentration was .45 deltaA_400 / minute. Convert this velocity to a change in concentration/minute (microM/minute). HINT Use the pKa of p-nitrophenyl (7.0) and the henderson hasselbalch equation to correct the concentration, since at pH 7.8 not all of the product is in the colored, visible form. Use 18,380 M^-1cm^-1 for the extinction coefficient.

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