|MadSci Network: Biochemistry|
The answer to your question is relatively simple although not an exact answer. Assuming that all in Nature is designed to perform optimally, the most obvious explanation is that in order for the enzyme catalase to function optimally in citrus fruits it has a pH optimum that fits the normal condition of citrus fruits i.e. acid. As most of the acid nature is due to citric acid it makes sense that catalase from citrus fruit functions best in this natural environment. So the only answer is that it would not make sense if it would be otherwise. The differences inpH optima between different catalases can be explained that there are small differences in amino acid sequence and protein structure that allow the enzyme to operate at other pH then catalases from other sources that have optima that fit THEIR environment.
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