|MadSci Network: Biochemistry|
Firstly and foremostly, all chemical reactions are affected by temperature, according to the laws of thermodynamics. One only should bear in mind that high temperatures cause thermal denaturation of the enzyme and that freezing of the solution damages enzymes as well. Henceforth thremal diapazone of enzyme-catalyzed reactions lies in between -4 to 100 C (for aqueous solutions) and could be extended somewhat in both directions by application of various cryo- and pyro- protectors.
pH effects are more complex, in case of reactions involving ions, effects are related to the ionic concetnrations , but one should not forget that enzymes, being very complex macromolecular entities can function only within very specific pH range. pH of the media is responcible for the charged state of the charge-bearing amino acids in the polypeptide chain, which can be directly or indirectly linked to specific activity, at extreme pH's enzyme can lose activity irreversibly due to denaturation as well. Hydration effects are affected by pH as well, and one should not forget that substrates of the enzymatic reactions are oftenly reversibly or irreversibly affected by pH of the medium.
The question is very broad, for further insights I suggest referring to any standard enzymology textbook. Some General Chemistry textbooks also contain brief explainations of the pH effects in enzymatic catalysis.
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