Yeah good answer from Robb
why do enzymes remain unchanged...
Because they are bioloigcal catalysts
If they were changed they would be co-factors or substrates, and as such couldnt work in the way they are designed to.
Evidence to prove this - always acive - and so if ou separate out the reactants which are always smaller so you could do this by utra fiters,
the activity persists.
Funnily enought the guy has just died - lst week - obituary - who popularised the surprising idea that enzymes have evolved into the most perfect forms and can't be bettered.