Dear Sebastien,
Nobody knows for sure, but there are a couple of ideas:
1) It is difficult to evolve something that will distinguish between
CO2 and O2 which "look" rather similar in molecular terms.
2) It is thought that rubisco was developed when O2 levels in the
atmosphere were much lower, and the competition between O2 and CO2 at
the active site of the enzyme would be won by the latter. On this idea
the oxygenase activity of rubisco is an unecessary bit of evolutionary
baggage.
But these are just ideas!
Best Wishes,
Martin Hodson

(Sebastian) wrote in message . com...
Hi,
why is rubisco not optimized on its carboxylase activity? Or: why does
rubisco still have got oxygenase activity, which doesn't provide any
advantage (or does it?)?
Complicated ways like C4 or CAM had to be invented to compensate
rubiscos disadvantage (i.e. oxygenase activity), but the next way,
modification, was never succesfully performed. Why that?
Thank you
SMS