Substrate isomerization inhibits ribulosebisphospate carboxylase‐oxygenase during catalysis

Abstract

The inhibition of purified spinach ribulosebisphosphate carboxylase‐oxygenase which occurs progressively during catalysis in vitro is caused by accumulation of at least two tight‐binding inhibitors at the catalytic site. Reduction of these inhibitors with NaB 3 H 4 , followed by dephosphorylation, produced a mixture of xylitol and arabinitol, thus identifying one of them as D‐xylulose 1,5‐bisphosphate. It was formed during carboxylation, presumably by a stereochemically incorrect reprotonation of the 2,3‐enediolate intermediate bound at the catalytic site. Under the conditions used, this epimerization occurred approximately once for every 400 carboxylation turnovers. Another inhibitor may be 3‐keto‐D‐arabinitol 1,5‐bisphosphate which would also be formed by misprotonation of the enediolate intermediate, but at C‐2 rather than at C‐3.