INACTIVATION OF RIBULOSE-1,5-BISPHOSPHATE CARBOXYLASE OXYGENASE DURING CATALYSIS - A THEORETICAL-STUDY OF RELATED TRANSITION STRUCTURES/

Possible mechanistic paths for self-inhibition of rubisco have been th eoretically characterized by using analytical gradients with both AM1 semiempirical and HF/3-21G level calculations. Starting from the frame work of an enediol moiety previously obtained from characterizations o f the saddle point of index 1 (SPi-1) for the carboxylation and oxygen ation reactions, the formation of xylulose, 3-ketoribitol, and 3-ketoa rabinitol inhibitors is made possible by following specific intramolec ular hydrogen rearrangements. The xylulose is attained from the SPi-1 describing intramolecular enolization via an intermediate made by prot onation of the hydroxyl group Linked to the third carbon (C3) of the m odel substrate 3,4-dihydroxy-2-pentanone. One of these two hydrogens c an migrate toward C3 with the correct stereochemistry to form xylulose inhibitor. 3-Ketoribitol and 3-ketoarabinitol inhibitors can be obtai ned after the enediol moiety is formed. Another minimum energy structu re is found which is derived from the enediol via a SPi-1 correspondin g to a retroenolization. This process finishes by forming a protonated hydroxyl group at C2. From this, and following the SPi-1, the 3-ketor ibitol and the 3-ketoarabinitol inhibitors can be formed. The carbon a nd oxygen frameworks of the stationary geometries characterized in vac uo fit well at the active site of rubisco, except perhaps for xylulose inhibitor. This geometric overlap with an experimentally determined t ransition state analog suggests that the active site can accommodate t he interconversion chemistry found with the present approach which lea ds to self-inhibition. This is compatible with the hypothesis that the loss of activity is due to the products of substrate isomerization fo rmed during catalysis.