Transition structures for D-ribulose-1,5-bisphosphate carboxylase/oxygenase-catalyzed oxygenation chemistry: Role of carbamylated lysine in a model magnesium coordination sphere

The oxygenation chemistry catalyzed by D-ribulose-1,5-bisphosphate carboxyl ase/oxygenase (Rubisco) is theoretically characterized with transition stru ctures (T-Ss) describing enolization, oxygen fixation, hydration, and conce rted O-O and C2-C3 bond breaking. These T-Ss are obtained at HF/3-21G, HF/6 -31G**, and B3LYP/6-31G** levels of theory. Hydroxypropanone models the sub strate embedded in the Mg coordination shell including a model of the carba mylated lysine. The enolization transition vector describes the intramolecu lar hydrogen transfer from C3 to the carbonyl oxygen (O2). The carbamylated lysine shows a striking catalytic effect by modulating the dihedral angle of the fragment O2-C2-C3-O3. For the isolated hydroxypropanone, the angle i s ca. -5 degrees, decreasing to ca. -60 degrees in the Mg-embedded model. T he torsion diminishes the hydroxypropanone singlet-triplet energy gap and p rompts the interaction with O-3(2). In turn, an intersystem crossing channe l is opened along the reaction path. The lowest energy T-S for oxygen fixat ion is a spin singlet with a hydroxyperoxy structure; the precursor describ es activated oxygen (peroxide) hydrogen-bonded to H-O3. The best electronic description of the hydrogen shuttling in both transition structures is via homolytic bond breaking/forming processes. For hydration, the transition s tructure leads to a gem-diol at C3. The final step is a concerted O-O and C 2-C3 bond rupture, represented by a six-center transition structure. It des cribes the simultaneous hydrogen shuttling from one O-H of the gem-diol (O3 -H) to the hydrogenated oxygen of the peroxy function to form water, and th e bond ruptures. The resultant water molecule is directly located in the fi rst coordination shell of magnesium. The successor complex in this step rep resents the products of the global chemistry. Once they leave, the Mg activ e site would be ready to go for a new catalytic run, which is the hallmark of an enzyme. It is therefore most encouraging to find it as a result of th e present model; thus, one of the gas oxygen atoms is transformed into a so lvent molecule and the other is incorporated in the model glycolate moiety in agreement with experiment. The model correlates well with hydrogen and o xygen isotope labeling experimental results. According to our model, no Mic haelis complex is necessary as O2 binds to form a precursor complex via int ersystem crossing with the successor complex of the enolization step.