Rubisco Synthesis, Assembly, Mechanism, and Regulation.

lon - H cls -enedlol o=o ribulose 1,5-P, / OPO', + 2P glycolate OPO', 3P Dglycerate OPO', 3-keto arabinitol 1 ,5-P2 Figure 4. The Reactions Catalyzed by Rubisco. The first intermediate of catalysis is the C2,C3 cis-enediol form of ribulose-bisphosphate (ribulose 1,5-P2) after abstraction of the C3 proton. The enediol can partition a number of ways, the majority into the products of carboxylation (upper reactions) or oxygenation (lwer reactions). However, a number of misprotonated isomers of ribulose-bisphosphate, for example, xylulose-bisphosphate, have been detected with the wild-type enzyme that are produced in quantity by mutations of specific amino acids involved in proton transfer. Phosphate elimination of the carbanion forms of intermediates are also produced by some mutants (see Morrell et al., 1994). R, -CHOH-CH20P03= ; 3P D-glycerate, 3-phosphoglycerate; 2P glyco- late, 2-phosphoglycolate. to bind. Coordination of the metal does not complete the ac- tive site but simply positions the carbamate and Mg2+ relative to those centers of the bisphosphate substrate, namely, the C2 carbonyl oxygen and C3 hydroxyl, that are involved in the catalytic events. With substrate bound in the correct orienta- tion (Lorimer et al., 1989), the C2 and C3 oxygen atoms complete the coordination sphere (Gutteridge and Lundqvist, 1994). The catalytic mechanism of the carboxylation of ribulose- bisphosphate can be depicted as five discrete partia1 reactions (Andrews and Lorimer, 1987). The first step has been estab- lished as the formation of an enediol intermediate of the bisphosphate substrate that is catalyzed by the enzyme in the absence of the second substrates,