Bifunctional Catalysis by CDP-ribitol Synthase: Convergent Recruitment of Reductase and Cytidylyltransferase Activities in Haemophilus influenzae and Staphylococcus aureus

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Abstract
CDP-ribitol synthase catalyzes the formation of CDP-ribitol from ribulose 5-phosphate, NADPH, and CTP. CDP-ribitol is an activated precursor for the synthesis of virulence-associated polysaccharides in the capsule of the Gram-negative pathogen Haemophilus influenzae and in the cell walls of Gram-positive pathogens including Staphylococcus aureus. We showed previously that CDP-ribitol synthase activity in H. influenzae is catalyzed by the bifunctional enzyme Bcs1 in a two-step reaction with reduction preceding cytidylyl transfer [Zolli, M., et al. (2001) Biochemistry 40, 5041−5048]. In the work reported here, we predicted a CDP-ribitol synthesis locus in S. aureus tandemly arranged as tarI, encoding an orthologue of the cytidylyltransferase domain of Bcs1, and tarJ, coding for an analogue of the reductase domain of Bcs1. We have shown the formation of a functional CDP-ribitol synthase complex between TarI and TarJ. Steady-state mechanistic studies of the CDP-ribitol synthases TarIJ and Bcs1 revealed that the analogous reductases and orthologous cytidylyltransferases undergo ordered mechanisms. The sequence of substrate binding and product release of the orthologous cytidylyltransferases differed. Steady-state analysis of the reductase and cytidylyltransferase activities of TarIJ indicated a 100-fold difference in the turnover where the primary reductase was rate limiting. Rapid mixing experiments revealed the presence of ∼12 μM ribitol 5-phosphate at steady state, 100-fold lower than the observed Km for this intermediate. Analysis of the approach to steady state suggested that channeling was not occurring in the coupled enzyme complex and was an unlikely driving force in the convergent recruitment of reductase and cytidylyltransferase activities in the two CDP-ribitol synthases.