Decaprenylphosphoryl Arabinofuranose, the Donor of the d -Arabinofuranosyl Residues of Mycobacterial Arabinan, Is Formed via a Two-Step Epimerization of Decaprenylphosphoryl Ribose

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Abstract
The major cell wall polysaccharide of mycobacteria is a branched-chain arabinogalactan in which arabinan chains are attached to the 5 carbon of some of the 6-linked galactofuranose residues; these arabinan chains are composed exclusively of d-arabinofuranose (Araf) residues. The immediate precursor of the polymerized Araf is decaprenylphosphoryl-d-Araf, which is derived from 5-phosphoribose 1-diphosphate (pRpp) in an undefined manner. On the basis of time course, feedback, and chemical reduction experiment results we propose that decaprenylphosphoryl-Araf is synthesized by the following sequence of events. (i) pRpp is transferred to a decaprenyl-phosphate molecule to form decaprenylphosphoryl-β-d-5-phosphoribose. (ii) Decaprenylphosphoryl-β-d-5-phosphoribose is dephosphorylated to form decaprenylphosphoryl-β-d-ribose. (iii) The hydroxyl group at the 2 position of the ribose is oxidized and is likely to form decaprenylphosphoryl-2-keto-β-d-erythro-pentofuranose. (iv) Decaprenylphosphoryl-2-keto-β-d-erythro-pentofuranose is reduced to form decaprenylphosphoryl-β-d-Araf. Thus, the epimerization of the ribosyl to an arabinosyl residue occurs at the lipid-linked level; this is the first report of an epimerase that utilizes a lipid-linked sugar as a substrate. On the basis of similarity to proteins implicated in the arabinosylation of the Azorhizobium caulidans nodulation factor, two genes were cloned from the Mycobacterium tuberculosis genome and expressed in a heterologous host, and the protein was purified. Together, these proteins (Rv3790 and Rv3791) are able to catalyze the epimerization, although neither protein individually is sufficient to support the activity.

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