Mycobacterium tuberculosis Ribose-5-phosphate Isomerase has a Known Fold, but a Novel Active Site

Competitive Inhibitors of Mycobacterium tuberculosis Ribose-5-phosphate Isomerase B Reveal New Information about the Reaction Mechanism

“…6). This serine is highly conserved, or replaced with the similar threonine, in all RpiBs so far identified (3,7), underlining its functional importance. The main chain nitrogen atoms in the 70 -74 region form an anion hole that is expected to assist catalysis by stabilizing the transient negative charges during the reaction.…”

“…Although a mechanism can be envisioned in which both isomerization and ring opening of cyclic R5P occur in a concerted manner, leading directly to the enediolate intermediate species, we believe this is very unlikely because of the much lower acidity of the C2 hydrogen in cyclic compared with linear R5P. Kinetic studies of the unrelated spinach RpiA (17) suggested that this enzyme binds the furanose form and assists in its opening, a conclusion that was supported by structural studies combined with computer docking (7). Because the ␣-and ␤-furanose forms are more plentiful in solution than the linear aldehyde (18), ring opening by the enzymes would have clear advantages for efficient processing of substrate, providing that this ring opening step is not ratedetermining.…”

“…Data collection statistics are summarized in Table I. Both crystals had C2 symmetry with unit cell dimensions nearly identical to those of the original phosphate complex crystals (7).…”

“…The two E. coli enzymes have been shown to have very different structures and active site residues, indicating that they represent an example of independent evolution (3,6). We have more recently solved the structure of RpiB from Mycobacterium tuberculosis (MtRpiB) and demonstrated that it can catalyze the isomerization of R5P and Ru5P with an efficiency very similar to that of the E. coli RpiB, despite differences in the active site residues (7). As for most isomerases, the reaction catalyzed by Rpis is believed to proceed through an enediolate intermediate (Fig.…”

“…1). By comparisons of the two RpiB structures and by docking experiments, roles for the various residues in this class of enzyme were proposed (7). However, the catalytic mechanism could not actually be established from this information alone.…”