Purine nucleoside phosphorylase from Mycobacterium tuberculosis. Analysis of inhibition by a transition-state analogue and dissection by parts

Purine salvage pathways are predicted to be present from the genome sequence of Mycobacterium tuberculosis. The M. tuberculosis deoD gene encodes a presumptive purine nucleoside phosphorylase (PNP). The gene was cloned, expressed, purified, and found to exhibit PNP activity. Purified M. tuberculosis PNP is trimeric, similar to mammalian PNP's but unlike the hexameric Escherichia coli enzyme. Immucillin-H is a rationally designed analogue of the transition state that has been shown to be a potent inhibitor of mammalian PNP's. This inhibitor also exhibits slow-onset inhibition of M. tuberculosis PNP with a rapid, reversible inhibitor binding (K_i of 2.2 nM) followed by an overall dissociation constant (K_i*) of 28 pM, yielding a K_m/K_i* value of 10⁶. Time-dependent tight binding of the inhibitor occurs with a rate of 0.1 s^-1, while relaxation of the complex is slower at 1.4 × 10^-3 s^-1. The pH dependence of the K_i value of immucillin-H to the M. tuberculosis PNP suggests that the inhibitor binds as the neutral, unprotonated form that is subsequently protonated to generate the tight-binding species. The M. tuberculosis enzyme demonstrates independent and equivalent binding of immucilin-H at each of the three catalytic sites, unlike mammalian PNP. Analysis of the components of immucillin-H confirms that the inhibition gains most of its binding energy from the 9-deazahypoxanthine group (K_is of 0.39 μM) while the 1,4-dideoxy-1,4-iminoribitol binds weakly (K_is of 2.9 mM). Double-inhibition studies demonstrate antagonistic binding of 9-deazahypoxanthine and iminoribitol (β = 13). However, the covalent attachment of these two components in immucillin-H increases equilibrium binding affinity by a factor of > 14 000 (28 pM vs 0.39 μM) compared to 9-deazahypoxanthine alone, and by a factor of > 10⁸ compared to iminoribitol alone (28 pM vs 2.9 mM), from initial velocity measurements. The structural basis for M. tuberculosis PNP inhibition by immucillin-H and by its component parts is reported in the following paper [Shi, W., Basso, L. A., Santos, D. S., Tyler, P. C., Furneaux, R. H., Blanchard, J. S., Almo, S. C., and Schramm, V. L. (2001) Biochemistry 40, 8204-8215].