Ribose 5-phosphate isomerase type B from Trypanosoma cruzi: Kinetic properties and site-directed mutagenesis reveal information about the reaction mechanism

Trypanosoma cruzi, the human parasite that causes Chagas disease, contains a functional pentose phosphate pathway, probably essential for protection against oxidative stress and also for R5P (ribose 5-phosphate) production for nucleotide synthesis. The haploid genome of the CL Brener clone of the parasite contains one gene coding for a Type B Rpi (ribose 5-phosphate isomerase), but genes encoding Type A Rpis, most frequent in eukaryotes, seem to be absent. The RpiB enzyme was expressed in Escherichia coli as a poly-His tagged active dimeric protein, which catalyses the reversible isomerization of R5P to Ru5P (ribulose 5-phosphate) with K_m, values of 4 mM (R5P) and 1.4 mM (Ru5P). 4-Phospho-D-erythronohydroxamic acid, an analogue to the reaction intermediate when the Rpi acts via a mechanism involving the formation of a 1,2-cis-enediol, inhibited the enzyme competitively, with an IC₅₀ value of 0.7 mM and a K_i of 1.2 mM. Site-directed mutagenesis allowed the demonstration of a role for His¹⁰², but not for His ¹³⁸, in the opening of the ribose furanosic ring. A major role in catalysis was confirmed for Cys⁶⁹, since the C69A mutant was inactive in both forward and reverse directions of the reaction. The present paper contributes to the knowledge of the mechanism of the Rpi reaction; in addition, the absence of RpiBs in the genomes of higher animals makes this enzyme a possible target for chemotherapy of Chagas disease.