title: Ribose 5-phosphate isomerase type B from Trypanosoma cruzi: kinetic properties and site-directed mutagenesis reveal information about the reaction mechanism.
abstract: 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 Km 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 Ki of 1.2 mM. Site-directed mutagenesis allowed the demonstration of a role for His102, but not for His138, 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 know-ledge 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.
authors.affiliation: Instituto de Investigaciones Biotecnologicas-Instituto Tecnologico de Chascomus (IIB-INTECH), Universidad Nacional de General San Martin-CONICET, Av General Paz 5445, INTI, edificio 24, 1650 San Martin, Buenos Aires, Argentina.
authors.forename: Ana L
authors.initials: AL
authors.lastname: Stern
authors.forename: Emmanuel
authors.initials: E
authors.lastname: Burgos
authors.forename: Laurent
authors.initials: L
authors.lastname: Salmon
authors.forename: Juan J
authors.initials: JJ
authors.lastname: Cazzulo
mesh_terms: D019747:Aldose-Ketose Isomerases / Q000235:genetics / Q000378:metabolism*; D000595:Amino Acid Sequence; D000818:Animals; D003001:Cloning, Molecular; D017124:Conserved Sequence; D007700:Kinetics; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D015800:Protozoan Proteins / Q000378:metabolism*; D011994:Recombinant Proteins / Q000378:metabolism; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D014349:Trypanosoma cruzi / Q000201:enzymology*
chemical_list: D015800:Protozoan Proteins; D011994:Recombinant Proteins; D019747:Aldose-Ketose Isomerases; C022548:ribosephosphate isomerase