A flexible loop as a functional element in the catalytic mechanism of nucleoside hydrolase from Trypanosoma vivax.

The nucleoside hydrolase of Trypanosoma vivax hydrolyzes the N-glycosidic bond of purine nucleosides. Structural and kinetic studies on this enzyme have suggested a catalytic role for a flexible loop in the vicinity of the active sites. Here we present the analysis of the role of this flexible loop via the combination of a proline scan of the loop, loop deletion mutagenesis, steady state and pre-steady state analysis, and x-ray crystallography. Our analysis reveals that this loop has an important role in leaving group activation and product release. The catalytic role involves the entire loop and could only be perturbed by deletion of the entire loop and not by single site mutagenesis. We present evidence that the loop closes over the active site during catalysis, thereby ordering a water channel that is involved in leaving group activation. Once chemistry has taken place, the loop dynamics determine the rate of product release.

New insights into the mechanism of nucleoside hydrolases from the crystal structure of the Escherichia coli YbeK protein bound to the reaction product.

Nucleoside hydrolases (NHs) are enzymes that catalyze the excision of the N-glycosidic bond in nucleosides to allow recycling of the nitrogenous bases. The fine details of the catalytic mechanism and the structural features imposing the substrate specificity of the various members of the NH family are still debated. Here we present the functional characterization of the Escherichia coli YbeK (RihA) protein as a pyrimidine nucleoside-preferring NH and its first crystal structure to 1.8 A resolution. The enzyme active site is occupied by either the alpha or beta anomer of ribose and provides the first structural description of the binding of the NH reaction product. While the amino acid residues involved in ribosyl binding are strictly conserved in pyrimidine-specific NHs, the residues involved in specific interactions with the nitrogenous bases differ considerably. Further comparison of the active site architecture of YbeK with the related NHs establishes structural determinants involved in triggering the conformational transition between the open and closed structures and suggests a mechanism for product release.

Pre-steady-state analysis of the nucleoside hydrolase of Trypanosoma vivax. Evidence for half-of-the-sites reactivity and rate-limiting product release.

The nucleoside hydrolase (NH) of the Trypanosoma vivax parasite catalyzes the hydrolysis of the N-glycosidic bond in ribonucleosides according to the following reaction: beta-purine (or pyrimidine) nucleoside + H(2)O --> purine (pyrimidine) base + ribose. The reaction follows a highly dissociative nucleophilic displacement reaction mechanism with a ribosyl oxocarbenium-like transition state. This paper describes the first pre-steady-state analysis of the conversion of a number of purine nucleosides. The NH exhibits burst kinetics and behaves with half-of-the-sites reactivity. The analysis suggests that the NH of T. vivax follows a complex multistep mechanism in which a common slow step different from the chemical hydrolysis is rate limiting. Stopped-flow fluorescence binding experiments with ribose indicate that a tightly bound enzyme-ribose complex accumulates during the enzymatic hydrolysis of the common purine nucleosides. This is caused by a slow isomerization between a tight and a loose enzyme-ribose complex forming the rate-limiting step on the reaction coordinate.

Cloning, preliminary characterization and crystallization of nucleoside hydrolases from Caenorhabditis elegans and Campylobacter jejuni.

The nucleoside hydrolases (NHs) are a family of nucleoside-modifying enzymes. They play an important role in the purine-salvage pathway of many pathogenic organisms which are unable to synthesize purines de novo. Although well characterized in protozoan parasites, their precise function and mechanism remain unclear in other species. For the first time, NHs from Caenorhabditis elegans and Campylobacter jejuni, which are representatives of mesozoa and bacteria, respectively, have been cloned and purified. Steady-state kinetics indicate a different substrate-specificity profile to previously described hydrolases. Native diffraction data sets were collected from crystals of NH from each organism. The hexagonal crystals (space group P6(2)22 or P6(4)22) of NH from C. elegans diffracted to a resolution of 2.8 A, while the data set from the orthorhombic crystals (space group I222 or I2(1)2(1)2(1)) of NH from C. jejuni could be processed to 1.7 A resolution. The unit-cell parameters were a = b = 102.23, c = 117.27 A in the former case and a = 101.13, b = 100.13, c = 81.37 A in the latter.

Enzyme-substrate interactions in the purine-specific nucleoside hydrolase from Trypanosoma vivax.

Nucleoside hydrolases are key enzymes in the purine salvage pathway of Trypanosomatidae and are considered as targets for drug design. We previously reported the first x-ray structure of an inosine-adenosine-guanosine preferring nucleoside hydrolase (IAG-NH) from Trypanosoma vivax (). Here we report the 2.0-A crystal structure of the slow D10A mutant in complex with the inhibitor 3-deaza-adenosine and the 1.6-A crystal structure of the same enzyme in complex with a genuine substrate inosine. The enzyme-substrate complex shows the substrate bound to the enzyme in a different conformation from 3-deaza-adenosine and provides a snapshot along the reaction coordinate of the enzyme-catalyzed reaction. The chemical groups on the substrate important for binding and catalysis are mapped. The 2'-OH, 3'-OH, and 5'-OH contribute 4.6, 7.5, and 5.4 kcal/mol to k(cat)/K(m), respectively. Specific interactions with the exocyclic groups on the purine ring are not required for catalysis. Site-directed mutagenesis indicates that the purine specificity of the IAG-NHs is imposed by a parallel aromatic stacking interaction involving Trp(83) and Trp(260). The pH profiles of k(cat) and k(cat)/K(m) indicate the existence of one or more proton donors, possibly involved in leaving group activation. However, mutagenesis of the active site residues around the nucleoside base and an alanine scan of a flexible loop near the active site fail to identify this general acid. The parallel aromatic stacking seems to provide the most likely alternative mechanism for leaving group activation.