Crystallization and preliminary crystallographic studies of Helicobacter pylori arginase

Illuminating the structure-function landscape of an evolutionary nonconserved motif in the arginases of Helicobacter gastric pathogens

The bimetallic enzyme arginase catalyses the conversion of L-arginine to L-ornithine and urea. In Helicobacter pylori (a known human gastric pathogen), this enzyme is an important virulence factor. In spite of the conservation of the catalytic and the metal-binding residues, the H. pylori homolog possesses a 13-residue motif (-153 ESEEKAWQKLCSL165 -) present in the middle of the protein sequence, whose role was recently elucidated. Despite several reviews available on arginases, no report has thoroughly illustrated the underlying basis for the importance of the above motif of the H. pylori enzyme in structure and function. In this review, we systematically describe a mechanistic basis for its importance in structure and function based on the known data. This motif of the H. pylori enzyme is present exclusively in the arginases of other Helicobacter gastric pathogens, where the critical residues are conserved, implying that the nonconserved stretch has been selected during the evolution of the enzyme in these gastric pathogens in a specific manner to perform its role in the structure and function. The combined information can be useful for understanding the function of arginases in other Helicobacter gastric pathogens. Additionally, this knowledge can be utilised to screen and design new small molecule inhibitors, specific to the arginases of these pathogens.

Structural, enzymatic and biochemical studies on Helicobacter pylori arginase

Arginase is an enzyme involved in the last step of the urea cycle, where it catalyses the hydrolysis of l-arginine to generate l-ornithine and urea. Compared to the well-characterised arginases from animals, yeast and other bacteria, Helicobacter pylori arginase, or RocF, is unique in at least three aspects. Firstly, it has been identified as an important factor in evasion of the host's immune system and thus contributes to persistent infection by the bacterium. Secondly, the optimal catalytic conditions of RocF are different from those of other arginases. Finally, sequence alignment indicates that RocF possesses considerable differences at its N- and C-terminal from other arginases and harbours an insertion of 13 residues in the middle of the sequence. To better understand these unique biochemical and enzymatic properties, we therefore have embarked on determining the structure of RocF. In this study, the crystal structure of RocF was solved with the molecular replacement method. Based on the structure and systematic mutagenesis studies, we confirmed that the inserted residues form a helix that was not observed in other arginases and was able to raise the arginase activity by 30% probably by change the conformation of the substrate binding pocket. Six residues were involved in Mn(2+) binding, all of which were essential for arginase activity. The C-terminal motif is not sufficient in establishing the oligomeric state of RocF, and no disulphide bonds were observed in RocF.

Expression, purification and characterization of arginase from Helicobacter pylori in its apo form

Arginase, a manganese-dependent enzyme that widely distributed in almost all creatures, is a urea cycle enzyme that catalyzes the hydrolysis of L-arginine to generate L-ornithine and urea. Compared with the well-studied arginases from animals and yeast, only a few eubacterial arginases have been characterized, such as those from H. pylori and B. anthracis. However, these enzymes used for arginase activity assay were all expressed with LB medium, as low concentration of Mn²⁺ was detectable in the medium, protein obtained were partially Mn²⁺ bonded, which may affect the results of arginase activity assay. In the present study, H. pylori arginase (RocF) was expressed in a Mn²⁺ and Co²⁺ free minimal medium, the resulting protein was purified through affinity and gel filtration chromatography and the apo-form of RocF was confirmed by flame photometry analysis. Gel filtration indicates that the enzyme exists as monomer in solution, which was unique as compared with homologous enzymes. Arginase activity assay revealed that apo-RocF had an acidic pH optimum of 6.4 and exhibited metal preference of Co²⁺>Ni²⁺>Mn²⁺. We also confirmed that heat-activation and reducing regents have significant impact on arginase activity of RocF, and inhibits S-(2-boronoethyl)-L-Cysteine (BEC) and Nω-hydroxy-nor-Arginine (nor-NOHA) inhibit the activity of RocF in a dose-dependent manner.