| Summary: | Arginine kinase (AK) is a reversible enzyme that regulates invertebrates’ phosphagen arginine phosphate levels. AK also elicits an immune response in humans, and it is a major food allergen in crustacea and may be a target for novel antiparasitic drugs. Although AK has been primarily described in the shrimp, it is also present in other invertebrates, such as the brown tick <i>Rhipicephalus sanguineus</i> (<i>Rs</i>), the vector for Rocky Mountain Spotted Fever. Here we report the enzymatic activity and the crystal structure of AK from <i>Rhipicephalus sanguineus</i> (<i>Rs</i>AK) in an open conformation without substrate or ligands and a theoretical structure of <i>Rs</i>AK modeled bound with the substrate/product (Arg-ADP) in a closed conformation. The Michaelis-Menten kinetics confirmed that <i>Rs</i>AK is an efficient biocatalyst due to its high <i>k<sub>cat</sub></i>/<i>K<sub>m</sub></i> parameter. The recombinant enzyme was expressed in bacteria and purified to a 20 mg/L culture yield. AK is an essential enzyme in invertebrates. Future work will be focused on the <i>Rs</i>AK enzymatic inhibition that may lead to novel strategies to control this pest, a burden to animal and human health.
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