The role of arginine deiminase pathway in the pathophysiology of Borrelia burgdorferi

Borrelia burgdorferi, the causative agent of Lyme disease is a spirochetal pathogen with limited metabolic capabilities. It does not have members of tricarboxylic acid cycle, amino acid and fatty acid biosynthetic pathways but has a few members of the arginine deiminase pathway suggesting that these homologs could contribute to synthesis of components not readily acquired from the host environment. The arginine deiminase pathway (ADI) consists of three enzymes that facilitate the catabolic reaction of converting arginine to ornithine, ammonia and CO2.Arginine deiminase (ADI, E.C. 3.5.3.6) is one of the well characterized enzymes of the arginine degradation enzymes. Sequence analysis of borrelial genome revealed presence of homologs of arcA, arcB and arcD, while,arcC was conspicuously absent. Arginine deiminase (arcA) is the first enzyme encountered in the arginine deiminase (ADI) pathway. This enzyme catalyzes the reversible catabolism of arginine to citrulline in the reaction: L-arginine + H2O > L-citrulline + NH3. The citrulline is then converted to ornithine and carbamoyl phosphate by arcB which codes for a carbamoyl transferase. Finally, arcD is an antiporter that functions by pumping out ornithine while acquiring arginine from the external environment. In order to evaluate the significance of this pathway in the patho-physiology of B. burgdorferi we over expressed and partially purified the members of the ADI pathway. Mono-specific antibodies generated against ArcA were able to detect an approximately 46 kDa protein in B. burgdorferi suggesting there is synthesis of this member of the ADI pathway. Several constructs that would facilitate either the deletion or constitutive expression of the members of ADI pathway have been generated that will facilitate the analysis of this pathway in the survival and infectivity of B. burgdorferi in the tick vector or the mammalian host. The genetic and biochemical analysis of the ADI pathway will facilitate a greater understanding of the patho-physiology of B. burgdorferi and help identify potential targets for control of Lyme disease