Arsenic, a toxic metalloid is naturally found in the environment but can exist as a harmful pollutant generated from industrial waste waters and gold mines. This poses a great threat to human health by contaminating the ground water system [1]. Arsenic can exist in both organic and inorganic forms and in four oxidation states, arsines and methyl arsines (As3-), elemental arsenic (As0), arsenite (AsO33-) and arsenate (AsO43-). Although arsenic is toxic and hazardous to human health, some prokaryotes have developed unique mechanisms that utilise inorganic forms of arsenic, such as arsenite (AsO33-) and arsenate (AsO43-) for respiration [2].
Such prokaryotes include Rhizobium NT-26 and Chrysiogenes arsenatis which uses arsenite oxidase enzyme (Aio) and arsenate reductase enzyme (Arr) respectively, for their crucial respiratory activities. In these bacteria, the periplasmic binding proteins (PBPs) AioX and ArrX bind to arsenite (AsO33-) and arsenate (AsO43-) respectively and trigger, through sensor histidine kinase signalling, the expression of their respective respiratory enzymes [3]. The structure of the AioX protein has been determined in the presence and absence of arsenite (AsO33-) [4]. However, the structural basis of arsenate (AsO43-) binding to the ArrX protein is unknown. Recent studies by ITC have shown that the ArrX protein bind to arsenate (AsO43-) with a Kd value of 1.67 μM. This presentation will discuss the structural comparison between the ArrX protein and the AioX protein by providing an insight, to how both the proteins discriminate between other chemically similar substrates and bind to its respective substrates (arsenate (AsO43-) and arsenite (AsO33-) respectively).