230 Biochemical Society Transactions (2011) Volume 39, part 1 Nitrate and (per)chlorate reduction pathways in (per)chlorate-reducing bacteria Margreet J. Oosterkamp* 1 , Farrakh Mehboob† 1 , Gosse Schraa*, Caroline M. Plugge* and Alfons J.M. Stams* 2 *Laboratory of Microbiology, Wageningen University, Dreijenplein 10, 6703 HB Wageningen, The Netherlands, and †Ecotoxicology Research Programme, National Institute of Bioremediation, National Agriculture Research Centre, Park Road, Islamabad, Pakistan Abstract The reduction of (per)chlorate and nitrate in (per)chlorate-reducing bacteria shows similarities and differences. (Per)chlorate reductase and nitrate reductase both belong to the type II DMSO family of enzymes and have a common bis(molybdopterin guanine dinucleotide)molybdenum cofactor. There are two types of dissimilatory nitrate reductases. With respect to their localization, (per)chlorate reductase is more similar to the dissimilatory periplasmic nitrate reductase. However, the periplasmic, unlike the membrane-bound, respiratory nitrate reductase, is not able to use chlorate. Structurally, (per)chlorate reductase is more similar to respiratory nitrate reductase, since these reductases have analogous subunits encoded by analogous genes. Both periplasmic (per)chlorate reductase and membrane-bound nitrate reductase activities are induced under anoxic conditions in the presence of (per)chlorate and nitrate respectively. During microbial (per)chlorate reduction, molecular oxygen is generated. This is not the case for nitrate reduction, although an atypical reaction in nitrite reduction linked to oxygen formation has been described recently. Microbial oxygen production during reduction of oxyanions may enhance biodegradation of pollutants under anoxic conditions. Introduction Perchlorate and chlorate, termed (per)chlorate, are com- pounds that are used as solid rocket fuel and in road flares, fireworks, matches, blasting agents, explosives and lubricating oils [1−3]. Besides their anthropogenic origin, (per)chlorate may also occur in the environment by natural production [4−6]. Natural deposits of large amounts of (per)chlorate can be found in the hyper-arid region of the Atacama desert of Chile [7]. Owing to their high solubility, (per)chlorate salts are readily transported and can be detected in surface water and groundwater [3,8]. Bacteria are able to use (per)chlorate as a terminal electron acceptor for growth [9,10]. Characteristically, molecular oxygen is produced upon (per)chlorate reduction [9,11]. The microbial reduction of perchlorate proceeds as follows: ClO 4 − → ClO 3 − → ClO 2 − → Cl − + O 2 Perchlorate (ClO 4 − ) is reduced by perchlorate reductase to chlorate (ClO 3 − ), which in turn is reduced to chlorite (ClO 2 − ) by chlorate reductase. In (per)chlorate-reducing bacteria, one enzyme may reduce both perchlorate and chlorate [12]. Chlorite is then split into Cl − and O 2 by a chlorite dismutase [9,10]. Recently, oxygen formation was also found in a culture that degraded methane Key words: chlorite dismutase, molybdenum-containing enzyme, nitrate reductase, nitrate reduction, (per)chlorate reductase, (per)chlorate reduction. Abbreviations used: MGD, molybdopterin guanine dinucleotide; Moco, molybdenum cofactor; Tat, twin-arginine translocation. 1 These authors contributed equally to this work. 2 To whom correspondence should be addressed (email fons.stams@wur.nl). anaerobically with nitrite. The proposed pathway involves a yet hypothetical ‘NO dismutase’, which, like chlorite dismutase, releases the molecular oxygen [13]. Oxygen produced in the (per)chlorate pathway is proposed to be utilized for biodegradation of anaerobically recalcitrant compounds, such as aromatic hydrocarbons [11,14,15]. In this respect, it is highly important to study the mechanisms involved in anaerobic (per)chlorate and nitrate reduction. Nitrate can also be used for bioremediation in anaerobic environments. Dechloromonas aromatica is a (per)chlorate-reducing bacterium that has been described to degrade benzene with nitrate [16]. The previously described Alicycliphilus denitrificans also degrades benzene with chlorate, but not with nitrate [15]. In the present review, we compare (per)chlorate and nitrate reduction in (per)chlorate-reducing micro-organisms. (Per)chlorate reductase (Per)chlorate reductases belong to the type II DMSO family of enzymes [17,18]. Type II DMSO reductases have a common Moco (molybdenum cofactor) known as bis(MGD)Mo [bis(molybdopterin guanine dinucleotide)- molybdenum] [19]. (Per)chlorate reductases are similar to other type II DMSO reductases, such as nitrate and selenate reductase and ethylbenzene dehydrogenase. (Per)chlorate reductases can also reduce nitrate. The α-subunit containing the molybdopterin is the catalytic subunit of (per)chlorate reductase. The β -subunit contains the Fe−S cluster and may be involved in electron C  The Authors Journal compilation C  2011 Biochemical Society Biochem. Soc. Trans. (2011) 39, 230–235; doi:10.1042/BST0390230