Invited Review Acetate formation in the energy metabolism of parasitic helminths and protists Aloysius G.M. Tielens a, * , Koen W.A. van Grinsven b,1 , Katrin Henze c , Jaap J. van Hellemond a , William Martin c a Department of Medical Microbiology and Infectious Diseases, Erasmus MC University Medical Center, ‘s Gravendijkwal 230, 3015 CE Rotterdam, The Netherlands b Department of Biochemistry and Cell Biology, Faculty of Veterinary Medicine, Utrecht University, Yalelaan 2, 3584 CM Utrecht, The Netherlands c Institute of Botany III, Heinrich Heine University, Düsseldorf, Universitätsstrasse 1, 40225 Düsseldorf, Germany article info Article history: Received 27 October 2009 Received in revised form 8 December 2009 Accepted 9 December 2009 Keywords: Acetate Hydrogenosomes Mitochondrial metabolism Anaerobic metabolism CoA-transferase Acetate:succinate CoA-transferase (ASCT) Acetyl-CoA synthetase (ADP-forming) abstract Formation and excretion of acetate as a metabolic end product of energy metabolism occurs in many pro- tist and helminth parasites, such as the parasitic helminths Fasciola hepatica, Haemonchus contortus and Ascaris suum, and the protist parasites, Giardia lamblia, Entamoeba histolytica, Trichomonas vaginalis as well as Trypanosoma and Leishmania spp. In all of these parasites acetate is a main end product of their energy metabolism, whereas acetate formation does not occur in their mammalian hosts. Acetate produc- tion might therefore harbour novel targets for the development of new anti-parasitic drugs. In parasites, acetate is produced from acetyl-CoA by two different reactions, both involving substrate level phosphor- ylation, that are catalysed by either a cytosolic acetyl-CoA synthetase (ACS) or an organellar acetate:suc- cinate CoA-transferase (ASCT). The ACS reaction is directly coupled to ATP synthesis, whereas the ASCT reaction yields succinyl-CoA for ATP formation via succinyl-CoA synthetase (SCS). Based on recent work on the ASCTs of F. hepatica, T. vaginalis and Trypanosoma brucei we suggest the existence of three subfam- ilies of enzymes within the CoA-transferase family I. Enzymes of these three subfamilies catalyse the ASCT reaction in eukaryotes via the same mechanism, but the subfamilies share little sequence homol- ogy. The CoA-transferases of the three subfamilies are all present inside ATP-producing organelles of par- asites, those of subfamily IA in the mitochondria of trypanosomatids, subfamily IB in the mitochondria of parasitic worms and subfamily IC in hydrogenosome-bearing parasites. Together with the recent charac- terisation among non-parasitic protists of yet a third route of acetate formation involving acetate kinase (ACK) and phosphotransacetylase (PTA) that was previously unknown among eukaryotes, these recent developments provide a good opportunity to have a closer look at eukaryotic acetate formation. Ó 2010 Australian Society for Parasitology Inc. Published by Elsevier Ltd. All rights reserved. 1. Introduction Most parasites have a complex life cycle, which can include free-living stages as well as distinct stages inhabiting one or more host organisms. Oxygen availability is often limited during all or part of the parasite life cycle, therefore they must possess path- ways of ATP synthesis that are independent of O 2 as the terminal electron acceptor. The formation of acetate from acetyl-CoA as a metabolic end product is a metabolic route that is present in many parasites, especially in those that inhabit or encounter hypoxic or anoxic habitats (Table 1)(Köhler, 1985; Tielens, 1994; Sanchez and Müller, 1996; Tielens et al., 2002). Furthermore, acetate pro- duction is also present in many non-parasitic eukaryotes, including flowering plants (Zeiher and Randall, 1990) and marine inverte- brates (De Zwaan, 1991), but it is absent in mammals. Since acetate is an important end-product of energy metabolism among many parasites but not among their mammalian hosts, acetate formation is an attractive target for the development of novel anti-parasitic drugs. Thus far, four different chemical reactions have been identified in which acetate is produced from acetyl-CoA, catalysed by either (i) a CoA-transferase, (ii) a synthetase, (iii) a hydrolase or (iv) a phosphate-acetyltransferase in combination with a kinase reaction (Fig. 1). For all four reactions, corresponding enzymes can be found among the eukaryotes. In parasites, however, only acetyl-CoA syn- thetases (ADP-forming) and acetate:succinate CoA-transferases (ASCTs) have been identified to date. Therefore, this review will only briefly address the characteristics of acetate kinases and acet- yl-CoA hydrolases and will focus on acetyl-CoA synthetases (ADP- forming) and ASCTs present in parasites. The similarities and dif- ferences in acetate formation between parasites will be discussed, as well as the biochemical characteristics, the sub-cellular localisa- tions, and some remarks will be presented on the evolutionary ori- gins of the enzymes responsible for acetate production. 0020-7519/$36.00 Ó 2010 Australian Society for Parasitology Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijpara.2009.12.006 * Corresponding author. Tel.: +31 10 703 2193; fax: +31 10 703 3875. E-mail addresses: a.tielens@erasmusmc.nl, a.g.m.tielens@uu.nl (A.G.M. Tielens). 1 Present address: Department of Molecular Cell Physiology, Faculty of Earth and Life Sciences, Vrije Universiteit Amsterdam, de Boelelaan 1085, 1081 HV Amsterdam, The Netherlands. International Journal for Parasitology 40 (2010) 387–397 Contents lists available at ScienceDirect International Journal for Parasitology journal homepage: www.elsevier.com/locate/ijpara