REVIEW ARTICLE Jose R.PeÂrez-CastinÄeira á Rosario GoÂmez-GarcõÂa Rosa L. LoÂpez-MarqueÂs á Manuel Losada Aurelio Serrano Enzymatic systems of inorganic pyrophosphate bioenergetics in photosynthetic and heterotrophic protists: remnants or metabolic cornerstones? Received: 30 April 2001 /Accepted: 15 June 2001 / Published online: 1 November 2001 Ó Springer-Verlag and SEM 2001 Abstract An increasing body of biochemical and genetic evidence suggests that inorganic pyrophosphate PPi) plays an important role in protist bioenergetics. In these organisms, two types of inorganic pyrophosphatases [EC 3.6.1.1, namely soluble PPases sPPases) and pro- ton-translocating PPases H + -PPases)] that hydrolyse the PPi generated by cell anabolism, thereby replenish- ing the orthophosphate pool needed for phosphoryla- tion reactions, are present in dierent cellular compartments. Photosynthetic and heterotrophic pro- tists possess sPPases located in cellular organelles plastids and mitochondria), where many anabolic and biosynthetic reactions take place, in addition to H + - PPases, which are integral membrane proteins of the vacuolysosomal membranes and use the chemical energy of PPi to generate an electrochemical proton gradient useful in cell bioenergetics. This last category of proton pumps was considered to be restricted to higher plants and some primitive photosynthetic bacteria, but it has been found recently in many protists microalgae and protozoa) and bacteria, thus indicating that H + -PPases are much more widespread than previously thought. No cytosolic sPPase in bacteria, fungi and animal cells) has been shown to occur in these lower eukaryotes. The widespread occurrence of these key enzymes of PPi metabolism among evolutionarily divergent protists strongly supports the ancestral character of the bioen- ergetics based on this simple energy-rich compound, which may play an important role in survival under dierent biotic and abiotic stress conditions. Keywords Inorganic pyrophosphate á Soluble inorganic pyrophophatase á Proton-translocating pyrophosphatase á Photosynthetic protists á Parasitic protists Introduction Inorganic pyrophosphate PPi) is a simple molecule composed of two metaphosphate groups PO 3 ± ) linked by an oxygen anion O 2 ± ), thus forming the P-O-P structure, also known as a ``pyrophosphate bridge''. P-O-P is a chemical group that stores readily usable energy for biochemical reactions and it can be found not only in PPi but also in ATP, the well-known ``energy currency''oflivingcells.PPiisproducedinlargeamounts by a variety of vital biosynthetic reactions, such as the synthesis of biopolymers polysaccharides, proteins, nu- cleic acids, lipids). PPi hydrolysis is important to pull these anabolic reactions most of which are reversible) in the direction of biosynthesis. Moreover, an ecient PPi hydrolysis is also essential to replenish the orthophos- phate Pi) needed for phosphorylation [2] Fig. 1). Although PPi was until recently considered a waste product of anabolism, an increasing body of evidence indicates that it can play a relevant role in cellular bio- energetics. Moreover, it has been suggested that PPi may have been the ancestor of ATP as the ``energy currency'' during the early stages of biochemical evolution [4]. Two major types of PPi-hydrolysing enzymes, known as in- organic pyrophosphatases PPases, EC 3.6.1.1), have been characterized to date: soluble and membrane- embedded. Soluble PPases sPPases) are ubiquitous pro- teins whose role is the removal of the PPi produced by anabolic reactions, so that they can eciently proceed in the correct biosynthetic) direction [20]. Membrane- bound, proton-translocating, inorganic pyrophosphata- ses H + -PPases) belong to a recently identi®ed category Int Microbiol 2001) 4: 135±142 DOI 10.1007/s10123-001-0028-x J.R. PeÂrez-CastinÄeira á R. GoÂmez-GarcõÂa 1 R.L. LoÂpez-MarqueÂs á M. Losada á A. Serrano &) Instituto de BioquõÂmica Vegetal y FotosõÂntesis, Centro de Investigaciones Cientõ®cas ``Isla de la Cartuja'', Universidad de Sevilla±CSIC, Avenida Americo Vespucio s/n, 41092 Seville, Spain E-mail: aurelio@cica.es Tel.: +34-954489524 Fax: +34-954460065 Presentaddress: 1 Instituto de ParasitologõÂa y Biomedicina ``LoÂpez-Neyra'', Granada, Spain