Metabolism parasitic stages

Parasitic stages, on the other hand, generally do not use oxygen as the final electron acceptor but use fermentative processes to obtain most of their ATP. For these stages, an uneconomical energy metabolism is not detrimental, as the host provides the nutrients. Most adult flatworms inside the final host produce end products of a fermentative carbohydrate breakdown, such as succinate, acetate, propionate and lactate. These end products are formed via malate dismutation, a fermentative pathway, which is present in all types of parasitic worms (flatworms as well as many nematodes), but which is also present in animals like freshwater snails, mussels, oysters and other marine organisms. Malate dismutation is linked to a specially... 

Table 20.1. Generalized scheme of the main differences in behaviour and metabolism between free-living and parasitic stages of parasitic flatworms.

All parasitic flatworms capable of anaerobic metabolism favour malate as the primary mitochondrial substrate and the oxidative decarboxylations of first malate and then pyruvate generate intramitochondrial reducing power in the form of NADH (Fig. 20.1). In contrast, the pathways used to reoxidize intramitochondrial NADH are quite diverse and depend on the stage or species of parasite under examination, but in all cases, redox balance is maintained and electron-transport associated ATP is generated by the NADH-reduction of fumarate to succinate. In the cestode, hi. diminuta, succinate and acetate are the major end products of anaerobic malate dismutation and are excreted in the predicted 2 1 ratio. In the trematode F. hepatica, succinate is then further decarboxylated to propionate with an additional substrate level phosphorylation coupled to the decarboxylation of methylmalonyl CoA. F. hepatica forms primarily propionate and acetate as end products, again in a ratio of 2 1 to maintain redox balance. 

As expected, rhodoquinone is also present in H. diminuta, a cestode known to produce succinate as end product. Surprisingly, rhodoquinone is also present in the homolactic fermenter, 5. mansoni, where it is especially present in sporocysts, the snail stage of this parasite, which produces succinate under anaerobic conditions (see section on Transitions in Energy Metabolism during the Life Cycle ). 

In the different stages of their life cycle, parasitic flatworms have to adapt their behaviour and metabolism to the different environments they encounter (Table 20.1). The most important variations in the environment are the availability of food and oxygen. Free-living stages do not gather food and are therefore... 

The anaerobic mode of protein utilization is entirely possible in theory and in practice. Oxygen is not required for protein and nitrogen catabolism until the final stages of amino acid deamination have been reached. Complete anaerobic catabolism of proteins and nitrogen compounds (to the point where the final products C02, HjO and NH3 appear) has been known for a long time in prokaryotic organisms, but in eukaryotes only in parasitic worms, which are obligate anaerobes (von Brand, 1946). However, in recent decades, anaerobic metabolism of proteins has been found in some aquatic... 

The Malaria Parasite Metabolic Pathway (MPMP) is a website (http // sites.huji.ac.il/malaria last accessed 16 July 2008) that contains over 120 maps that encompasses not only classical biochemical pathways but next to each entry there is a 48-h clock that depicts stage-dependent transcription of a particular gene in P. falciparum. At the clock s zero hour, merozoites invade and at 12 o clock the 48-h cycle terminates. In the middle of the clock appears the hour of maximal transcript level. Clicking on the clock links to a transcriptomic database (http //malaria. ucsf.edu last accessed 16 July 2008). The enzymes are also linked to the International Union of Biochemistry and Molecular Biology website (http //www.iubmb.org last accessed 16 July 2008) where there is a detailed chemistry of the enzymatic reaction. Unlike other databases MPMP has been developed, curated and updated manually (Ginsburg, 2006). 

Sherman, I. W. (1998b). Purine and pyrimidine metabolism of asexual stages. In "Malaria. Parasite Biology, Pathogenesis and Protection" (Sherman, ed.), pp. 177-184. ASM Press, Washington, DC. 

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