Parasites have developed a variety of physiological functions necessary for existence within the specialized environment of the host. Regarding energy metabolism, which is an essential factor for survival, parasites adapt to low oxygen tension in host mammals using metabolic systems that are very different from that of the host. The majority of parasites do not use the oxygen available within the host, but employ systems other than oxidative phosphorylation for ATP synthesis. In addition, all parasites have a life cycle. In many cases, the parasite employs aerobic metabolism during their free-living stage outside the host. In such systems, parasite mitochondria play diverse roles. In particular, marked changes in the morphology and components of the mitochondria during the life cycle are very interesting elements of biological processes such as developmental control and environmental adaptation. Recent research has shown that the mitochondrial complex II plays an important role in the anaerobic energy metabolism of parasites inhabiting hosts, by acting as quinol-fumarate reductase.

Mitochondrial respiratory enzymes play a central role in energy production in aerobic organisms. They differentiated from the ␣-proteobacteria-derived ancestors by adding noncatalytic subunits. An exception is Complex II (succinate: ubiquinone reductase), which is composed of four ␣-proteobacteria-derived catalytic subunits (SDH1-SDH4). Complex II often plays a pivotal role in adaptation of parasites in host organisms and would be a potential target for new drugs. We purified Complex II from the parasitic protist Trypanosoma cruzi and obtained the unexpected result that it consists of six hydrophilic (SDH1, SDH2 N , SDH2 C , and SDH5-SDH7) and six hydrophobic (SDH3, SDH4, and SDH8 -SDH11) nucleus-encoded subunits. Orthologous genes for each subunit were identified in Trypanosoma brucei and Leishmania major. Notably, the iron-sulfur subunit was heterodimeric; SDH2 N and SDH2 C contain the plant-type ferredoxin domain in the N-terminal half and the bacterial ferredoxin domain in the C-terminal half, respectively. Catalytic subunits (SDH1, SDH2 N plus SDH2 C , SDH3, and SDH4) contain all key residues for binding of dicarboxylates and quinones, but the enzyme showed the lower affinity for both substrates and inhibitors than mammalian enzymes. In addition, the enzyme binds protoheme IX, but SDH3 lacks a ligand histidine. These unusual features are unique in the Trypanosomatida and make their Complex II a target for new chemotherapeutic agents.The parasitic protist Trypanosoma cruzi is the etiological agent of Chagas disease, a public health threat in Central and South America. These parasites are normally transmitted by reduviid bugs via the vector feces after a bug bite and also via transfusion of infected blood. About 16 -18 million people are infected, and 100 million are at risk, but there are no definitive chemotherapeutic treatments available (1). Despite having potential pathways for oxidative phosphorylation (2), all trypanosomatids (Trypanosoma and Leishmania species) analyzed so far are characterized by incomplete oxidation of glucose with secretion of end products, such as succinate, alanine, ethanol, acetate, pyruvate, and glycerol (3, 4) (Fig. 1). Major routes for formation of succinate in Trypanosoma brucei are via NADH-dependent fumarate reductase in glycosomes and mitochondria (5, 6). In trypanosomatid mitochondria, the Krebs cycle is inefficient, and pyruvate is principally converted to acetate via acetate:succinate CoA transferase (7). A part of the Krebs cycle operates the utilization of histidine in the insect stage of T. cruzi (8).Mitochondrial Complex II (succinate:quinone reductase (SQR) 5 and succinate dehydrogenase (SDH)) serves as a membrane-bound Krebs cycle enzyme and often plays a pivotal role in adaptation of parasites to environments in their host (9, 10). In general, Complex II consists of four subunits (11). A flavoprotein subunit (SDH1, Fp) and an iron-sulfur subunit (SDH2, Ip) form a soluble heterodimer, which then binds to a membrane anchor heterodimer, SDH3 (CybL) and SDH4 (CybS)....

Complex II (succinate-ubiquinone oxidoreductase) is an important enzyme complex in both the tricarboxylic acid cycle and the aerobic respiratory chains of mitochondria in eukaryotic cells and prokaryotic organisms. In this study, the amino acid sequences of the large (cybL) and small (cybS) subunits of cytochrome b in human liver complex II were deduced from cDNAs isolated by homology probing with mixed primers for the polymerase chain reaction. The mature cybL and cybS contain 140 and 103 amino acids, respectively, and show little similarity to the amino acid sequences of the subunits from other species in contrast to the highly conserved features of the flavoprotein (Fp) subunit and iron-sulfur protein (Ip) subunit. From hydrophobicity analysis, both cybL and cybS appear to have three transmembrane segments, indicating their role as membrane-anchors for the enzyme complex. Histidine residues, which are possible heme axial ligands in cytochrome b of complex II, were found in the second transmembrane segment of each subunit. The genes for cybL (SDHC) and cybS (SDHD) were mapped to chromosome 1q21 and 11q23, respectively by fluorescent in situ hybridization (FISH).

We have mapped large (cybL) and small (cybS) subunits of cytochrome b in the succinate-ubiquinone oxidoreductase (complex II) of human mitochondria to chromosome 1q21 and 11q23, respectively (H. Hirawake et al., Cytogenet. Cell Genet. 79 (1997) 132-138). In the present study, the human SDHD gene encoding cybS was cloned and characterized. The gene comprises four exons and three introns extending over 19 kb. Sequence analysis of the 5' promoter region showed several motifs for the binding of transcription factors including nuclear respiratory factors NRF-1 and NRF-2 at positions -137 and -104, respectively. In addition to this gene, six pseudogenes of cybS were isolated and mapped on the chromosome.

Succinate-ubiquinone reductase (complex II) is an important enzyme complex in both the tricarboxylic acid cycle and aerobic respiration. A recent study showed that defects in human complex II are associated with cancers as well as mitochondrial diseases. Mutations in the four subunits of human complex II are associated with a wide spectrum of clinical presentations. Such tissue-specific clinical symptoms suggest the presence of multiple isoforms of the subunits, but subunit isoforms have not been previously reported. In the present study, we identified two distinct cDNAs for the human flavoprotein subunit (Fp) from a single individual, and demonstrated expression of these two isoforms in skeletal muscle, liver, brain, heart and kidney. Interestingly, one of the Fp isoforms was encoded as an intronless gene.