Reports on the participation of NDH along with other respira
Reports on the participation of NDH2 along with other respiratory complexes in the formation of a large supercomplex (Grandier-Vazeille et al. 2001) or even a respirosome-like structure in S. cerevisiae mitochondria (Matus-Ortega et al. 2015) are very intriguing. Similarly, in Y. lypolytica mitochondria, it has been suggested that formation of the supercomplex promotes substrate and electron channeling, ROS sequestering, and the stabilization of labile, multisubunit mitochondrial complexes (Guerrero-Castillo et al., 2009, Guerrero-Castillo et al., 2012). In addition, studies on the N. crassa NDH2 have indicated a possible partial compensation of particular dehydrogenases by each other in different stages of development, indicating a higher level of regulation of the respiratory chain components (Carneiro et al. 2007). These findings suggest the presence of “respiratory strings”, precisely regulated systems in mitochondria, which can both facilitate electrons channeling between respiratory complexes and control the abundance and activities of the respiratory chain components. Therefore, considering the functions of the alternative NAD(P)H dehydrogenases, the general dynamics of the mitochondrial respiratory chain should be taken into account.
It has been suggested that type II dehydrogenases are involved in programmed cell death in fungi. In A. niger, a decrease in ATP production as a consequence of enhanced alternative NADH dehydrogenase activity leads to a decrease in cellular viability and subsequently cause earlier senescence and culture death (O’Donnell et al. 2011). In yeast, NDI1 may be involved in the cell death induced by different stimuli, such as hydrogen peroxide, acetic acid, and manganese ions (Cui et al. 2012). Interestingly, alternative NAD(P)H dehydrogenases are phylogenetically related to cell death-promoting proteins of the apoptosis-inducing factor (AIF)-family (Goncalves and Videira 2015).
In apicomplexans, the internal NADH dehydrogenase is suggested to be a critical component of the mitochondrial electron transport chain lacking Complex I (Biagini et al. 2006) and an evolutionary ARCA Cy5 EGFP mRNA to a microaerophilic lifestyle of parasites that enable uncoupled NADH oxidation and therefore a lowering of mitochondrial reducing power, as well as a reduction of mitochondrial superoxide generation (Fisher et al. 2007). On the other hand, internal NDH2 has been found to be a potential source of superoxide production in procyclic trypanosome mitochondria (Fang and Beattie 2002b). Recent studies have shown that in T. brucei proliferating bloodstream forms, internal NDH2 is advantageous, but not essential, in this parasite stage when the mitochondrial NAD+/NADH balance is not very important (Surve et al. 2017). Similarly, it has been observed that in N. crassa, alternative dehydrogenases are beneficial but not indispensable during fast growth, when carbon sources are abundant in the environment (Carneiro et al. 2012).
Parasitic protists use mitochondria or the mitochondrion-derived organelle (mitosomes) and enzymes for the aerobic (or anaerobic) oxidation of substrates and for energy production (Mogi and Kita 2010). Due to absence of NDH2 in the mammalian mitochondria, alternative dehydrogenases are potential drug targets to treat diseases caused by Trypanosoma, parasite amoeba or apicomplexans. Unfortunately, it still remains a large challenge to find specific inhibitors of NDH2 (Dong et al. 2009).
Final Remarks Alternative NAD(P)H dehydrogenases are present in the branched respiratory chain not only in mitochondria of plants but also in non-photosynthesizing unicellular eukaryotes, including amoeboid protists, as well as in filamentous fungi. NDH2 are also present in the reduced mitochondrial respiratory chain of fermentative yeast and parasite protists, including apicomplexans. The genomic data available indicate that, with the exception of animals, most eukaryotes contain genes that potentially encode NDH2. Their partial conservation throughout evolution and widespread presence makes them a particularly interesting subject of research. The physiological role of the alternative rotenone-insensitive dehydrogenases is still incomprehensible, especially in the mitochondria of fungi and protists. Compared with plant NDH2, the molecular and functional properties, regulation, and physiological impact of fungal and protozoan alternative NAD(P)H dehydrogenases are much less studied and surveyed. It can be assumed that their physiological role lies in the modulation of the mitochondrial energy metabolism in response to stress situations. They may prevent the overreduction of the electron transport chain components and thereby decrease production of ROS. Alternative respiratory pathways, including NDH2, may flexibly change in response to various environmental factors. Therefore, energy-dissipating systems, such as NDH2, may play an important role in the metabolic and energetic adaptation to the changes in the surrounding environment that occur during the life of fungi and protists.