br NADH and NADPH Turnover and
NADH and NADPH Turnover and the Putative Role of Alternative NAD(P)H Dehydrogenases Still very little is known about the metabolic function of external and internal alternative NADH:UQ oxidoreductases, and almost nothing is known about the mechanisms underlying their metabolic regulation, especially in fungi and protists. External NADH dehydrogenases are obviously involved in feeding electrons from NADH generated in the cytoplasm into the mitochondrial respiratory chain. Internal alternative NADH enzymes may compete with Complex I for the substrates matrix NADH and UQH2 (ubiquinol, the reduced form of UQ). Although, electron transport via an internal alternative nonproton-pumping dehydrogenase results in a lower transmembrane potential compared to electron transport via Complex I, the possibility to build (or complement) a functional electron transport chain by the expression of a single polypeptide instead of the at least 35 subunits of Complex I seems to be advantageous under conditions where the carbon source is abundant and rapid growth is essential (Melo et al. 2004). Despite a large amount of new information on NDH2 that has been collected, many questions remain unanswered, especially those concerning the function of the enzymes in cellular metabolism. Since the pyridine nucleotides are central mediators of the reducing power flow between different cellular processes and compartments (Rasmusson and Wallstrom 2010), the presence of several NDH2 enzymes could possibly improve the catalytic flexibility of respiratory NAD(P)H oxidation and therefore thereby the redox balancing or sensing (Geisler et al. 2007). These factors merge energy metabolism with carbon metabolism and stress defense. Mitochondrial NAD(P)H oxidation may participate in the prevention of ROS formation (Fernie et al., 2004, Moller, 2001) and in decreasing the excess of reducing equivalents feeding the mitochondrial respiratory chain (Raghavendra and Padmasree 2003). However, some data indicate that NDH2 could increase ROS production causing apoptosis of the cell (Carneiro et al., 2012, Fang and Beattie, 2002b, Fang and Beattie, 2003a). The external NAD(P)H dehydrogenases are involved in the modulation of the cytoplasmic NAD(P)H pool. In mitochondria, the major citric Riluzole receptor cycle enzymes and the metabolite exchangers together mediate reducing fuel shuttling across the inner membrane resulting in redox separation between the mitochondrial and cytosol compartments (Moller 2001). The malate/oxaloacetate shuttle maintains a sharp NADH gradient between the cytosol and mitochondrial matrix. Therefore, the primary task in maintaining the NADH redox balance belongs to the internal NADH dehydrogenases including both Complex I and alternative internal dehydrogenases. In turn, the level of NAD(P)H in the cytosol is the effect of a variety of metabolic pathways involved, including the oxidative pentose phosphate pathway (PPP) and the participation of cytosolic NAD(P)H kinases. The NADPH molecule is crucial for many biological pathways such as cellular antioxidative system-mediated reactions (Rasmusson and Wallstrom 2010). Thus, the mitochondrial alternative NAD(P)H dehydrogenases may participate in maintaining the appropriate redox balance. Considering their Ca2+ dependence, it has been suggested that some type II dehydrogenases will be inactive in unstressed cells (Moller 2001). Interestingly, in plants, the genes encoding type II NAD(P)H dehydrogenases and the AOX have been demonstrated to be expressed simultaneously under stress conditions and during development (Clifton et al., 2005, Ho et al., 2007, Rasmusson et al., 2009), indicating the coupling of these two alternative pathways. Alternative NAD(P)H dehydrogenases do not participate in a proton electrochemical gradient generation in the inner mitochondrial membrane. Their action leads to the gradient dissipation. Thus, they are energy-dissipating systems. Because the proton electrochemical gradient powers transport processes through the membrane, it is essential to maintain it at the appropriate level. In addition, mitochondrial ROS production depends on the level of the reduction of mitochondrial electron carriers that is related to the level of the proton electrochemical gradient across the inner membrane (Dominiak et al. 2018). Therefore, by influencing the proton electrochemical gradient, NDH2 may modulate transport processes across the inner membrane and mitochondrial ROS generation. It has been proposed that during evolution, the involvement of the alternative NAD(P)H dehydrogenases in the mitochondrial respiratory chain may be one of mechanisms that enables them to adjust the cellular metabolism to changing environmental conditions and protect against ROS formation (Moller 2001). As described in this review, alternative NAD(P)H dehydrogenases are present in the branched respiratory chain not only in plants but also in some non-photosynthesizing unicellular eukaryotes, including amoeboid protists, as well as in filamentous fungi (Table 1). Interestingly, they are also present in the reduced mitochondrial respiratory chain of fermentative yeast and parasite protists, including apicomplexans.