Kluveromyces lactisSimilar to S cerevisiae K lactis is a
Kluveromyces lactisSimilar to S. cerevisiae, K. lactis is a facultative anaerobic respiratory yeast, which lacks Complex I in its mitochondrial respiratory chain. However, both yeasts differ regarding their respiratory-fermentative metabolism as K. lactis exhibits a greater use of the pentose phosphate pathway (PPP) to metabolize glucose instead of the fermentation process. Therefore, K. lactis respiration is not subject to glucose repression (Gonzalez-Siso et al. 2000). Based on the homology to the S. cerevisiae genes encoding alternative dehydrogenases, two genes, one for internal NADH dehydrogenase and one for external NADPH dehydrogenase, have been identified in the K. lactis genome and designated KlNDI1 and KlNDE1, respectively (Tarrio et al. 2005). An expression profile of the internal dehydrogenase KlNDI1 demonstrates flexibility dependent on the carbon source used by the cells. The level of the KlNDI1 transcript increases significantly when the cells grow in a low glucose concentration and in non-fermentable carbon sources. A similar effect has been observed in S. cerevisiae for its NDI1 protein that is more abundant in cells grown in non-fermentable carbon sources than in glucose. Importantly, the cloned KlNDI1 gene can complement the ndi1 mutation of S. cerevisiae in vivo. In turn, KlNDE1 is external dehydrogenase that uses NADPH as a substrate. Its expression increases simultaneously with the activation of the oxidative branch of the pentose phosphate pathway. However, the absence of the downregulation of KlNDE1 at a high glucose concentration supports the Crabtree-negative phenotype of K. lactis. It has been proposed that the presence of the KlNDI1 and KINDE1 dehydrogenases enables rapid glycerol phosphate to the different levels of carbon source and provides glucose metabolism via the pentose phosphate pathway at the expense of glycolysis (Tarrio et al. 2005). Similar to S. cerevisiae, in K. lactis mitochondria, the second external dehydrogenase, KlNDE2, has also been identified and characterized (Tarrio et al. 2006). In contrast to KlNDE1, the KlNDE2 protein uses both NADH and NADPH as substrates. A comparison of both enzymes shows that the Michaelis constant (KM) of KlNDE2 for NADPH is almost twice as high as that of KlNDE1, indicating that KlNDE2 has a lower affinity of for NADPH. Thus, the cytosolic concentration of NADPH may impact the relative activity of each of these proteins. This fact suggests that K. lactis can modulate the synthesis of the two external alternative dehydrogenases in relation to the requirements of the cell. Surprisingly, in spite of the EF-like motif present in the KlNDE2 sequence, there is no evidence of calcium regulation of the enzyme (Tarrio et al. 2006). Considering the ability to oxidize cytosolic NADPH in the mitochondria, both fermentative yeast described, S. cerevisiae and K. lactis, exhibit a substantially different physiology. In contrast to S. cerevisiae, K. lactis, which is highly dependent on glucose oxidation via the pentose phosphate pathway that produces NADPH, requires the oxidation of cytosolic NADPH in the mitochondria. Filamentous FungiNeurospora crassaFour alternative NAD(P)H dehydrogenases have been cloned and characterized in the mitochondria of the filamentous fungus N. crassa (Carneiro et al., 2004, Carneiro et al., 2007, Duarte et al., 2003, Melo et al., 2001). One of these enzymes is directed towards mitochondrial matrix (NDI1), while three others are directed externally (NDE1-3). However, one of them seems to be present in the cytosol as well. An NDI1 protein of ∼57kDa is located internally in the inner mitochondrial membrane, which was confirmed using digitonin fractionation and proteinase K treatment of the mitochondria (Duarte et al. 2003). Based on experiments on the activity of the isolated NDI1 protein, a KM value of 56μM for NADH was determined using Q2 as electron acceptor. In the presence of rotenone, the ndi1 mutant almost completely lacked NADH oxidation ability, suggesting that NDI1 is the sole internal alternative dehydrogenase in the N. crassa mitochondria. Interestingly, NDI1 may play a key role in the initial steps of spore germination because its absence delays the process (Duarte et al. 2003).