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  • br Type II NAD P H

    2021-05-06


    Type II NAD(P)H Dehydrogenases in Fungi Fermentative YeastSaccharomyces cerevisiaeBecause S. cerevisiae is a facultative ARQ 197 yeast capable of meeting its energy requirements with ATP generated by fermentation, only relatively few mitochondrial proteins are essential for cell viability. The mitochondrial respiratory chain of baker’s yeast does not contain Complex I. Instead of Complex I, it possesses an internal alternative NADH dehydrogenase NDI1 that functions as a matrix NADH-oxidizing enzyme and thus maintains matrix NAD+/NADH homeostasis (Rigoulet et al. 2004). Due to the lack of transhydrogenase activity in yeast, the enzyme is essential for the yeast cell to preserve the correct redox balance both in the mitochondrial matrix and in the cytosol. An analysis of the crystal structure indicates that the yeast NDI1 forms a globular α/β structure that can be divided into three domains: an active part with two domains, one for FAD and one for NADH binding, and a C-terminal domain (Feng et al., 2012, Iwata et al., 2012). The C-terminal domain is usually highly conserved among various species. It consists of three β-strands and two α-helices, and the α-helices form a helix-turn-helix structure that interacts extensively with the active domain of NDI1. In addition, C-terminal domain participates through helix α16 in homodimer formation of NDI1 of approximate size ∼150kDa (Feng et al. 2012). Moreover, deletion analysis indicates that helices α15 and α16 are strongly involved in anchoring the protein to the membrane. Like many enzymes that react with UQ, NDI1 has two UQ-binding sites, UQI and UQII. UQI may not readily exchange with the UQ pool because it forms extensive interactions with the CTD of NDI1. However, in contrast, UQII is more likely to serve this purpose because it has much fewer contacts with NDI1. The structures obtained suggest that there is an electron transfer pathway from NADH to UQII, with UQI acting as an intermediate together with FAD. FAD first accepts two electrons from NADH to form FADH2 and subsequently transfers them to UQI and UQII (Feng et al. 2012). However, the exact mechanism of the electron transfer has not still been elucidated in detail. Recently, Yamashita et al. (2018) have reported that stigmatellin (STG) acts as a first known competitive inhibitor of NDI1. It has been hypothesized that the stigmatellin binding site overlaps with the UQI site during the enzymatic reaction. The identification of binding UQ as a substrate at the STG-1a/UQI site implies that the binding sites for two substrates, UQ and NADH, are distinct. According to the authors STG-1b/UQII may not be functional in the enzymatic reaction of NDI1 (Yamashita et al. 2018). In the yeast mitochondria, external and internal alternative NADH dehydrogenases have been reported to form a large supercomplex with succinate dehydrogenase (Complex II) (Grandier-Vazeille et al. 2001). Alternatively, the yeast mitochondrial NDI1 has recently been described to associate with Complexes III and IV, forming a respirosome-like structure, which can facilitate electron channeling between individual respiratory complexes (Matus-Ortega et al. 2015). NDI1 has garnered extensive attention due to experiments using the enzyme to restore the mitochondrial electrochemical potential in Caenorhabditis elegans and mammalian cells with Complex I defects (DeCorby et al., 2007, Perales-Clemente et al., 2008) and opens the possibility of genetic treatment for Parkinson’s and LEBER optic neuropathic diseases (Marella et al., 2007, Marella et al., 2008). In addition, cells transfected with the ndi1 gene decreased the production of reactive oxygen species (ROS) (DeCorby et al., 2007, Marella et al., 2007, Seo et al., 2006). Notably, the S. cerevisiae mitochondria do also possess external NADH dehydrogenases (Luttik et al. 1998). There are two enzymes present in the yeast inner mitochondrial membrane facing intermembrane space, NDE1 and NDE2. These dehydrogenases and glycerol-3-phosphate shuttle supply excess cytosolic NADH to the mitochondrial respiratory chain. In the null mutant Δnde1Δnde2, the ability to oxidize external NADH was abolished but did not result in lethality. This observation suggests that some other systems that sustain the mitochondrial reoxidation of cytosolic NADH must be present in the yeast mitochondria (Luttik et al. 1998). What is tremendously intriguing is that both NDE enzymes in yeast were confirmed to associate in a supramolecular complex with other dehydrogenases, thus likely influencing the activity of each other depending on substrate availability (Rigoulet et al. 2004). This finding indicates that in the yeast mitochondria, the metabolic pathway of cytosolic NADH is highly organized and regulated. For instance, activity of the external NADH dehydrogenases has an inhibitory effect on the external glycerol-3-phosphate dehydrogenase. The exact function of the external NADH dehydrogenases in yeast, apart from maintaining the NAD+/NADH balance, still remains elusive. According to Fang and Beattie, external NADH dehydrogenases are responsible for half of the superoxide radicals produced in the yeast mitochondria during the transfer of electrons to oxygen (Fang and Beattie 2003a).