Competition of Electrons to Enter the Respiratory Chain

Abstract

In the yeast Saccharomyces cerevisiae, the most im-portant systems for conveying excess cytosolic NADH to the mitochondrial respiratory chain are the external NADH dehydrogenases (Nde1p and Nde2p) and the glyc-erol-3-phosphate dehydrogenase shuttle. In the latter system, NADH is oxidized to NAD ؉ and dihydroxyac-etone phosphate is reduced to glycerol 3-phosphate by the cytosolic Gpd1p. Subsequently, glycerol 3-phosphate donates electrons to the respiratory chain via mitochon-drial glycerol-3-phosphate dehydrogenase (Gut2p). At saturating concentrations of NADH, the activation of external NADH dehydrogenases completely inhibits glycerol 3-phosphate oxidation. Studies on the function-ally isolated enzymes demonstrated that neither Nde1p nor Nde2p directly inhibits Gut2p. Thus, the inhibition of glycerol 3-phosphate oxidation may be caused by competition for the entrance of electrons into the respi-ratory chain. Using single deletion mutants of Nde1p or Nde2p, we have shown that glycerol 3-phosphate oxida-tion via Gut2p is inhibited fully when NADH is oxidized via Nde1p, whereas only 50% of glycerol 3-phosphate oxidation is inhibited when Nde2p is functioning. By comparing respiratory rates with different respiratory substrates, we show that electrons from Nde1p are fa-vored over electrons coming from Ndip (internal NADH dehydrogenase) and that when electrons come from ei-ther Nde1p or Nde2p and succinodehydrogenase, their use by the respiratory chain is shared to a comparable extent. This suggests a very specific competition for electron entrance into the respiratory chain, which may be caused by the supramolecular organization of the respiratory chain. The physiological consequences of such regulation are discussed. The yeast Saccharomyces cerevisiae lacks transhydrogenase activity (1, 2), and the redox couple NAD ϩ /NADH cannot pass through the mitochondrial membrane. Hence, systems for NADH turnover in mitochondria as well as in the cytosol are required under both aerobic and anaerobic conditions. The reason for this is that several processes result in production of NADH, i.e. several processes are, contrary to ethanol fermen-tation, not redox neutral. The synthesis of 1 mol of glycerol, the second major by-product of S. cerevisiae cells fermenting glu-cose, results in the consumption of 1 mol of NADH, whereas other by-products such as acetate lead to the production of cytosolic NADH. The largest part of excess cytosolic NADH formation is connected to biomass production (3, 4). The syn-thesis of proteins and nucleic acids and even the highly reduced lipids is associated with assimilatory NADH production. In particular, NADH is generated in the biosynthetic pathways of amino acid synthesis (3, 4). Anaerobically, the only means by which S. cerevisiae can reoxidize surplus production of NADH is by glycerol production (2, 5). Aerobically, several systems exist for conveying excess cytosolic NADH to the mitochondrial electron transport chain in S. cerevisiae (6). The two most important systems in this respect seem to be the external NADH dehydrogenase (Nde1p/Nde2p) (7, 8) and the glycerol 3-phosphate shuttle (9). The Nde1p/Nde2p system, which is localized in the inner mitochondrial membrane with the cata-lytic sites projecting toward the intermembrane space, has been shown to directly oxidize cytosolic NADH (7, 8). The glycerol 3-phosphate shuttle system, which involves the FAD-dependent Gut2p (10), is situated in the inner membrane of the mitochondria with the catalytic site projecting toward the cy-tosol and has been shown to be active in maintaining a cytosolic redox balance (9). In this system, the co-factor NADH is oxi-dized to NAD ϩ by the cytosolic glycerol-3-phosphate