D-2-hydroxyglutaric aciduria is a neurometabolic disorder, characterized by the accumulation of
D-2-hydroxyglutarate (D-2HG) in human mitochondria. Increased levels of D-2HG are detected in humans exhibiting point mutations in the genes encoding
isocitrate dehydrogenase,
citrate carrier, the
electron transferring flavoprotein (ETF) and its downstream electron acceptor ETF-
ubiquinone oxidoreductase or
D-2-hydroxyglutarate dehydrogenase (hD2HGDH). However, while the pathogenicity of several
amino acid replacements in the former four
proteins has been studied extensively, not much is known about the effect of certain point mutations on the biochemical properties of hD2HGDH. Therefore, we recombinantly produced wild type hD2HGDH as well as two recently identified disease-related variants (hD2HGDH-I147S and -V444A) and performed their detailed biochemical characterization. We could show that hD2HGDH is a
FAD dependent
protein, which is able to catalyze the oxidation of D-2HG and D-
lactate to α-ketoglutarate and
pyruvate, respectively. The two variants were obtained as apo-
proteins and were thus catalytically inactive. The addition of
FAD failed to restore enzymatic activity of the variants, indicating that the cofactor binding site is compromised by the single
amino acid replacements. Further analyses revealed that both variants form aggregates that are apparently unable to bind the
FAD cofactor. Since,
D-2-hydroxyglutaric aciduria may also result from a loss of function of either the ETF or its downstream electron acceptor ETF-
ubiquinone oxidoreductase, ETF may serve as the cognate electron acceptor of reduced hD2HGDH. Here, we show that hD2HGDH directly reduces recombinant human ETF, thus establishing a metabolic link between the oxidation of
D-2-hydroxyglutarate and the mitochondrial electron transport chain.