Mitochondrial Energy Metabolism : Kgd4 is a novel subunit of the α-ketoglutarate dehydrogenase complex

Sammanfattning: Mitochondria are essential organelles of eukaryotic cells and are responsible for their energy metabolism. The citric acid cycle, a chain of redox reactions in the mitochondrial matrix, is a central metabolic hub. The energy of these redox reactions is preserved in reducing equivalents, which are fed into the electron transport chain of the inner mitochondrial membrane. This process ultimately produces ATP.Mitochondrial ribosomes are responsible for the synthesis of the hydrophobic core components of the respiratory chain complexes. Using the baker’s yeast Saccharomyces cerevisiae as a eukaryotic model organism, the composition of the polypeptide tunnel exit of mitochondrial ribosomes was defined. We identified novel mitochondria-specific ribosomal proteins and generated insights into how they contribute to the specialization of mitochondrial ribosomes.In a second approach, Kgd4 was identified as a novel structural component of the a-ketoglutarate dehydrogenase complex (KGDH), which is part of the citric acid cycle. We demonstrate that this protein is responsible for recruiting one essential subunit to the catalytic core of the complex. By ensuring the structural integrity, Kgd4 indirectly maintains the catalytic function of the enzyme complex. Kgd4 also represents a rare exception from the standard protein synthesis process. There are two isofroms of the protein, which differ in the length of the amino acid sequence. Both Kgd4 forms contribute to KGDH stability and activity. Translation of the long isoform starts from a non-canonical UUG codon upstream of the commonly used start-codon AUG. This provides insights into the flexibility of translation initiation in yeast. Finally, we purified the fully intact and functional KGDH complex from isolated yeast mitochondria for future structural studies. These experiments also revealed details about the interaction of the complex with the inner mitochondrial membrane.