The function of glutaredoxin GRXS15 is required for lipoyl-dependent dehydrogenases in mitochondria.
Iron-sulfur (Fe-S) clusters are ubiquitous cofactors in all life and are used in a wide array of diverse biological processes, including electron transfer chains and several metabolic pathways. Biosynthesis machineries for Fe-S clusters exist in plastids, the cytosol and mitochondria. A single monothiol glutaredoxin (GRX) is involved in Fe-S cluster assembly in mitochondria of yeast and mammals. In plants, the role of the mitochondrial homologue GRXS15 has only partially been characterized. Arabidopsis (Arabidopsis thaliana) grxs15 null mutants are not viable, but mutants complemented with the variant GRXS15 K83A develop with a dwarf phenotype similar to the knockdown line GRXS15amiR. In an in-depth metabolic analysis of the variant and knockdown GRXS15 lines, we show that most Fe-S cluster-dependent processes are not affected, including biotin biosynthesis, molybdenum cofactor biosynthesis, the electron transport chain and aconitase in the TCA cycle. Instead, we observed an increase in most TCA cycle intermediates and amino acids, especially pyruvate, glycine and branched-chain amino acids (BCAAs). Additionally, we found an accumulation of branched-chain a-keto acids (BCKAs), the first degradation products resulting from transamination of BCAAs. In wild-type plants, pyruvate, glycine and BCKAs are all metabolized through decarboxylation by mitochondrial lipoyl cofactor-dependent dehydrogenase complexes. These enzyme complexes are very abundant, comprising a major sink for lipoyl cofactor. Because biosynthesis of lipoyl cofactor depends on continuous Fe-S cluster supply to lipoyl synthase, this could explain why lipoyl cofactor-dependent processes are most sensitive to restricted Fe-S supply in grxs15 mutants.