13C NMR based profiling unveils different α-ketoglutarate pools involved into glutamate and lysine synthesis in the milk yeast Kluyveromyces lactis

Background: The construction of efficient cell factories for the production of metabolites requires the rational improvement/engineering of the metabolism of microorganisms. The subject of this paper is directed towards the quantitative understanding of the respiratory/fermentative Kluyveromyces lactis yeast metabolism and its rag8 casein kinase mutant, taken as a model for all rag gene mutations. Methods: 13C-NMR spectroscopy and [1,2-13C2]glucose was used as metabolic stable-isotope tracer to define the metabolic profiling of a K. lactis yeast and its derivative mutants. Results: Rag8 showed a decrease of all 13C glutamate fractional enrichments, except for [4- 13C]glutamate that was higher than wild type ones. A decrease of TCA cycle flux in rag8 mutants and a contribute of a [4-13C]ketoglutarate pool not originating from mitochondria was suggested. 13C lysine enrichments confirmed the presence of two compartmentalized α-ketoglutarate (α-KG) pools participating to glutamate and lysine synthesis. Moreover, an increased transaldolase, as compared to transketolase activity, was observed in the rag8 mutant by 13C-NMR isotopomer analysis of alanine. Conclusions: 13C NMR-based isotopomer analysis showed the existence of different α-KG metabolic pools for glutamate and lysine biosynthesis. In the rag8 mutant, 13C labeled pentose phosphate intermediates participated in the synthesis of this compartmentalised α-KG pool. General significance: A compartmentalization of the α-KG pools involved in lysine biosynthesis has been revealed for the first time in K. lactis. Given its great impact in metabolic engineering field, its existence should be validated/compared with other yeasts and/or fungal species.