In Kluyveromyces lactis a Pair of Paralogous Isozymes Catalyze the First Committed Step of Leucine Biosynthesis in Either the Mitochondria or the Cytosol

Combinatory breeding of sake yeast strains with mutations that enhance Ginjo aroma production

Isoamyl acetate and ethyl caproate are the primary aroma compounds responsible for the fruity fragrance characteristic of Ginjo sake. Simultaneous high-level production of both compounds is crucial to achieving a balanced aroma and complex flavor. Isoamyl acetate is predominantly produced by hda1∆/hda1∆ and LEU4(G516S)/LEU4(G516S), while ethyl caproate is produced in high quantities by FAS2(G1250S)/FAS2(G1250S). In this study, to maximize the production of both aroma compounds, genome editing was employed to generate sake yeast strains combining these mutations. After small-scale fermentation tests were conducted to evaluate the production of aroma compounds, we found that the isoamyl acetate-enhancing effect of hda1∆/hda1∆ was almost completely masked by FAS2(G1250S)/FAS2(G1250S). In contrast, the effects of LEU4(G516S)/LEU4(G516S) were not entirely masked by FAS2(G1250S)/FAS2(G1250S), resulting in 2.4- and 5.4-fold greater production of isoamyl acetate and ethyl caproate, respectively. This study highlights the utility of genome editing in the combinatorial breeding of sake yeast.

Branched-chain amino acid biosynthesis in fungi

Branched-chain amino acids (BCAAs)-isoleucine, leucine, and valine-are synthesized by fungi. These amino acids are important components of proteins and secondary metabolites. The biochemical pathway for BCAA biosynthesis is well-characterized in the yeast Saccharomyces cerevisiae. The biosynthesis of these three amino acids is interconnected. Different precursors are metabolized in multiple steps through shared enzymes to produce isoleucine and valine, and the valine biosynthesis pathway branches before the penultimate step to a series of leucine biosynthesis-specific steps to produce leucine. Recent efforts have made advances toward characterization of the BCAA biosynthesis pathway in several fungi, revealing diversity in gene duplication and functional divergence in the genes for these enzymatic steps in different fungi. The BCAA biosynthesis pathway is regulated by the transcription factor LEU3 in S. cerevisiae, and LeuB in Aspergillus nidulans and Aspergillus fumigatus, and the activity of these transcription factors is modulated by the leucine biosynthesis pathway intermediate α-isopropylmalate. Herein, we discuss recent advances in our understanding of the BCAA pathway and its regulation, focusing on filamentous ascomycete fungi and comparison with the well-established process in yeast.

Two alpha isopropylmalate synthase isozymes with similar kinetic properties are extant in the yeast Lachancea kluyveri

The first committed step in the leucine biosynthetic pathway is catalyzed by α-isopropylmalate synthase (α-IPMS, EC 2.3.3.13), which in the Saccaromycotina subphylum of Ascomycete yeasts is frequently encoded by duplicated genes. Following a gene duplication event, the two copies may be preserved presumably because the encoded proteins diverge in either functional properties and/or cellular localization. The genome of the petite-negative budding yeast Lachancea kluyveri includes two SAKL0E10472 (LkLEU4) and SAKL0F05170g (LKLEU4BIS) paralogous genes, which are homologous to other yeast α-IPMS sequences. Here, we investigate whether these paralogous genes encode functional α-IPMS isozymes and whether their functions have diverged. Molecular phylogeny suggested that the LkLeu4 isozyme is located in the mitochondria and LkLeu4BIS in the cytosol. Comparison of growth rates, leucine intracellular pools and mRNA levels, indicate that the LkLeu4 isozyme is the predominant α-IPMS enzyme during growth on glucose as carbon source. Determination of the kinetic parameters indicates that the isozymes have similar affinities for the substrates and for the feedback inhibitor leucine. Thus, the diversification of the physiological roles of the genes LkLEU4 and LKLEU4BIS involves preferential transcription of the LkLEU4 gene during growth on glucose and different subcellular localization, although ligand interactions have not diverged.

The chemical mechanisms of the enzymes in the branched-chain amino acids biosynthetic pathway and their applications

l-Valine, l-isoleucine, and l-leucine are three key proteinogenic amino acids, and they are also the essential amino acids required for mammalian growth, possessing important and to some extent, special physiological and biological functions. Because of the branched structures in their carbon chains, they are also named as branched-chain amino acids (BCAAs). This review will highlight the advance in studies of the enzymes involved in the biosynthetic pathway of BCAAs, concentrating on their chemical mechanisms and applications in screening herbicides and antibacterial agents. The uses of some of these enzymes in lab scale organic synthesis are also discussed.