Abstract
BackgroundThe l-arabinose isomerase is an intracellular enzyme which converts l-arabinose into l-ribulose in living systems and d-galactose into d-tagatose in industrial processes and at industrial scales. d-tagatose is a natural ketohexose with potential uses in pharmaceutical and food industries. The d-galactose isomerization reaction is thermodynamically equilibrated, and leads to secondary subproducts at high pH. Therefore, an attractive l-arabinose isomerase should be thermoactive and acidotolerant with high catalytic efficiency. While many reports focused on the set out of a low cost process for the industrial production of d-tagatose, these procedures remain costly. When compared to intracellular enzymes, the production of extracellular ones constitutes an interesting strategy to increase the suitability of the biocatalysts.ResultsThe l-arabinose isomerase (l-AI) from Lactobacillus sakei was expressed in Lactococcus lactis in fusion with the signal peptide of usp45 (SPUsp45). The l-AI protein and activity were detected only in the supernatant of the induced cultures of the recombinant L. lactis demonstrating the secretion in the medium of the intracellular L. sakeil-AI in an active form. Moreover, we showed an improvement in the enzyme secretion using either (1) L. lactis strains deficient for their two major proteases, ClpP and HtrA, or (2) an enhancer of protein secretion in L. lactis fused to the recombinant l-AI with the SPUsp45. Th l-AI enzyme secreted by the recombinant L. lactis strains or produced intracellularly in E. coli, showed the same functional properties than the native enzyme. Furthermore, when mice are fed with the L. lactis strain secreting the l-AI and galactose, tagatose was produced in vivo and reduced the glycemia index.ConclusionsWe report for the first time the secretion of the intracellular l-arabinose isomerase in the supernatant of food grade L. lactis cultures with hardly display other secreted proteins. The secreted l-AI originated from the food grade L. sakei 23 K was active and showed the same catalytic and structural properties as the intracellular enzyme. The L. lactis strains secreting the l-arabinose isomerase has the ability to produce d-tagatose in vivo and conferred an anti-hyperglycemic effect to mice.
Highlights
The l-arabinose isomerase is an intracellular enzyme which converts l-arabinose into l-ribulose in living systems and d-galactose into d-tagatose in industrial processes and at industrial scales. d-tagatose is a natural ketohex‐ ose with potential uses in pharmaceutical and food industries
The proteins belonging to the isomerase family are intracellular, in this study we report for the first time an efficient secretion in the extracellular medium of the l-arabinose isomerase (l-AI) from Lactobacillus sakei 23 K by the food grade bacterium Lactococcus lactis
Results and discussion l‐AI secretion in L. lactis clpP‐htrA strain l-AI is an intracellular protein, which is an efficient bioconverter of galactose into tagatose
Summary
The l-arabinose isomerase is an intracellular enzyme which converts l-arabinose into l-ribulose in living systems and d-galactose into d-tagatose in industrial processes and at industrial scales. d-tagatose is a natural ketohex‐ ose with potential uses in pharmaceutical and food industries. In order to improve the l-AIs suitability for biotechnological applications, many tools have been used such as: the screening of biodiversity to identify relevant enzymes with interesting properties, protein isolation, molecular modeling and rational design [12,13,14] In this context, the thermoactivity, the metallic ions requirement and the catalytic efficiency of several enzymes were probed [15, 16]. While several studies revealed that numerous efforts have been made to set out a low cost procedure for the industrial production of tagatose, these processes remain costly [19]. This is mainly due to the biocatalyst production costs. Compared to the intracellular enzymes, the production of extracellular biocatalysts is an attractive alternative to improve the industrial process profitability
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