Identification and characterization of novel D-lyxose isomerases from the goat rumen metagenome

Biocatalytic technology has emerged as a powerful tool for synthesizing functional sugars with a wide range of physiological functions. D-lyxose isomerase is an important aldose-ketose isomerase used for synthesizing functional sugars such as L-ribose and D-mannose; however, at present, only a few D-lyxose isomerases have been studied. In this study, a metagenomic approach was employed to mine novel D-lyxose isomerases from the goat rumen microbiome. Eleven full-length D-lyxose isomerase genes were identified using sequence alignment and phylogenetic analysis. All 11 genes were classified as Group II D-lyxose isomerases and display low similarity to previously characterized D-lyxose isomerases. Five of the genes were selected for heterologous expression in Escherichia coli, and all were expressed successfully and had detectable enzyme activity. Further characterization revealed that optimum temperatures for the five enzymes range from 45 degrees C to 60 degrees C, and the optimum pH range is 7.0-8.0, with high relative residual activities under weakly acidic conditions. The recombinant enzymes have a broad substrate spectrum and are active against L-ribose, L-ribulose, D-fructose, and D-mannose, in addition to exhibiting the highest activity against D-lyxose. Among the five enzymes, GR-LI2 displays the highest activity toward L-ribulose, and GR-LI5 displays the highest activity against fructose, suggesting that these two enzymes have significant potential for L-ribose and D-mannose synthesis, respectively.