Characterization of two extracellular arabinanases in Lactobacillus crispatus

Extracellular glycosyl hydrolases are uncommon in lactobacilli and include amylases and fructosidases mediating starch and fructan utilization, respectively. Extracellular arabinanases have not been described in lactobacilli. This study is aimed at identifying the function of an arabinan utilization operon in Lactobacillus crispatus DSM29598 and at characterizing two putative extracellular arabinanases that are located on that operon. The arabinan utilization operon of L. crispatus DSM29598 encodes enzymes for degradation of arabinan, α-galactosidases, β-galactosidases, and enzymes and for utilization of arabinose including phosphoketolase. The two putative extracellular arabinanases, AbnA and AbnB, are homologous to family GH43 endo-arabinanases. In Lactobacillaceae, homologs of these enzymes were identified exclusively in vertebrate-adapted species of the genus Lactobacillus. L. crispatus grew with arabinan from sugar beet pectin as sole carbon source, indicating extracellular arabinanase activity, and produced lactate and acetate, indicating metabolism via the phosphoketolase pathway. The two arabinanases AbnA and AbnB were heterologously expressed and purified by affinity chromatography. AbnA hydrolyzed linear and branched arabinan, while AbnB hydrolyzed only linear arabinan. The optimum pH for AbnA and AbnB was 6 and 7.5, respectively; 40 °C was the optimum temperature for both enzymes. The application of arabinan degrading L. crispatus as probiotic or as synbiotic with pectins may improve the production of short-chain fatty acids from pectin to benefit host health. KEY POINTS: • An arabinan utilization operon in L. crispatus encodes two extracellular arabinanases. • The same operon also encodes metabolic genes for arabinose conversion. • In Lactobacillaceae, extracellular arabinanases are exclusive to Lactobacillus species.