A heterodimeric β-galactosidase was discovered in the novel strain Lactobacillus curieae M2011381. The gene encoding the enzyme was expressed in Escherichia coli BL21 (DE3). The specific enzyme activities of the recombinant holoenzyme (LacLM) and large subunit (LacL) measured 11.4 U/mg and 3.8 U/mg, respectively. The k cat /K m values of LacLM and LacL were 740 mM −1 s −1 and 1.40 mM −1 s −1 , respectively. LacLM showed maximum activity at pH 8.0 and 55 °C, and it could maintain its activity at a neutral pH and below 45 °C. LacLM displayed both hydrolysis and transgalactosylation activity on 200 g/L lactose. When LacLM was added to milk, the lactose was hydrolyzed after 6 h without galactooligosaccharide generation. The sequence alignment and homology modeling of the structures of the holoenzyme and subunits revealed that LacL has a catalytic domain with a catalytic dyad, Glu470 and Glu538, and small subunit LacM is a β-sheet domain with a conserved Trp294. The molecular docking of LacLM helped to illustrate the roles of both subunits in the reaction with lactose.
Lactobacillus curieae CCTCC M2011381 is a novel strain isolated from stinky tofu brine. In order to evaluate the potential of the strain in fermenting plant foodstuff and possible bioactivity produced in fermentation, the growth of the strain in soymilk, soy protein isolate and ginkgo nut beverages was studied firstly. Cell counts of the strain could increase 4 log CFU/mL after 20 h of growth in all materials. The scavenging ratio of diphenyl picryl hydrazinyl radical was improved by the fermentation due to total flavonoids content increase, peptides formation, and efficient transformation of soy isoflavone glycoside to aglycone. Meanwhile, the fermentation significantly enhanced the inhibitions of 3-hydroxy-3-methylglutaryl-coenzyme A reductase of all the three materials. The highest inhibition of 67.2% was achieved by the fermented ginkgo nut beverage. The fermentation also significantly raise the inhibition of angiotensin I converting enzyme of soy protein isolate and ginkgo nut beverages. Finally, the strain was verified survival in mice gastrointestinal tract within all the fermented matrices. The highest cell count, 5.14 log CFU/g faeces, was detected at 5 d after the gavage of fermented soy protein isolate beverage. Accordingly, L. curieae has the prospect as the starter culture in fermentation of plant foodstuff.
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