Abstract
D-Galactose-specific L-arabinose isomerase (L-AI) would have much potential for the enzymatic conversion of D-Galactose into D-tagatose, while most of the reported L-AIs are L-arabinose specific. This study explored a highly D-Galactose-specific L-AI from Bifidobacterium adolescentis (BAAI) for the production of D-tagatose. In the comparative protein-substrate docking for D-Galactose and L-arabinose, BAAI showed higher numbers of hydrogen bonds in D-Galactose-BAAI bonding site than those found in L-arabinose-BAAI bonding site. The activity of BAAI was 24.47 U/mg, and it showed good stability at temperatures up to 65°C and a pH range 6.0–7.5. The Km, Vmax, and Kcat/Km of BAAI were found to be 22.4 mM, 489 U/mg and 9.3 mM–1 min–1, respectively for D-Galactose, while the respective values for L-arabinose were 40.2 mM, 275.1 U/mg, and 8.6 mM–1 min–1. Enzymatic conversion of D-Galactose into D-tagatose by BAAI showed 56.7% conversion efficiency at 55°C and pH 6.5 after 10 h.
Highlights
D-tagatose is a rare ketohexose in nature found mostly in gums and lichens, which is widely used as a low caloric sweetener and substitute of sucrose for its 92% sweetness of sucrose in a 10% aqueous solution with only one-third calories of sucrose (Guo et al, 2018)
We report a highly D-Galactose-specific L-arabinose isomerase (L-AI) from Bifidobacterium adolescentis (BAAI) for the first time, which worked under acidic condition
The phylogenetic analysis of BAAI with the other L-AIs demonstrated that it had closeness with B. longum (I3ZR32), Thermoanaerobacter mathranii (Q70G56), and Thermoanaerobacterium saccharolyticum (K7SW59)
Summary
D-tagatose is a rare ketohexose in nature found mostly in gums and lichens, which is widely used as a low caloric sweetener and substitute of sucrose for its 92% sweetness of sucrose in a 10% aqueous solution with only one-third calories of sucrose (Guo et al, 2018). It is known for its prebiotic action, capability of reducing cholesterol, preventing colon cancer, treatment of type II diabetes, and so on (Lu et al, 2008). Biological method offers an eco-friendly conversion approach, in which L-arabinose isomerase (L-AI, EC 5.3.1.4) is used for isomerizing D-Galactose into D-tagatose (Zhang et al, 2017, 2019)
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