*2.2. Identification of the Biotransformation Product*

To resolve the chemical structure of compound (**2**), the biotransformation was scaled up to 25 mL, with 1 mg/mL GAA, 15 μg/mL BsGT110, 10 mM MgCl2, and 10 mM UDP-glucose in 50 mM acetate buffer of pH 6 and 40 ◦C for a 30-min incubation. A total of 5.4 mg of compound (**2**) in the 25-mL reaction was purified with preparative high-performance liquid chromatography (HPLC). The chemical structure of the purified compound was then resolved using mass and nucleic magnetic resonance (NMR) spectral analyses. The molecular formula of compound (**2**) was established as C36H53O12 by the electrospray ionization mass (ESI-MS) at m/z 679.67 [M + H]+, indicating the presence of a glucose residue. The NMR spectra exhibit characteristic glucosyl signals: the anomeric carbon signal at δ<sup>C</sup> 95.9, one CH2 signal at δ<sup>C</sup> 61.8, and four CH signals at δ<sup>C</sup> 70.6, 73.9, 78.2, 79.1. The large coupling constant (8.1 Hz) of the anomeric proton H-1 (6.33 ppm) indicated the β-configuration. The cross peak of H-1 with C-26 (6.33/174.6 ppm) in the HMBC spectrum demonstrated the structure of compound (**2**) to be GAA-26-*O*-β-glucoside. The NMR spectra data are shown in Table S1 and Figures S1–S4. Figure 4 illustrated the biotransformation process of GAA by BsGT110.

**Figure 4.** Biotransformation of GAA by BsGT110 in the acidic condition.
