Study on oligomerization of glutamate decarboxylase from Lactobacillus brevis using asymmetrical flow field-flow fractionation (AF4) with light scattering techniques

In this work, asymmetrical flow field-flow fractionation (AF4) coupled with UV/Vis, multi-angle light scattering (MALS), and differential refractive index (dRI) detectors (AF4-UV-MALS-dRI) was employed for analysis of glutamate decarboxylase (LbGadB) from Lactobacillus brevis (L. brevis). AF4 provided molecular weight (MW) (or size)-based separation of dimer, hexamer, and aggregates of LbGadB. The effect of pH on oligomerization of LbGadB was investigated, and then AF4 results were compared to those from molecular modeling. The MWs measured by AF4-UV-MALS-dRI for dimeric and hexameric forms of LbGadB were 110 and 350 kDa, respectively, which are in good agreements with those theoretically calculated (110 and 330 kDa). The molecular sizes determined by AF4-UV-MALS-dRI were also in good agreement with those obtained from molecular modeling (6 and 10 nm, respectively, for dimeric and hexameric from AF4-UV-MALS-dRI and 6.4 × 7.6 and 7.6 × 13.1 nm from molecular modeling). The effects of temperature, salt type, and salt concentration on oligomerization of LbGadB were also investigated using dynamic light scattering (DLS). It was found that the hexameric form of LbGadB was most stable at pH 6 and in presence of NaCl or KCl. The results indicate that AF4, in combination of various online detectors mentioned above, provides an effective tool for monitoring of oligomerization of LbGadB under different conditions, such as temperature, pH, type of salts, and salt concentrations. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (10.1007/s00216-017-0735-6) contains supplementary material, which is available to authorized users.