Residual structures in the unfolded state of starch-binding domain of glucoamylase revealed by near-UV circular dichroism and protein engineering techniques

Biocatalytic synthesis of L-ascorbyl palmitate using oleic acid imprinted Aspergillus niger lipase immobilized on resin

In order to improve the esterification efficiency of the enzymatic synthesis of l-ascorbic acid palmitate, the substrate analogue imprinting of the Aspergillus niger lipase-catalyzed esterification process was studied. Oleic acid was selected as the imprinting molecule, oleic acid imprinting immobilized lipase was prepared at pH 8.0, 0.1 g oleic acid, 1.5 mL of 95 % ethanol, and 0.1 g Tween-20. Through solubilization and supersaturation of Vitamin C, the reaction concentration of Vitamin C reached 5.00 % (m/v) in dioxane with 93.99 % esterification rate and 110.72 g/L of product concentration. Moreover, the Vitamin C reaction concentration can reach 8.00 % by using staged substrate feeding, and the esterification rate and product concentration of esterification after 28 h was 156.34 g/L and 82.96 %. Besides, the imprinting-induced conformational changes in enzyme proteins was characterized by fluorescence and infrared spectroscopy. This method provides a pathway for enzymatic production of l-ascorbic acid palmitate.

Resolving Site-Specific Heterogeneity of the Unfolded State under Folding Conditions

Understanding the properties of the unfolded state under folding conditions is of fundamental importance for gaining mechanistic insight into folding as well as misfolding reactions. Toward achieving this objective, the folding reaction of a small protein, monellin, has been resolved structurally and temporally, with the use of the multisite time-resolved FRET methodology. The present study establishes that the initial polypeptide chain collapse is not only heterogeneous but also structurally asymmetric and nonuniform. The population-averaged size for the segments spanning parts of the β-sheet decreases much more than that for the α-helix. Multisite measurements enabled specific and nonspecific components of the initial chain collapse to be discerned. The expanded and compact intermediate subensembles have the properties of a nonspecifically collapsed (hence, random-coil-like) and specifically collapsed (hence, globular) polymer, respectively. During subsequent folding, both the subensembles underwent contraction to varying extents at the four monitored segments, which was close to gradual in nature. The expanded intermediate subensemble exhibited an additional very slow contraction, suggestive of the presence of non-native interactions that result in a higher effective viscosity slowing down intrachain motions under folding conditions.