Both Acidic pH Value and Binding Interactions of Tartaric Acid With α-Glucosidase Cause the Enzyme Inhibition: The Mechanism in α-Glucosidase Inhibition of Four Caffeic and Tartaric Acid Derivates.

Wenyue Li,Lijun Sun,Wanshu Sun,Xuebo Liu,Yi Song,Xi Yang

doi:10.3389/fnut.2021.766756

Abstract

The inhibition mechanism of four caffeic and tartaric acid derivates, including caffeic acid (CA), tartaric acid (TA), caftaric acid (CFA) and chicoric acid (CHA) against α-glucosidase was characterized by substrate depletion, fluorescence quenching, isothermal titration calorimetry (ITC) and molecular docking. TA and CA were found with the highest and no inhibition effect respectively, and caffeoyl substitution at 2 and/or 3-OH of TA significantly decreased its inhibition. The enzyme inhibition effects of organic acids were not in an inhibitor concentration-dependent mode, and there was a rush increase in inhibition at a respective acidic pH value, especially for CFA and CHA, suggesting the important role of acidic pH in the enzyme inhibition for both compounds. Besides, CA, CFA and CHA were shown with strong quenching effects on α-glucosidase fluorescence because of π-conjugations between aromatic ring of caffeoyl moiety and that of enzyme fluorescent residues. However, no fluorescence quenching effect was observed for TA due to lack of aromatic ring. Additionally, a direct binding interaction behavior was observed for TA with α-glucosidase according to the fitted independent binding model in ITC, but not for CFA and CHA. Therefore, both acidic pH and binding interactions of TA with α-glucosidase resulted in the enzyme inhibition.

Highlights

Postprandial hyperglycaemia has been considered as one essential factor inducing disorder symptoms of carbohydrate metabolisms [1]
The threshold concentration of tartaric acid (TA) for the enzyme inhibition was determined as 0.625 mg/mL, and the values of caftaric acid (CFA) and chicoric acid (CHA) were 2.5 mg/mL and 5 mg/mL, respectively (Figure 1C)
This indicates that caffeoyl substitution at 2-OH and/or 3-OH of TA gradually decreased the inhibitory activity of the organic acid against α-glucosidase

Summary

Introduction

Postprandial hyperglycaemia has been considered as one essential factor inducing disorder symptoms of carbohydrate metabolisms [1]. As for flavonoids (one kind of polyphenols with C3-C6-C3 skeleton structures), the hydroxyl groups (-OH), especially that at 3- position of ring C and 5’-position at ring B play an important role in hydrogen bondings of myricetins with the active site of αglucosidase and in the enzyme inhibition [9]. The presence of galloyl moiety has been reported to enhance the inhibitory activity of tea polyphenols (catechins and theaflavins) against α-glucosidase by increasing the polyphenol-enzyme binding interactions. This is attributed to the fact that the three -OHs can form hydrogen bondings with the enzyme active site, and that the benzene ring can form π-stacking with the aromatic ring(s) of fluorescent amino acids of the enzyme [12]. It is necessary to explore more polyphenol structure-inhibitory relationships in order to increase the efficiency in discovery of natural inhibitors of α-glucosidase

Methods

Results

Conclusion