Kinetic Analysis and Mechanism on the Inhibition of Chlorogenic Acid and Its Components against Porcine Pancreas α-Amylase Isozymes I and II

Chlorogenic acid (5-caffeoylquinic acid, 5-CQA) is a kind of polyphenol and is richly included in green coffee beans. The inhibitory effects of 5-CQA and its components, caffeic acid (CA) and quinic acid (QA), on the two porcine pancreas alpha-amylase (PPA) isozymes, PPA-I and PPA-II, were investigated using p-nitrophenyl-alpha-D-maltoside as substrate at pH 6.9 and 30 degrees C. The inhibition potencies of the respective inhibitors against both PPA isozymes were almost the same and in the order of 5-CQA > CA >> QA. Their IC(50) values were 0.07-0.08 mM, 0.37-0.40 mM, and 25.3-26.5 mM, respectively. The inhibition mechanisms of 5-CQA and CA were investigated by kinetic analyses, and the inhibitor constants K(i) and K(i)' (for the free enzyme and enzyme-substrate complex, respectively) were determined. It was indicated that 5-CQA and CA showed mixed-type inhibition with K(i) > K(i)' against both PPA-I and PPA-II. The binding of PPA-I or PPA-II with 5-CQA or CA was all exothermic and enthalpy-driven. QA is a poor inhibitor, and its inhibitory mode was unique and hardly analyzed by a simple Michaelis-Menten-type interaction between the enzyme and inhibitor. However, it was shown that the inhibitory activity of CA was enhanced 5 times by ester-bond formation with QA in the form of 5-CQA. These results provide us with significant hints for the development of alpha-amylase inhibitors useful for the prevention of diabetes and obesity.