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• W2005641892 abstract "The kinetic constants, Km, kcat, and KI of two series of peptic substrates and inhibitors have been determined at pH 2.0 and 4.5. In one series, the substrate or inhibitor contained an α-carboxyl group. Measurements of the pKa of this group on each compound indicated that the group is extensively titrated between the two pH values. In the other series, the substrate or inhibitor contained no group capable of being significantly titrated between pH 2.0 and 4.5. In addition, pH-activity profiles of representative substrates from both series were made under conditions where the initial velocity was approximately proportional to the initial substrate concentration. Comparisons of the kinetic constants at the two different pH values and of the pH-activity profiles indicated that little conformational change occurs in the binding region in the active center of pepsin when the pH is raised from 2.0 to 4.5 but that substrates and inhibitors with an α-carboxyl group are bound much less strongly at the higher pH because of the significant amount of ionization of this group that takes place at pH 4.5. In addition, Kcat values of substrates with α-carboxyl groups decrease more rapidly with pH increases than substrates without this moiety. The binding and Kcat data therefore show that substrates without an α-carboxyl group are much better substrates at high pH values than those with the acid group. Based on an assumed mechanism, calculation of pKa values of functional groups on the enzyme from the kinetic data indicates that there is a group in the enzyme-substrate complex with a pKa of 4.1 for substrates with an α-carboxyl group and 4.7 for substrates without this group that must be protonated for hydrolysis to take place. The data also indicate that there is a group (s) on the enzyme molecule of pKa 4.0–4.4 that must be protonated for substrate to be bound. Modification of the mechanism, for ionizable substrates, has the effect of bringing the pKa values of these groups closer to those calculated for nonionizable substrates and suggests that the intrinsic pKa values are intrinsic and the same for both types of substrates. The inter-pretation of the data in this publication and the data itself have been compared with results from other laboratories and explanations are given for the differences that have been noted between the findings of different laboratories." @default.
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• W2005641892 date "1969-05-01" @default.
• W2005641892 modified "2023-09-27" @default.
• W2005641892 title "The effect of pH on the kinetic constants of peptic substrates and inhibitors" @default.
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• W2005641892 doi "https://doi.org/10.1016/0003-9861(69)90410-x" @default.
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