KINETICS OF BINDING OF METHYL ALPHA-D-GALACTOPYRANOSIDE AND BETA-D-GALACTOPYRANOSIDE TO PEANUT AGGLUTININ - A C-13 NUCLEAR MAGNETIC-RESONANCE STUDY

The binding kinetics of methyl .alpha.- and methyl .beta.-D-galactopyranoside to the anti-T lectin from peanuts were studied by 13C NMR, employing methyl galactopyranosides specifically enriched in 13C at C-1. Association and dissociation rate constants and their activation parameters, are reported. The association rate constants, 4.6 .times. 104 M-1 s-1 for the .alpha.-galactopyranoside and 3.6 .times. 104 M-1 s-1 for the .beta.-galactopyranoside, are several orders of magnitude below those expected for a diffusion-controlled process. For both anomers, the association rate constant was temperature independent, implying that the association process occurs without a significant activation enthalpy. A considerable association activation entropy was found for both ligands. The dissociation rate constants were in the range of 9-46 s-1 within a temperature range of 5.degree.-35.degree. C for the .alpha.-galactopyranoside, and in the range of 9-39 s-1 within a temperature range of 5-25.degree. C for the .beta.-galactopyranoside. A considerable dissociation activation enthalpy of .apprx. 10 kcal mol-1 was found for both anomers. A 2-step binding model, consistent with the present NMR data and with previous UV and CD [circular dichroism] spectroscopic data, is presented.