ELECTROSTATIC COUPLING TO PH-TITRATING SITES AS A SOURCE OF COOPERATIVITY IN PROTEIN-LIGAND BINDING

Citation
V. Spassov et D. Bashford, ELECTROSTATIC COUPLING TO PH-TITRATING SITES AS A SOURCE OF COOPERATIVITY IN PROTEIN-LIGAND BINDING, Protein science, 7(9), 1998, pp. 2012-2025
Citations number
34
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Biology
Journal title
ISSN journal
0961-8368
Volume
7
Issue
9
Year of publication
1998
Pages
2012 - 2025
Database
ISI
SICI code
0961-8368(1998)7:9<2012:ECTPSA>2.0.ZU;2-A
Abstract
This paper describes an alternative mechanism for the cooperative bind ing of charged ligands to proteins. The ligand-binding sites are elect rostatically coupled to protein side chains that can undergo protonati on and deprotonation. The binding of one ligand alters the protein's p rotonation equilibrium in a manner that makes the the binding of the s econd ligand more favorable. This mechanism requires no conformational change to produce a cooperative effect, although it is not exclusive of conformational change. We present a theoretical description of the mechanism, and calculations on three kinds of systems: A model system containing one protonation site and two ligand-binding sites; a model system containing two protonation sites and two Ligand-binding sites; and calbindin D-9k, which contains two Ca2+-binding sites and 30 proto nation sites. For the one-protonation-site model, it is shown that the influence of the protonation site can only be cooperative. The compet ition of this effect with the anticooperative effect of ligand-ligand repulsion is studied in detail. For the two-protonation site model, th e effect can be either cooperative or, in special cases, anticooperati ve. For calbindin D-9k, the calculations predict that six protonation sites in or near the ligand-binding sites make a cooperative contribut ion that approximately cancels the anticooperative effect of Ca2+-Ca2 repulsion, accounting for more than half of the total cooperative eff ect that is needed to overcome repulsion and produce the net cooperati vity observed experimentally. We argue that cooperative mechanisms of the kind described here are likely when there is more than one ligand- binding site in a protein domain.