COOPERATIVE CYCLIC INTERACTIONS INVOLVED IN METAL-BINDING TO PAIRS OFSITES IN EF-HAND PROTEINS

This study focuses on a closed net of electron-pair donor-acceptor int eractions, present in the core of all metal-bound EF-hand pairs, that link both metal ions across a short two-stranded beta-sheet, A molecul ar model based on the above cycle of interactions was studied using se mi-empirical molecular orbital quantum mechanical methods. The calcula tions indicate that the interactions in the model cycle are cooperativ e, that is, that the interaction energy of the cyclic structure is gre ater than that of the sum of isolated interactions between its compone nts. The cooperativity in this cycle can be attributed to an increase in the stability of the interactions resulting from a mutual polarisat ion of the associated groups, The predicted polarisation of the amide groups in the cycle is in agreement with experimental NMR N-15 deshiel ding observed for these amide groups upon metal binding, Experimental observations of strengthening of the beta-sheet hydrogen bonds are als o consistent with the model calculations. By this mechanism, the bindi ng of the first metal ion would enhance the binding of the second meta l ion, and thus, the intradomain cooperativity in cation binding of ca lmodulin and related EF-hand proteins can be ascribed, at least partly , to this short-range molecular mechanism. (C) 1998 Federation of Euro pean Biochemical Societies.