Thermodynamic and kinetic studies of macromolec structure and enzymic mechanisms

National Institute Of Diabetes And Digestive And Kidney Diseases

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Abstract

(i) Optical studies on the structures of amyloid peptides and proteins are continuing.We are particularly interested in origin of the very intense CD spectrum which has been associated with the cross-beta structure of amyloid fibrils. (ii) In collaboration with the group of T. Dayie (Univ. of MD), we are investigating the effect of a small inert cosolute, trimethylamine oxide (TMAO), on the conformational changes of the SAM II riboswitch in the presence of increasing concentrations of the metabolite S-adenosyl-methionine (SAM) and Mg++. The free riboswitch was thought to be in an open or semi-random conformation in the absence of SAM and Mg++ and to become more folded and compact upon binding SAM and/or Mg++. Extensive measurements of the dependence of circular dichroism spectra of the riboswitch upon the concentrations of these small ligands may be quantitatively accounted for by a model, according to which SAM binds exclusively to a compact conformation. In the absence of SAM, the riboswitch seems to be a mixture of open and compact states. Low concentrations of Mg++ or TMAO bind to the open form(s), inhibiting the binding of SAM. At higher concentrations, cooperative binding of Mg++ or molecular crowding by TMAO, transform the riboswitch into the compact form, facilitating the binding of SAM.