Induced changes in protein secondary structure due to internal sequence modifications and ligand binding

Abstract

Secondary structure of protein sequences is dependent on both internal and external interactions of amino acid residues and ligand-binding partners. Internal residue features can leave a protein in an ordered, folded state, or in a disordered, unfolded state. Structural characteristics can be further influenced by protein-ligand binding interactions with a lipid membrane surface. Structural features can be altered upon membrane binding, causing disordered proteins to become more ordered in structure. The stabilizing influence of methionine (Met) oxidation in an aromatic-Met hydrogen bonding interaction, within a small, 15-residue peptide was studied using Differential Scanning Calorimetry (DSC) and Circular Dichroism (CD) Spectroscopy to observe changes in structural strength. By observing the ordered to disordered transition of this peptide, changes in enthalpy and transition temperature were determined. This added aromatic-oxidized Met interaction causes a stronger and more stable ordered peptide structure due to internal residue interaction. Multiple intrinsically disordered proteins were studied upon binding to a membrane surface to determine the influence that physiological membrane surfaces and curvature have on appropriate conformer formation of synaptic vesicle (SV) binding proteins. Various C2 domains of synaptotagmin I (Syt I) and α-Synuclein (αS) were studied using DSC, CD and Carboxyfluorescein (CF) release assays. The proper folding of these proteins is important for their necessary function, and misfolding or sequence mutation can significantly alter their functionality within neuronal environments. These studies are vital to enhance understanding of the dependence of internal residue and membrane binding interactions on structural properties of proteins, as these specific interactions are not limited to individual systems.