Interactions and Conformations of α-Helical Human Apolipoprotein CI on Hydrophilic and on Hydrophobic Substrates

Abstract

Interactions between amphiphilic R-helical human apolipoprotein CI (APO CI) adsorbed on hydrophilic and on hydrophobic surfaces were studied using an interferometric surface force apparatus in an effort to understand the surface conformation and the binding activity of this protein. We used mica as the hydrophilic substrate and polymerized octadecyltriethoxysilane (OTE)-covered mica as the hydrophobic substrate. The OTE monolayer and the OTE Langmuir-Blodgett film were studied using Brewster angle microscopy and atomic force microscopy, respectively. We found that interaction forces between layers of APO CI adsorbed on hydrophilic and on hydrophobic surfaces are mainly due to electrostatic double-layer forces at large surface distances and to steric repulsive forces at small distances. In some cases, no force was measured prior to finding a steric wall, suggesting that a complete neutralization of the surface charge was achieved by the protein adsorption. Protein layer thickness values allow us to give an image of the organization and conformation of the APO CI protein on surfaces. The adhesion obtained in both kinds of surfaces indicates that the interaction between the hydrophobic sides of the APO CI proteins is stronger than that between the hydrophilic sides of the protein.

Conformational and Disorder to Order Transitions in Proteins: Structure / Function Correlation in Apolipoproteins

“…These protein segments could take part in bridging between the two surfaces and thus be responsible for the attractive force. This kind of attractive force was also observed in adsorbed surfaces with Apo CI (Campos-Terán et al, 2004). In addition, studies of Apo AII and Apo CI monolayers have shown that it is possible to form a layer with protruding segments at an interface (Bolaños-García et al, 2001;Ruíz-García et al, 2003).…”

“…This experiment showed the reversibility of Apo AII adsorption process that produces in each stage different protein conformations. Also, it is noteworthy that quite similar force curves were observed in Apo CI (Campos-Terán et al, 2004), which also has a similar secondary structure but with a different peptide sequence, different net charge, and it is only monomeric. Therefore, the observed force curves seem to be a consequence of the particular features of the amphiphilic -helices.…”

“…Confirmation of this process was obtained by ellipsometry measurements done by our group (Ramos et al, 2008), which showed that sequential addition of protein (at least at high ionic strength) leads to an increase in the adsorbed amount of protein, as well as the protein layer thickness. However, for the case of Apo AII, the presence of a repulsive interaction showed that protein adsorption do not lead to charge neutralization of the mica surface charge as it was found for Apo CI (Campos-Terán et al, 2004). This is most likely due to the structural difference between both proteins, where the Apo CI monomer can more efficiently arrange so that it better match the surface charge compare to the Apo AII dimer.…”

“…As described below, this has been achieved employing Langmuir monolayers in conjunction with Brewster angle microscopy (BAM), atomic force microscopy (AFM) of Apos LB films (Bolaños-García et al, 1999Mas-Oliva et al, 2003;Xicohtencatl-Cortes et al, 2004a), grazing incidence X-ray diffraction on protein monolayers (Ruíz-García et al, 2003), and surface force measurements (SFA) (Campos-Terán et al, 2004;Ramos et al, 2008). Because at that time, we were unable to define whether the secondary structure of specific segments of Apolipoprotein CI (ApoCI) and AII (Apo AII) remained stable independently of their position at air/water and lipid/water interfaces, recently we have addressed the possibility that these segments responding to specific environmental changes and following disorder-to-order transitions might function as molecular switches that trigger function (Mendoza-Espinosa et al, 2008, 2009.…”

Biological physics in México

“…With regards to BP, Cinvestav's physics department is already developing a research core [34][35][36][37][38][39], which other departments at Cinvestav are also doing research in biological physics and related issues [40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55][56][57]. We also should account for the work at the biophysics group in the Instituto de Física y Matemáticas of the Universidad Michoacana San Nicolás Hidalgo [58], as well as the Instituto de Ciencias Físicas [59][60][61][62][63][64][65][66][67][68][69][70][71][72], Instituto de Física [73][74][75][76], and Facultad de Ciencias [77,78], all three at UNAM. Some very recent endeavors in biological physics research are under gestation at the División de Ciencias e Ingeniería at Universidad de Guanajuato in León (DCI-UG, formerly UG Physics Institute, IFUG...…”