Peptide and protein interactions with biological membranes and membrane-like systems

Biological membranes are among the most important of structures, defining the boundaries that separate the inside and outside of a cell. They are dynamic structures, mostly composed by lipids and proteins, and are able to fulfill several roles which are indispensible for life, from systems of pumps and channels that allow the transport of specific molecules, to energy storage or information transduction.

Given that the thermodynamic stability of protein conformations depends critically on the interaction between the polypeptide chain and the solvent, structural analysis of transmembrane proteins often requires the use of organic solvents to function as isotropic membrane mimetics [1].

The growing availability of increasingly powerful computers combined with ongoing methodological developments, allow the computational simulation of membrane systems to be a reality and an important complement to experimental approaches.

Given our specialized methodologies, we are particularly interested in membrane and membrane-like systems where the effect of pH is also a key factor.

One example of such a system is the neuropeptide kyotorphin (KTP). This endogenous dipeptide is capable of triggering an analgesic response in the brain, and could potentially be an alternative to substances like morphine, if the mechanism by which it interacts with a receptor were better understood and if an effective way for it to cross the blood brain barrier were devised. Although its interaction with a receptor is fairly unknown, it has been proposed that the interaction between KTP and the lipid bilayer prior to receptor recognition may play a role in helping the peptide reach a conformation that favors receptor binding, something that has been previously studied both experimentally [2] and computationally [3].

We have found that the peptide favors the membrane phase, which is in accordance with previous experimental results [2]. Additionally, the effect of pH in the conformation of KTP is similar in water [4] and in the membrane.

It was also found that the presence of the membrane restricts the conformational freedom of KTP, whilst allowing it to satisfy the constraints necessary for binding to an opioid-like receptor [3]. In fact, the membrane seems to stabilize the conformers which are better suited for receptor binding.

Another example is the surfactant protein C (SP-C). SP-C is a component of the pulmonary surfactant film, a surface dynamic material which lines the alveolar epithelium, being important for reduction of the surface tension in lungs during the breathing cycle. This highly hydrophobic protein with three positively charged residues, more than 70% of non-polar residues and two covalently linked fatty acyl chains, adopts a mainly alpha-helix structure while associated with the membrane, presumably via its palmitoylated tails [5]. A chloroform/methanol/water mixture was used to resolve SP-C structure by NMR [6] and to perform several in vitro studies.

A comprehensive conformational analysis and the elucidation of the mode of association between SP-C and the membrane is essential to understand the interactions involved in structure stabilization and in some specific functions of the protein. Hence, we aim at studying the conformational behavior of SP-C in a chloroform/methanol/water mixture and the structural properties that are important in the association between SP-C and a model membrane, considering their dependence on pH.