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# 4: Membrane-Protein Interactions

Membrane proteins are common proteins that are part of, or interact with, biological membranes. Membrane proteins fall into several broad categories depending on their location. Integral membrane proteins are a permanent part of a cell membrane and can either penetrate the membrane (transmembrane) or associate with one or the other side of a membrane (integral monotopic). Peripheral membrane proteins are transiently associated with the cell membrane. Membrane proteins are often medically important—about a third of all human proteins are membrane proteins, and these are targets for more than half of all drugs. Nonetheless, compared to other classes of proteins, determining membrane protein structures remains a challenge in large part due to the difficulty in establishing experimental conditions that can preserve the correct conformation of the protein in isolation from its native environment.

• 4.1: Membrane Permeability
All cells are contained by a cell membrane (biomembrane) selectively open to some chemicals and ions but acts as a barrier to undesired components. Here the focus would be on biological membranes in the form of cell membranes, often consist of a phospholipid bilayer with embedded, integral, and/or peripheral proteins responsible for communication and transportation of chemicals and ions.
• 4.2: Insertion of Membrane Proteins into Lipid Membranes
Integral membrane proteins are ubiquitous throughout living organisms, ranging from prokaryotes to mammals, accounting for approximately 20-30% of all proteins. They perform a diverse set of functions ranging from signal transduction, to ion transport or even photosynthetic reaction centers. While their activity might vary dramatically, all these proteins experience a similar challenge. They must traverse the amphiphilic lipid membrane to reach their correctly folded state. The ways in which nat
• 4.3: Protein-lipid Interactions
lipids also have integral roles such as storing energy or being a major component of a membrane. Despite their individual importance, interaction of lipids and proteins  can provide functions that would not be possible individually. The greatest number of these interactions are seen in membranes, which are composed of a wide variety of lipids and proteins.
• 4.4: Physical Lipid Protein Interactions
In 1972, Singer and Nicolson proposed the fluid mosaic model which describes the lipids as a neutral environment to the activities of proteins. However, recent studies suggest that lipids are strongly coupled to membrane-proteins and at times the lipid bilayer has the leading role determining the function of membrane-proteins. One specific case of study highlighting the physical lipid-protein interactions concerns the mechanosensitive (MS) ion channels.
• 4.5: Nanoparticle Spontaneous Penetration and Assembly in and Through Membranes
Nanomaterial science is a rapidly evolving field of study to approach many scientific questions across fields. From sensors, drug delivery systems, cellular augmentation, and probes to highlight a handful of uses. However, these particles across application often come into contact with lipid membranes and can interact with the berries in a verity of ways. Nanoparticles (NPs) have been shown to restructure lipid membranes to penetrate or embed themselves into the lipids spontaneously. How they ar
• 4.6: Non-Membrane Lipid Assemblies
Amphipathic compounds (detergents) are a unique set of molecules with the ability of manipulation (distortion or formation) of the hydrophobic-hydrophilic interactions in biological samples. Detergents in an aqueous solution self-associate to colloid particles. The formation of the colloidal aggregates by detergents are termed micelles. In research micelle-forming detergents provide an amphipathic environment that can mimic lipid bilayers.