Lipids transmembrane transport

In the human body choline is needed for the synthesis of phospholipids in cell membranes, methyl metabolism, transmembrane signaling and lipid cholesterol transport and metabolism [169]. It is transported into mammalian cells by a high-affinity sodium-dependent transport system. Intracellular choline is metabolized to phosphorylcholine, the reaction being catalyzed by the enzyme choline... 

Some microorganisms produce compounds that can become incorporated into lipid membranes and will facilitate the transmembrane transport of ions, notably K+. These natural products are antibacterial, killing bacteria by lethally altering the transmembrane ion flux. Such antibacterial molecules are called ionophores, or ion carriers, in contrast to other antibacterials, such as polyene antibiotics, which simply produce leakage through the cell membrane. 

One of the strategies described in Sect. 3 for BLM stabilization, mixing water-insoluble, nonlipid monomers with nonpolymerizable lipids, has also been applied to liposomes. Meier and coworkers [21] created stabilized, nanoscale bioreactors (Fig. 20) by incorporating OmpF, a channel-forming protein, into POPC vesicles to provide for passive transmembrane transport of low molecular weight compounds. p-Lactamase was entrapped during liposome formation, followed by addition of... 

Neutral lonophores. The relationship between equilibrium ionophore affinities and dynamic biological transmembrane transport is detailed in Figure 2. The transport cycle catalyzed by neutral ionophores is given on the left. Ionophore added to a biological membrane partitions predominately into the membrane. A portion of the ionophore diffuses to the membrane Interface where it encounters a hydrated cation. A loose encounter complex is formed followed by replacement of the cationic hydration sphere by engulfment of the cation by the ionophore. The dehydrated complex is lipid-soluble and hence can diffuse across the membrane. The cation is then rehydrated, released, and the uncomplexed lono-phore freed to return to its initial state within the membrane. 

The membranes surrounding each cell (plasma membrane) and the intracellular structures (endoplasmatic reticulum, Golgi apparatus, nuclear membrane, mitochondria membranes, other organelle membrane) are not only composed of lipids but incorporate proteins and steroids as well. Besides this, adsorbed proteins will greatly influence the functionality of the membrane. Integral proteins can span the entire membrane. A considerable fraction of these proteins facilitates transmembrane transport of species usually not permeating the membrane. The transport mechanism may be very different. One group involves carriers... 

This selective transport across cellular membranes is carried out by two broad classes of specialized proteins, which are assodated with or embedded in those lipid bilayers channels and transmembrane transporters. They work by different mechanisms Whereas channels catalyze the passage of ions (or water and gas in the case of the aquaporin channel) (Agre, 2006) across the membrane through a watery pore spanning the membrane-embedded protein, transporters are working via a cycle of conformational changes that expose substrate-binding sites alternately to the two sides of the membrane (Theobald Miller, 2010). 

Biological membranes, it should be noted, are far from homogeneous. They are usually a mixture of a variety of lipids and some smaller molecules, e.g., cholesterol, the exact composition of which is adjusted by the organism in response to environmental changes. In this context, proteins that perform membrane-based functions should truly be considered a part of the membrane, as many preserve neither their structure nor function outside of the membrane environment. Furthermore, although the biological functions performed at membranes are seemingly diverse, at some point in their action all make use of the relative impermeability of membranes. Thus, many of the proteins involved perform transmembrane transport of one type of solute or another. 

All of the transport systems examined thus far are relatively large proteins. Several small molecule toxins produced by microorganisms facilitate ion transport across membranes. Due to their relative simplicity, these molecules, the lonophore antibiotics, represent paradigms of the mobile carrier and pore or charmel models for membrane transport. Mobile carriers are molecules that form complexes with particular ions and diffuse freely across a lipid membrane (Figure 10.38). Pores or channels, on the other hand, adopt a fixed orientation in a membrane, creating a hole that permits the transmembrane movement of ions. These pores or channels may be formed from monomeric or (more often) multimeric structures in the membrane. 

The insulin receptor is a transmembrane receptor tyrosine kinase located in the plasma membrane of insulin-sensitive cells (e.g., adipocytes, myocytes, hepatocytes). It mediates the effect of insulin on specific cellular responses (e.g., glucose transport, glycogen synthesis, lipid synthesis, protein synthesis). 

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