Biomedical Engineering Reference
In-Depth Information
bilayer [ 6 ]. Chapter 5 is dedicated to discussing a comprehensive model of energetics
related to the channel formation and the general functions of MPs.
The transport properties of lipid bilayer membranes depend on the type and num-
ber of membrane constituents. However, the hydrophobic MP-lipid coupling which
generates certain lipid-protein, protein-lipid-protein, or lipid-protein-lipid complexes
truly controls most of the processes occurring across membranes. AMPs are active
in changing the biophysical properties of cell membranes. They often interact with
lipids [ 11 ] and create AMP-lipid complexes which lead to the creation of protein-
lined or lipid-lined well-structured ion channels/pores, less-structured ion flowing
pores, and localized disorders or defects, etc. In the formation of such special struc-
tures in a membrane involving AMPs and lipids, various mechanisms are observed,
which are mostly specific to AMPs. The complexes often appear with various dis-
tinguishable structures: a few of them are, for example, a linear β -helix created by
gramicidin A, a barrel-stave pore created by alamethicin, a toroidal pore created by
magainin, melittin, etc., lipidic channels created by ceramides (which is an example
of a non AMP-induced channel), and defects created by gramicidin S, etc. In this
chapter, we discuss the structures of these membrane-disrupting events, and the pri-
mary mechanisms which dictate their formation and functions. Based on the results
of various model studies, we create a complete platform to address the antimicrobial
effects of a group of AMPs. As mentioned earlier, this chapter will be organized
around the structural aspects of the events that change membrane transport proper-
ties, mainly induced by AMPs, but sometimes even by certain classes of lipids. All
these membrane events not only follow certain structural complexities due to the
biophysical coexistence between channel-forming agents and the lipid membrane,
as (for example) mentioned in Fig. 4.1 , but these phenomena often satisfy complex
energetics as well. We discuss the structural aspects of these processes in detail in
this chapter, and the energetic aspects will be explained in Chap. 5 .
4.1 Protein-Lined Ion Channels in Lipid Membranes
The membrane possesses pores or channels that allow a selective passage of metabo-
lites and ions into and out of the cell. They can even drag molecules from an area
of low concentration to an area of high concentration, working directly against
diffusion. An example of this is the sodium/potassium pump. Most of the work done
on the transport across membranes is done via ion pumps such as sodium-potassium
pumps. The energy required for the functioning of the pump comes from the hydrol-
ysis of ATP, in which a phosphorylated protein is identified as an intermediate in
the process. The hydrolysis of a phospho-protein usually causes a conformational
change that opens a pore that drives the sodium and potassium transport. Some mem-
brane proteins actively use energy from the ATP in the cell to perform mechanical
work. Here, the energy of a phosphate is used to exchange sodium atoms for potas-
sium atoms. It can be demonstrated that the free energy change in the hydrolysis of
a phospho-protein with a value of 9.3 kJ/mol will drive a concentration gradient of
 
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