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Membrane Proteins
All cells have membranes that are organized assemblies of proteins and lipids.
The presence of the membrane is essential for the cell life as they gives cells
their individuality from the environment. Prokaryotes have relatively simple
cell organization while eukaryotic cells contain internal membranes such as
the thylakoid membranes seen in the figure below (from Stryer, Biochemistry).
Biological membranes are fluid mosaics of the lipids and proteins (see figure below from Stryer, Biochemistry). The lipids are arranged as bilayers and the proteins are generally free to diffuse laterally in the lipid matrix. The function of the membranes are mediated by the integral membrane proteins that serve as channels, receptors, and energy transducers. The membranes are asymmetric with the inner and outer components having different components, however, this is not important for this section. Rather, we will first understand the properties that lead to the formation of bilayers and how the embedded membrane proteins can have so many critical functions.
As an example, membranes from photosynthetic purple bacteria have proteins that convert light energy into chemical energy. This essential function is performed by a series of coupled reactions involving a number of proteins as shown in the figure below (from Tony Croft’s web site http://www.life.uiuc.edu/crofts/ahab/home.html). Light is captured in the reaction center (far left) causing an electron to be transferred from the bacteriochlorophyll dimer to the quinone acceptors. Coupled to the electron transfer is proton transfer that is shuttled to the cytochrome bc1 complex (far right) by quinones and water soluble cytochromes. These reactions called the Q cycle lead to the transfer of electrons and protons across the cell membrane to be used to generate energy rich compounds such as NADPH and ATP.
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Photosynthesis Center Arizona State University Box 871604 Room PSD 209 Tempe, AZ 85287-1604
13 February 2006 |
phone: (480) 965-1963 fax: (480) 965-2747 |