Membrane proteins: structure and function

Proteins residing in the lipid bilayer of a membrane surrounding a biological compartment make it possible for the compartment to engage in specialized functions. Since the determination of the X-ray structure of the bacterial photoreaction centre, high-resolution structures for more than 15 integral membrane proteins have been elucidated, mainly by X-ray and-partly by electron diffraction methods. The energy-conversion systems, namely those involving photosynthesis and respiration, contain various kinds of integral membrane proteins. The transmembrane parts of these membrane proteins fold into alpha-helices. Proton and electron transfer within the molecules have been inferred by inspecting the structures. Channels through which chemical substances are translocated selectively have important roles in various biological processes, such as mass transfer and signalling. Two structural architectures are known for the channels. One is an antiparallel beta-barrel consisting of 14-22 strands and the other is alpha-helical consisting of eight to ten alpha-helices. Each channel has a structure suitable for selective transfer of a specific substance. Two types of anchoring architectures have been elucidated by the X-ray method. Prostaglandin H2 synthase-1 has helical anchors located with parallel orientation along and closely following the membrane surface. OmpA has an anchor domain arranged in the form of an antiparallel beta-barrel consisting of eight strands.