BIOCHEMISTRY - L. Stryer - 1984

VOLUME 1

PART I. CONFORMATION AND DYNAMICS

CHAPTER 10. AN INTRODUCTION TO BIOLOGICAL MEMBRANES

10.18. Three-dimensional model of membranes from electron microscopy

As we have seen from the Examples of oxygen carriers and Enzymes described in previous chapters, X-ray crystallography is a reliable method for studying the three-dimensional Structure of soluble Proteins. Is X-ray crystallography applicable to Membrane Proteins? The difficulty is that, until recently, it has not been possible to obtain integral membrane proteins in the form of three-dimensional crystals. However, some membrane proteins form a regular lattice in the plane of the membrane, i.e., two-dimensional crystals. Structural analysis of these crystalline forms can be carried out using Electron Cell/15.html">Microscopy; in particular, such a study was successfully performed on the purple membrane of Halobacterium halobium, a bacterium that lives in a salty environment. The purple membrane is a specialized region of The Cell membrane containing Bacteriorhodopsin, a 25-kDa protein that converts light energy into a transmembrane proton gradient used for ATP synthesis (Section 19.21). Crystalline sheets or discs up to 1 μm in diameter were obtained. Because each of them contained about 20,000 bacteriorhodopsin molecules, it was possible to obtain images using a very weak electron beam, thereby minimizing radiation damage. Furthermore, unstained specimens could be used to obtain high-resolution images. A single electron micrograph of a crystalline sheet of the purple membrane yields a view of the structure projected onto a plane. The next step is to obtain images of successive sections and process the information contained in about 20 micrographs using Fourier series.

Using this approach, Richard Henderson and Nigel Unwin (R. Henderson, N. Unwin) constructed a three-dimensional model of the purple membrane at 7 Å resolution (Fig. 10.32). The proteins of this membrane contain seven closely packed α-helices that run nearly perpendicular to the plane of the membrane, spanning a width of about 45 Å. The space between the protein molecules is filled with a lipid bilayer. It is highly probable that the structural principle of the purple membrane is also utilized in the Organization of other integral proteins. In particular, membrane pumps and channels are thought to contain α-helical regions spanning the bilayer.

Class="center">Fig. 10.32. A - model of bacteriorhodopsin constructed from a three-dimensional map at 7 Å resolution. B - explanatory diagram showing the arrangement of α-helical regions in The Lipid Bilayer. The connections between these helices have not yet been determined