Schulz Georg E
Institut für Organische Chemie und Biochemie, Albert-Ludwigs-Universität, Albertstr. 21, Freiburg im Breisgau, Germany.
Biochim Biophys Acta. 2002 Oct 11;1565(2):308-17. doi: 10.1016/s0005-2736(02)00577-1.
Integral membrane proteins come in two types, alpha-helical and beta-barrel proteins. In both types, all hydrogen bonding donors and acceptors of the polypeptide backbone are completely compensated and buried while nonpolar side chains point to the membrane. The alpha-helical type is more abundant and occurs in cytoplasmic (or inner) membranes, whereas the beta-barrels are known from outer membranes of bacteria. The beta-barrel construction is described by the number of strands and the shear number, which is a measure for the inclination angle of the beta-strands against the barrel axis. The common right-handed beta-twist requires shear numbers slightly larger than the number of strands. Membrane protein beta-barrels contain between 8 and 22 beta-strands and have a simple topology that is probably enforced by the folding process. The smallest barrels form inverse micelles and work as enzymes or they bind to other macromolecules. The medium-range barrels form more or less specific pores for nutrient uptake, whereas the largest barrels occur in active Fe(2+) transporters. The beta-barrels are suitable objects for channel engineering, because the structures are simple and because many of these proteins can be produced into inclusion bodies and recovered therefrom in the exact native conformation.
整合膜蛋白有两种类型，即α螺旋蛋白和β桶状蛋白。在这两种类型中，多肽主链的所有氢键供体和受体都得到完全补偿并被掩埋，而非极性侧链则指向膜。α螺旋类型更为常见，存在于细胞质（或内膜）中，而β桶状蛋白则存在于细菌的外膜中。β桶状结构由链的数量和剪切数来描述，剪切数是衡量β链相对于桶轴倾斜角度的一个指标。常见的右手β扭转需要剪切数略大于链的数量。膜蛋白β桶状结构包含8到22条β链，具有一种简单的拓扑结构，这种结构可能是由折叠过程所决定的。最小的桶状结构形成反胶束并作为酶发挥作用，或者与其他大分子结合。中等大小的桶状结构形成或多或少具有特异性的孔用于营养物质的摄取，而最大的桶状结构存在于活性Fe(2+)转运蛋白中。β桶状结构是通道工程的合适对象，因为其结构简单，而且许多这类蛋白质可以产生到包涵体中，并以精确的天然构象从中回收。
