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1、 (2) 鞘磷脂 (sphingomyelin, SM)不含甘油，而代之以 鞘氨醇(C18)，鞘氨醇的C-1羟基，被磷酸胆碱化，长 链的脂肪酸结合在鞘氨醇的C-2位的氨基上。 2 2 1 1 2 2 3 3 3 3 2 2 1 1 磷酸鞘氨醇 1 1 2 2 3 3 4 4 5 5 固醇（steroid）：质膜中的固醇，以胆固醇 (cholesterol)为主，胆固醇酯很少，主要起调节生 物膜中脂质的物理状态。 极性羟基极性羟基 翻转(flip-flop) 侧向移动 摇动旋动 膜脂质运动方式示意 (4) 膜质脂的相变和分相： 相变（phase transition）和分相是生物膜结构 的特征之
2、一。在生理温度下，膜脂双层中一部分表 现为流动态(液晶态)，另一部分表现为固态(结晶态 )。因此，在膜平面上看，显示分相现象。 从液态变为晶态成为相变。引起相变的温度称 为相变温度。 2、膜蛋白在膜内的组装： 按蛋白质在膜内的部位分两类： 外在蛋白 （外周蛋白）：脂双层的内、外表面， 占20-30%，主要在内表面，水溶性蛋白，通过温 和的方法与膜分离。 内在蛋白 （固有蛋白）：镶嵌于脂双层内，占70 -80%，膜生物功能的主要承担者，与膜结合紧密 ，只能用去污剂使膜崩解。 大多数固有蛋白分两大类： 通过一段小的疏水区域连接或定位于脂双层 膜上，其余部分伸展出膜的一侧或两侧。 类似于球形，大部分
3、片段包埋于膜中，膜外 侧仅暴露很小部分。 Various ways in which membrane proteins associate with the lipid bilayer. 特殊的固有蛋白脂锚定蛋白（lipid-anchored proteins） 脂锚定蛋白可与脂质分子形成共价键，而脂质分 子的一部分位于膜双层中间，由此有效的把共价相连 的蛋白质锚定在膜上，调节膜蛋白的活性。 四种常见的连接方式： 1、肉豆蔻酸的酰胺键锚定：cAMP依赖的蛋白激酶的催化亚 基，G蛋白的亚基等 2、脂肪酸的硫酯键锚定：G蛋白偶联的受体，一些病毒的表 面糖蛋白 3、含异戊二烯基的硫醚键锚定：p21
4、ras蛋白，核膜层蛋白等 4、糖基磷脂酰肌醇锚定：如乙酰胆碱酯酶，甲状腺球蛋白等 The structure is shown (A) schematically and (B) in an electron micrograph. The arrangement shown in the drawing has been deduced mainly from studies on the interactions of purified proteins in vitro. Spectrin dimers are linked together into a netlike meshwor
5、k by junctional complexes composed of short actin filaments (containing 13 actin monomers), band 4.1, adducin, and a tropomyosin molecule that probably determines the length of the actin filaments. The cytoskeleton is linked to the membrane by the indirect binding of spectrin tetramers to some band
6、3 proteins via ankyrin molecules, as well as by the binding of band 4.1 proteins to both band 3 and glycophorin (not shown). The electron micrograph shows the cytoskeleton on the cytosolic side of a red blood cell membrane after fixation and negative staining. The spectrin meshwork has been purposel
7、y stretched out to allow the details of its structure to be seen. In a normal cell, the meshwork shown would be much more crowded and occupy only about one-tenth of this area. The polypeptide chain crosses the lipid bilayer seven times as a helices. The location of the retinal chromophore (purple) a
8、nd the probable pathway taken by protons during the light-activated pumping cycle are shown. The first and key step is the passing of a H+ from the chromophore to the side chain of aspartic acid 85 (red) that occurs upon absorption of a photon by the chromophore. Subsequently, other H+ transfers uti
9、lizing the hydrophilic amino acid side chains that line a path through the membrane complete the pumping cycle and return the enzyme to its starting state. Color code: glutamic acid (orange), aspartic acid (red), arginine (blue). 1、溶酶体膜 溶酶体是存于胞浆中的内含大量酸性水解酶 的小囊泡, 泡内pH大约在5.0左右。溶酶体膜的 组成特殊，膜上有H+-ATP酶，在A
10、TP存在下， 将胞外的H+泵入泡内，以维持其酸性。溶酶体 膜在细胞浆和溶酶体内容物之间构成了重要而有 效的屏障：溶酶体膜把各种水解酶和胞浆中的各 种其他物质分离开，防止有用的物质被水解，也 防止破坏细胞自身，溶酶体膜成为至关重要的防 护警戒圈。 溶酶体内的水解酶平时处于非活性状态，表明酶和底物均不能通透溶酶体膜 ，只有破坏了膜的完整性，酶活性才能充分的表露出来。 Figure 17-10. Synthesis of catalase and its incorporation into peroxisomes. Step 1: Four monomers are assembled and h
11、eme is added, forming the mature tetrameric catalase molecule. Steps 2 and 3: The cytosolic receptor protein PTS1R binds an SKL signal and escorts the catalase tetramer to the Pex14p receptor on the peroxisome membrane. Step 4: PTS1R returns to the cytosol to pick up another peroxisome-destined prot
12、ein. Step 4: The polypeptide chain elongates; then the signal sequence is cleaved by a signal peptidase in the ER lumen and is rapidly degraded. Step 5: The peptide chain continues to elongate and is extruded into the ER lumen through the translocon. In yeasts but not in mammalian cells, the ER chap
13、erone protein Hsc70 binds to the growing chain on the luminal surface and then, powered by the hydrolysis of ATP, releases the nascent chain. This cycle of binding and release of the nascent chain by yeast Hsc70 is essential for translocation to continue; it also facilitates the eventual folding of
14、many secreted polypeptides. The ATP needed for functioning of luminal chaperones is imported from the cytosol by an ATP/ADP antiporter in the rough ER membrane, similar to the one in the inner mitochondrial membrane . Step 6: The peptide chain continues to elongate until translation is completed. Th
15、en, the ribosomes are released, the remaining C-terminus of the secreted protein is drawn into the ER lumen, the translocon gate shuts, and the secreted protein assumes its final conformation. See T. A. Rapoport, 1991, FASEB J. 5:2792; S. L. Sanders et al., 1992, Cell 69:353; T. Powers and P. Walter, 1996, Nature 381:191; B. Martoglio and B. Dobberstein, 1996, Trends Cell Biol. 6:142; and G. Bacher et al., 1996, Nature 381:248.