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1、概述生物膜的组成和特点细胞的内膜系统,一.概 述,生物膜（Biological Membrane）是生物体在漫长的进化过程中逐渐形成的，使细胞有了相对稳定的内环境。包括：细胞外层的质膜 细胞器的膜 核膜,质膜使细胞成为生命活动的基本单位。内膜系统使分隔开的各个细胞器具有独特的功能。本身参与多种生物反应过程。,基本功能：,与生命科学中许多基本问题都有密切关系，如细胞起源、遗传信息传递、生物能量转换、物质运转、激素作用、神经传导、细胞免疫、细胞识别、细胞分化和增殖等。生物膜的结构与功能的研究是细胞生物学、分子生物学、生物物理学、医学、仿生学等许多领域的热点。,Three views of a ce
2、ll membrane.(A)An electron micrograph of a plasma membrane(of a human red blood cell)seen in cross section.(B and C)These drawings show two-dimensional and three-dimensional views of a cell membrane.(A,courtesy of Daniel S.Friend.)The lipid molecules are arranged as a continuous double layer about 5
3、 nm thick.,二.生物膜的组成和特点,由蛋白质、脂质和糖组成。不同类型的生物膜其脂质与蛋白质所占的比例不同。,膜蛋白质约占30 一40，多是糖蛋白、脂蛋白、或糖脂蛋白。膜脂质约占40一50，为磷脂、糖脂及胆固醇。糖占5，有糖蛋白，糖脂及糖脂蛋白,生物膜化学组成之间的连接方式：膜蛋白与膜脂之间为非共价键连接，包括表在膜蛋白与膜脂之间以离子键或氢键方式进行连接。内在膜蛋白由于分子表面多为非极性氨基酸残基，疏水性较大，与膜脂之间以疏水键方式进行连接。,膜脂与糖之间为共价键连接，包括磷脂分子中磷酸的-OH或鞘磷脂分子中鞘氨酸的-OH与糖的-OH，形成O-糖苷键方式进行连接。膜蛋白与糖之间为共价
4、键连接，包括膜蛋白分子中的天门冬氨酰的氨基与糖的-OH形成N-糖苷键方式，膜蛋白分子中的丝氨酸或苏氨酸的-OH与糖的O-糖苷键方式进行连接。,Schematic diagram of typical membrane proteins in a biological membrane.The phospholipid bilayer,the basic structure of all cellular membranes,consists of two leaflets of phospholipid molecules whose fatty acyl tails form the hyd
5、rophobic interior of the bilayer;their polar,hydrophilic head groups line both surfaces.Most integral proteins span the bilayer as shown;a few are tethered to one leaflet by a covalently attached lipid anchor group.Peripheral proteins are primarily associated with the membrane by specific protein-pr
6、otein interactions.Oligosaccharides bind mainly to membrane proteins;however,some bind to lipids,forming glycolipids.,(1)甘油磷脂（磷酸甘油酯）包括甘油骨架，两个脂肪酸及磷酸化的醇。,甘油,L-磷脂酸,（一）、膜脂质的特点,极性部分（极性头）,胆碱磷脂（磷脂酰胆碱，卵磷脂）,磷脂结构示意,The effect of a double bond,磷酸基分别与丝氨酸、乙醇胺、胆碱或肌醇结合即形成:丝氨酸磷脂(Phosphatidylserine PS)，又称磷脂酰丝氨酸 乙醇胺磷
7、脂(Phosphatidylethanoamine PE)，磷脂酰乙醇胺 胆碱磷脂(Phosphatidylcholine PC)，磷脂酰胆碱 肌醇磷脂(phosphatidylinositol PI)，磷脂酰肌醇,组成膜主要成分的四种磷脂,磷脂酰丝氨酸（PS）,磷脂酰乙醇胺(PE),磷脂酰肌醇,(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,（神经）鞘氨醇,葡萄糖脑苷脂,油酸,磷脂酰胆碱,（神经）鞘磷脂,固醇（steroid）：质膜中的固醇，以胆固醇(cholesterol
8、)为主，胆固醇酯很少，主要起调节生物膜中脂质的物理状态。,胆固醇的量与磷脂有一定比例，常以测定胆固醇磷脂比例来鉴定膜是否有病变，此比值称cp比，各种细胞膜的cp值相差较多，约为0.030.1。,形成脂质双层结构 膜磷脂的极性头部通过疏水力、静电引力和氢键，对水有强烈的亲和力，因而排列在外，与外界(或胞浆)水溶性环境相邻；其非极性区互相聚集，尽量避免与水接触，所以排列在内部。两个分子磷脂的非极性区尾尾相联，决定了脂质双层的结构。,2.膜脂质的结构特点,Figure 10-10.Four major phospholipids in mammalian plasma membranes.Note
9、that different head groups are represented by different symbols in this figure and the next.All of the lipid molecules shown are derived from glycerol except for sphingomyelin,which is derived from serine.,Experimental formation of pure phospholipid bilayers.A preparation of biological membranes is
10、treated with an organic solvent,such as a mixture of chloroform and methanol(3:1),which selectively solubilizes the phospholipids and cholesterol.Proteins and carbohydrates remain in an insoluble residue.The solvent is removed by evaporation.If the lipids are mechanically dispersed in water,they spo
11、ntaneously form a liposome,shown in cross-section,with an internal aqueous compartment.(Bottom right)A planar bilayer,also shown in cross-section,can form over a small hole in a partition separating two aqueous phases;such bilayers are often termed“black lipid membranes”because of their appearance.,
12、(2)不对称性：膜脂质双层两侧分布的不对称性决定于磷脂的头部，脂质双层内侧是两个含有氨基的磷脂(PS、PE)，有较强的负电性;PC及SM在脂质双层的外侧。内外两侧磷脂的脂肪酸也不完全相同，PC及SM多为饱和脂肪酸，PS、PE含不饱和脂肪酸较多。,Figure 10-11.The asymmetrical distribution of phospholipids and glycolipids in the lipid bilayer of human red blood cells.The symbols used for the phospholipids are those introd
13、uced in Figure 10-10.In addition,glycolipids are drawn with hexagonal polar head groups(blue).Cholesterol(not shown)is thought to be distributed about equally in both monolayers.,50%40%30%20%10%0 10%20%30%40%50%,人红细胞膜中磷脂分布的不对称性,膜外侧,膜内侧,磷脂酰胆碱,磷脂酰乙醇胺,磷脂酰丝氨酸,鞘磷脂,总磷脂,(3)膜脂质的运动:测定磷脂不同部位的运动速度及偏转的角度，发现其极性头
14、部运动较快，脂肪酸梁最慢。膜脂质运动方式有五种：脂肪酰链的旋转异构化运动 磷脂分子围绕其长轴的旋转运动 磷脂侧向扩散运动 脂质分子在脂双层之间的翻转运动 脂肪酰链垂直于膜双分子层平面轴的振荡运动,翻转(flip-flop),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。,磷脂的相变与其成分和环境有密切关系:（1）脂肪酰链的饱和度及链的长短（愈短、
15、愈不饱和，烃链愈不易靠紧，相变温度愈低）（2）胆固醇对流动性的影响（增加力学稳定；防止低温引起的相变）（3）温度,Figure 10-7.Influence of cis-double bonds in hydrocarbon chains.The double bonds make it more difficult to pack the chains together and therefore make the lipid bilayer more difficult to freeze.,Heat induces transition from a gel to a fluid ov
16、er a temperature range of only a few degrees.The fluid phase is favored by the presence of short fatty acyl chains and by a double bond in the chains;thus these structural features reduce the melting temperature of bilayers.,Alternative forms of the phospholipid bilayer.,生物膜所有的生物活性都由蛋白质来实现。功能越复杂，膜上所
17、含蛋白质量就越大。膜内的蛋白有单纯的蛋白，但更多的是糖蛋白。糖蛋白占膜蛋白中的比例大，结构复杂，功能多，许多反应都是糖蛋白中糖链起着关键性的作用，它是膜的门户，有人称它为“天线”。,（二）、膜蛋白的特点,1、糖蛋白的结构：糖与多肽的结合有两种类型：N-糖苷型：葡萄糖与蛋白质的天门冬酰胺结合 O-糖苷型：N-乙酰氨基半乳糖与丝氨酸或苏氨酸结合,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
18、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.,a single a helix,have a covalently attached fatty acid chain inserted in the cytosolic lipid monolayer.as multiple a helices,as a rolled-u
19、p b sheet(a b barrel).Some of these single-pass and multipass proteins have a covalently like(1).Other membrane proteins are exposed at only one side of the membrane.Anchored to the cytosolic surface by an amphipathic a helix that partitions into the cytosolic monolayer of the lipid bilayer through
20、the hydrophobic face of the helix.Attached to the bilayer solely by a covalently attached lipid chain either a fatty acid chain or a prenyl group in the cytosolic monolayer Via an oligosaccharide linker,to phosphatidylinositol in the noncytosolic monolayer.(7,8)Many proteins are attached to the memb
21、rane only by noncovalent interactions with other membrane proteins.,已分离提纯的膜蛋白有几百种之多，这些蛋白质在膜内组装各不相同，大约可分为6种类型：,形成通道,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
22、二烯基的硫醚键锚定：p21ras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,Membrane protein attachment by a fatty acid chain or a prenyl group.,(A)A fatty acid chain(myristic acid)is attached via an amide linkage to an N-terminal glycine.,(B)A prenyl group,(C)a myristyl anchor,(D)a farnesyl anchor,酰胺键,硫醚键,The covalent at
23、tachment of either type of lipid can help localize a water-soluble protein to a membrane after its synthesis in the cytosol.A fatty acid chain(myristic acid)is attached via an amide linkage to an N-terminal glycine.A prenyl group is attached via a thioether linkage to a cysteine residue that is init
24、ially located four residues from the proteins C-terminus.After this prenylation,the terminal three amino acids are cleaved off,and the new C-terminus is methylated before insertion into the membrane.Palmitic acid,an 18 carbon saturated fatty acid,can also be attached to some proteins via thioester b
25、onds formed with internal cysteine side chains.This modification is often reversible,allowing proteins to become recruited to membranes only when needed.The structures of two lipid anchors are shown below:(C)a myristyl anchor(a 14-carbon saturated fatty acid chain),(D)a farnesyl anchor(a 15-carbon u
26、nsaturated hydrocarbon chain).,Only the a-carbon backbone of the polypeptide chain is shown,with the hydrophobic amino acids in green and yellow.The polypeptide segment shown is part of the bacterial photosynthetic reaction center,A segment of a transmembrane polypeptide chain crossing the lipid bil
27、ayer as an a helix.,(Based on data from J.Deisenhofer et al.,Nature 318:618 624,1985,and H.Michel et al.,EMBO J.5:1149 1158,1986.),Fig 22.A single-pass transmembrane protein.Note that the polypeptide chain traverses the lipid bilayer as a right-handed a helix and that the oligosaccharide chains and
28、disulfide bonds are all on the noncytosolic surface of the membrane.The sulfhydryl groups in the cytosolic domain of the protein do not normally form disulfide bonds because the reducing environment in the cytosol maintains these groups in their reduced(-SH)form.,The detergent disrupts the lipid bil
29、ayer and brings the proteins into solution as protein-lipid-detergent complexes.The phospholipids in the membrane are also solubilized by the detergent.,Fig24.Solubilizing membrane proteins with a mild detergent,Sodium dodecyl sulfate(SDS)is an anionic（阴离子）detergent,and Triton X-100 is a nonionic de
30、tergent.The hydrophobic portion of each detergent is shown in green,and the hydrophilic portion is shown in blue.The bracketed portion of Triton X-100 is repeated about eight times.,Fig 25.The structures of two commonly used detergents,In this example,functional Na+-K+pump molecules are purified and
31、 incorporated into phospholipid vesicles.The Na+-K+pump is an ion pump that is present in the plasma membrane of most animal cells;it uses the energy of ATP hydrolysis to pump Na+out of the cell and K+in.,Fig26.The use of mild detergents for solubilizing,purifying,and reconstituting functional membr
32、ane protein systems,Fig31.The spectrin-based cytoskeleton on the cytosolic side of the human red blood cell membrane.,(B,courtesy of T.Byers and D.Branton,Proc.Natl.Acad.Sci.USA 82:6153 6157,1985.National Academy of Sciences.),The structure is shown(A)schematically and(B)in an electron micrograph.Th
33、e 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 meshwork by junctional complexes composed of short actin filaments(containing 13 actin monomers),band 4.1,adducin,and a tropomy
34、osin 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 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
35、 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 purposely stretched out to allow the details of its structure to be seen.In a normal cell,the meshwork shown would be much more crowded a
36、nd occupy only about one-tenth of this area.,Fig32.Converting a single-chain multipass protein into a two-chain multipass protein.(A)Proteolytic cleavage of one loop to create two fragments that stay together and function normally.(B)Expression of the same two fragments from separate genes gives ris
37、e to a similar protein that functions normally.,(Adapted from H.Luecke et al.,Science 286:255 260,1999.),Fig37.The three-dimensional structure of a bacteriorhodopsin molecule.,The polypeptide chain crosses the lipid bilayer seven times as a helices.The location of the retinal chromophore(purple)and
38、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 utilizing the
39、 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).,Fig38.The three-dimensional structure of the photosynthetic reaction center of the bact
40、erium Rhodopseudomonas viridis.,(Adapted from a drawing by J.Richardson based on data from J.Deisenhofer,O.Epp,K.Miki,R.Huber,and H.Michel,Nature 318:618 624,1985.),The structure was determined by x-ray diffraction analysis of crystals of this transmembrane protein complex.The complex consists of fo
41、ur subunits L,M,H,and a cytochrome.The L and M subunits form the core of the reaction center,and each contains five a helices that span the lipid bilayer.The locations of the various electron carrier coenzymes are shown in black.Note that the coenzymes are arranged in the spaces between the helices.
42、,(Adapted from H.Luecke et al.,Science 286:255 260,1999.),The cell coat,or glycocalyx.This electron micrograph of the surface of a lymphocyte stained with ruthenium red emphasizes the thick carbohydrate layer surrounding the cell.(Courtesy of Audrey M.Glauert and G.M.W.Cook.),Fig45.Simplified diagra
43、m of the cell coat(glycocalyx).The cell coat is made up of the oligosaccharide side chains of glycolipids and integral membrane glycoproteins and the polysaccharide chains on integral membrane proteoglycans.In addition,adsorbed glycoproteins and adsorbed proteoglycans(not shown)contribute to the gly
44、cocalyx in many cells.Note that all of the carbohydrate is on the noncytosolic surface of the membrane.,3.膜蛋白的运动：膜蛋白与膜脂相似，在膜内是可以运动的。一方面它有本身的运动，另一方面它镶嵌在脂质之中，脂质运动对它有影响。膜蛋白自身运动有两种形式：在膜的平面作侧向扩散运动沿着膜的平面垂直轴作旋转运动,1膜脂组分对膜蛋白功能的影响：生物膜的内在蛋白需要一定量的膜脂才能维持其构象，表现其活性。,纯化的膜蛋白所含膜脂量如果降到一定程度，有些膜蛋白的活性就下降，甚至完全失活。,（三）膜脂与膜蛋
45、白的相互作用,2、膜蛋白的周围有着一层不可活动的“界面脂”。3、膜脂流动性对膜蛋白功能的影响 对物质运输的影响 对酶活性的影响 对受体活性的影响 4、膜蛋白对膜质的影响：蛋白质可使脂的疏水链排列趋于无序，使脂质更趋于流动。,如蛋白质加到豆蔻酸磷脂酰胆碱（DMPC）内之后，脂链侧向相互作用消失，破坏脂质自有序排列，,三.细胞的内膜系统,真核细胞中，细胞膜占很大的比重，形成各种细胞器执行各种生理功能。,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ATP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
46、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。,2.过氧化氢酶体-利用氧的场所 过氧化氢酶体是有单层膜包含着内容物，胞内无DNA或核糖体，所以它膜上的蛋白质及脂质都来自胞浆。过氧化氢酶体分布很广，各种细胞都有，特别是肝细胞含量极多。在肝细胞内可找到三种氧化酶：D-氨基酸氧化酶、尿酸氧化酶、过氧化氢酶。,Figure 17-10.Synthesis of ca
47、talase and its incorporation into peroxisomes.,Step 1:Four monomers are assembled and heme 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 membr
48、ane.,Step 4:PTS1R returns to the cytosol to pick up another peroxisome-destined protein.,Newly made catalase subunits are released from polyribosomes into the cytosol as apo-catalase,a monomer that contains a C-terminal SKL uptake-targeting sequence(red)but lacks an iron-containing heme group.In the
49、 model depicted here,the catalase-PTS1R complex is transported across the membrane and then dissociates in the lumen.As-yet uncharacterized proteins on the peroxisomal membrane probably form part of the receptor-transport channel.In an alternative model,PTS1R delivers catalase to the Pex14p receptor
50、 but does not enter the lumen itself.,See R.A.Ruchubinski and S.Subramani,1995,Cell 83:525;J.A.McNew and J.M.Goodman,1996,Trends Biochem.Sci.21:54;and M.Albertini et al.,1997,Cell 89:83.,3.核膜 核被膜：核被膜包括核膜，核孔复合物及核纤层蛋白。核膜是双层脂膜，外层膜的化学组成及功能与内质网相似，粗面内质网和核外膜都有核糖体，可合成蛋白，内质网与核外膜是连续的。内层膜，有许多特殊的内在蛋白被核纤层蛋白围着。,核
