Lipids and Biological Membranes PDF

Lipids and biological membranes: Lipids are the fourth major group of molecules found in all cells. Lipds are not polymeric unlike other macromolecules, however, they do aggregate. Perform 3 biological functions. They are essential components of biological membranes, lipids containing hydrocarbon c...

Lipids and biological membranes: Lipids are the fourth major group of molecules found in all cells. Lipds are not polymeric unlike other macromolecules, however, they do aggregate. Perform 3 biological functions. They are essential components of biological membranes, lipids containing hydrocarbon chains serves as energy storage, many cellular signaling involve lipid molecules. Above is *Mycobacterium tuberculosis*, that causes tuberculosis. It evades both human immune system and antibiotic drugs, due to thick cell wall containing enormous lipid known as mycolic acid. Lipids are substances of biological origin that are soluble in organic solvents (chloroform, methanol). They can be separated chromatographically and identified by mass spectrometry. Lets see structural and functional properties of major classes of lipids. Fatty acids: these are carboxylic acids with long-chain hydrocarbon side groups. They usually occur in esterified form as major component of the various lipids. Structural formula of some C~18~ fatty acids are shown here. The double bonds all have the cis configuration. The more common biological fatty acids are listed here. In higher plants and animals , the predominant fatty acid residues are those of the C~16~ and C~18~ species: palmitic acid, oleic acid, linoleic acid and stearic acid. Fatty acids with \20 carbon atoms are uncommon. Most fatty acids have an even no. of carbon atoms because they are biosynthesized by the catenation of C~2~ unit. Unsaturated fatty acids contain double bonds, and are often polyunsaturated (2 or more double bonds). ꞷ-3 and ꞷ-6 fatty acid are example of important polyunsaturated fatty acid (e.g, α-Linolenic acid and linoleic acid), nomenclature based on double bonded carbon atom as counter from methyl terminal (ꞷ) end of the chain. Saturated fatty acids are fully reduced, highly flexible and can assume wide range of conformations. The fats and oils that occur in plants and animals consist largely of mixtures of triacylglycerols (triglycerides). These nonpolar, water-insoluble substances are fatty acid triesters of glycerol. **Triacylglycerol** acts as **energy** **reservoir** in animals and are therefore their most abundant class of lipids. Triacylglycerol differ in the placement of their 3 fatty acid residues. Like shown here, 1-Palmitoleoyl-2-linoleoyl-3-stearoylglycerol. Triacylglycerol functions as energy reservoir (are less oxidized than carbohydrates and proteins and hence yield significantly more energy per unit mass on complete oxidation). Fat provides about six times the metabolic energy of an equal weight of hydrated glycogen. In animals adipocytes (fat cells) are specialized for the synthesis and storage of triacylglycerol. Figure shows scanning electron micrograph of adipocytes. Each adipocyte contain a fat globule that occupies nearly entire cell Glycerophospholipids (or phosphoglycerides) are the major lipid component of biological membranes. They consist of glycerol-3-phosphate whose C1 and C2 positions are esterified with fatty acids. Also, the phosphoryl group is linked to another usually polar group, X. In figure a) is backbone, glycerol-3-phosphate. In b) the the general formula of glycerophospholipids , R1 and R2 are long chain hydrocarbon chains of fatty acids. Glycerol-3-phosphate and glycerophospholipids are chiral compounds. So, glycerophospholipids are amphiphilic molecules, with nonpolar aliphatic (hydrocarbon) "tails" and polar phosphoryl --X heads. The simplest glycerophospholipids, in which X = H, are phosphatidic acids; present in small amount in biological membranes. Saturated C~16~ or C~18~ fatty acids usually occur at the C1 position and unsaturated C~16~ to C~20~ at C2 position. Individual glycerophospholipids are named according to the identities of these fatty acid residues. In figure. The glycerphospholipid 1-stearoyl-2-oleoyl-3-phosphatidylcholine is shown A) Molecular formula in Fischer projection. B) Energy minimized space filling model with C green, H white, N blue , O red, and P orange. Note that how the unsaturated oleoyl chain is bent compared to the saturated stearoyl chain. The glycerophospholipid containing two palmitoyl chains (Dipalmitoylphosphatidylcholine , DPPC)is an important component of lung surfactant (the surface of the cell that form alveoli of luns are coated with surfactants) and is essential for normal pulmonary function. Phospholipases Hydrolyze Glycerophospholipids. Figure shows the action of phospholipases. Phospholipase A~2~ hydrolytically excises the C2 fatty acid residues from a phospholipid to yield the corresponding lysophospholipid. The bonds hydrolyzed by other types of phospholipases, which are named according to their specifies are also indicated. 1,2 --Diacylglycerol, derived from membrane lipids by the action of phospholipase C, is an intracellular signal molecule that activates a protein kinase. Phospholipase A~2~ is a small about 14 kD, protein, of about 125 amino acid residues. Figure shows the model of phospholipase A~2~ and a glycerophospholipid. The X-ray structure of the enzyme from cobra venom is shown with space-filling model of dimyristoylphosphatidylethanolamine in its active site as located by NMR methods. A Ca^2+^ ion in the active site is shown in magenta. Plasmalogens are glycerophospholipids in which the C1 subsituent of the glycerol moiety is linked via an α, β- unsaturated ether linkage in the cis configuration rather than through an ester linkage. Ethanolamine, Choline and Serine form the most common plasmalogen head group. Sphingolipids are also major membrane components. Most of them are derivatives of the C18 amino alcohol sphingosine, whose double bond has the trans configuration. The N-acyl fatty acid derivatives of sphingosine are known as ceramides. Ceramides are the parent compounds of more abundant sphingolipids. Sphingomyelins are ceramides bearing either a phosphotidylcholine or a phophotidylethanolamine head group, (can be classified as sphingophospholipids). They typically make up 10 to 20 mol % of plasma membrane lipids. Figure shows a sphingomyelin. A) molecular formula , B) Energy-minimized space-filling model with C green , H white , N blue, O red and P orange. This is electron micrograph of myelinated nerve fibres. Thin cross-sectional view shows the spirally wrapped membranes around each nerve axon. The myelin sheath may be 10-15 layers thick. Its high lipid content makes it an electrical insulator. Cerebrosides are ceramides with head groups that consist of a single sugar residue. (like galactocerebrosides and glucocerebrosides) Gangliosides are the most complex glycosphingolipids. They are ceramides with attached oligosaccharides that include atleast one sialic acid residue. The structure of gangliosides G~M1~, G~M2~ and G~M3~ (3 of the 100s). Are shown here. G~M2~ and G~M3~ differ from G~M1~ only by the sequential absences of the terminal D-galactose and N-acetyl-D-galactosamine residues. Other gangliosides have different oligosaccharides head group. Energy minimized space-filling model of G~M1~ with C green, H white, N blue, and O red are shown here. [Steroids] are mostly of [eukaryotic origin]. They are derivatives of cyclopentanoperhydrophenanthrene, a compound that [consist of four fused nonplanar rings]. [Cholestero]l is the **most abundant steroid in animals**, is further [classified as sterol] because of its [C3-OH group]. A) Shows its [structural formula] with the [standard numbering system]. And B) is Energy minimized s[pace-fillin]g model with C green, H white, and O red. Cholesterol is a [major component] of animal [plasma membrane (constituting 30 -40%)] Its [polar OH group] give sit a [weak amphiphilic character] and [fused ring system provide greater rigidity]. Cholesterol [can] also [be] [esterified to long chain fatty acids] **to form cholesterol esters** such as cholesterol stearate. **Plant** contain [little cholesterol] but [synthesize other sterols.] [In mammals cholesterol] is the **[metabolic precursor of steroid hormones]**. They are classified according to physiological response they evoke. **Glucocorticoids** such as **[cortiso]l**, [effect carbohydrate], [lipid] and [protein] [metabolism], [influence inflammatory reactions] and [capacity to cope with stress]. **Aldosterone** and other **[mineralocorticoids]** [regulate excretion of salt and water by kidneys]. The **adrogens** and **estrogens** [effect sexual development and function]. Testosterone is an adrogen (male sex hormone) and β-estradiol is an estrogen (female sex hormone). [The various forms of vitamin D] are hormones. They [are sterol derivatives] in which the [steroid B ring is disrupted between C9 and C10]. **Vitamin D~2~ (ergocalciferol)** is [nonenzymatically formed in the skin of animals through the photolytic action of UV light on the plant sterol ergosterol (a common milk additive]). **Vitamin D~3~ (cholecalciferol**) is [similarly derived from 7-dehydrocholesterol] (hence the saying that [sunlight provides vitamin D]). [Vitamin D~2~ and D~3~ are inactive], the [active] [form] are [produced] by [addition of OH group] carried out [by liver and kidney] to yield 1 α[,25-Dihydroxycholecalciferol]. **Active vitamin D increases serum Ca^2+^** by promoting the intestinal [absorption of dietary Ca^2+^. This] [increases] the absorption of [Ca^2+^ in bones and teeth. ] [Vitamin D deficiency in children produces **rickets**], a disease characterized by [stunted growth and deformed bones]. [Excessive intake of vitamin D] over long period results in [vitamin D intoxication], the consequent [high serum Ca^2^]^+^ results in [calcification of soft tissues] and [development of kidney stones]. [Other lipids] perform a variety of metabolic roles. **Isoprenoids** are [not-structural components of membranes] built from [5-carbon units] with the [same carbon skeleton as isoprene]. Like, isoprenoid, [ubiquinone] (also known as coenzyme Q, [cofactor of mitochondrial respiratory complexe]s supporting cellular bioenergetics.) Mammalian ubiquinone [consists of 10 isoprenoids units.] [Over 50,000 isoprenoids] (called **terpenoids**), has been [characterized in bacteria, fungus and plants]. **[Fat soluble vitamins in vertebrae]** like (like vitamin D) is **rich in isoprenoid compounds**. ) (vitamins are organic substances that animal require in small amounts but cannot synthesize and hence must acquire in diet.) **Vitamin A, retinol**, is [derived] from [plant product β-carotene] (present in [green vegetables], [carrots], [tomatoes]). **Retinol is oxidized to retinal** which [functions as the eye's photoreceptor] at low light intensities. [Deficiency] of vitamin A [can lead to blindness]. [Retinoic acid] stimulates [tissue repair], used to [treat acne] and [skin ulcers] and [cosmetically to eliminate wrinkles]. **Vitamin K** is a lipid [synthesized by plants (as phylloquinone]) and [bacteria (as menaquinone]). [About half of the daily requirement for humans is supplied by intestinal bacteria]. Vitamin K has **role in blood clotting proteins**. Vitamin K [deficiency] prevents results [in inactive clotting proteins] leading [to excessive bleeding] ; [Anti-coagulant drugs] are compounds that [interfere] with [vitamin K function] (like [to prevent clot formation after surgery]). [**Vitamin** **E**] is **α-tocopherol**. It is incorporated into cell membranes and **functions as antioxidant** that [prevents oxidative damage] to membrane proteins and lipids. [**Eicosanoids** are **Prostaglandins**] (lipid compounds), [Thromboxanes], [Leukotrienes], and [Lipoxins]. Eicosanoids [acts at a very low concentrations] and are involved in the [production of pain and fever] and in the [regulation of blood pressure], [blood coagulation] and [reproduction]. In humans eicosanoids are [derived from Arachidonic acid a C~20~ compounds]. Arachidonate is stored in cell membranes as the C2 ester of phosphatidyl inositol. [Amphiphillic molecules] such as [soaps and detergents] **form micelles** (globular aggregates whose hydrocarbon groups are out of contact with water. **Single tailed amphiphiles**, such as [soap anions], [form spheroidal or ellipsoidal micelles] because of their tapered shapes, [there hydrated head groups are wider than their tails]. Figure shows aggregates of single-tailed lipids. [The tapered Van der Waals envelope of these lipids] a) [permits] them [to pack efficiently to form a spheroidal micelle], b) The [diameter of the micelles depends on the length of the tails]. Spheroidal micelles composed of many more lipid molecules than the optimal no. c) [Would have been unfavorable water-filled center (blue]). Such micelle [could flatten out to collapse] the hollow centre, but as these ellipsoidal micelles [become elongated].d) they [allow develop water filed spaces]. The [**two** hydrocarbon **tails** of **glycerophospholipids** and **sphingolipids** give these amphiphiles a somewhat **rectangular cross-section**]. The steric requirements of packing such molecules together yield large disklike micelles. These **lipid bilayers** are around 60 A◦ thick. [A suspension of phospholipids (glycerophospholipid or sphingomyelins) can form **liposomes** ]**(**[closed, self-sealing solvent-filled vesicles] that are bounded by only single bilayer). Figure shows electron micrograph, accompanying figure consist of a **lipid bilayer**. [Lipid bilayers have fluid-like properties]. Figure shows [phospholipid diffusion] in a lipid bilayer. a\) **Transverse diffusion ( a flip-flop**) is defined as the [transfer] of a [phospholipid molecule] from [one bilayer leaflet to the other]. (it is an [extremely rare event]), (have [half-time of several days or more]). b\) **[Lateral diffusion]** is defined as the [pairwise exchange] of neighboring phospholipid molecule [in the same bilayer leaflet]. Lipids are **highly mobile** in the plane of the bilayer. This is the model [snapshot of a lipid bilayer] at an instant in time. The conformations of dipalmitoylphosphatidylcholine molecules in a bilayer surrounded by water were [modelled by computer] (molecular dynamics simulation). The **fluidity** of bilayer is **temperature-dependen**t. As bilayer is [cooled down below a characteristic transition temperature], it [undergoes] a sort of [phase-chang]e. Figure shows phase transition in a lipid bilayer. A) [above the transition temperature] , both lipid molecules as a whole and their nonpolar tails are [highly mobile] in the plane of the bilayer. B) [Below the transition temperature] , the lipid molecules [form] a [much more orderly array to yield a gel like solid.] C**holester**ol, which does [itself] [does not form a bilayer], **decreases membrane fluidity** because its [rigid steroid ring system interferes] [with the motion of the fatty acid side chains] in other membrane lipids. This is [X-ray] structure of the [integral membrane protein] **aquaporin-O (AQPO)** in [association] with [lipids]. The [protein] is represented by its surface diagram, which is colored according to charge ([red negative, blue positive and white uncharged]). Tightly bound molecules of [dimyristoylphosphatidylcholin]e are drawn in s[pace-filli]ng. The arrangement of the two rows of lipid molecules, with phosphate-phosphate distance of 35 A◦, matches the dimensions of a lipid bilayer. [Human erythrocyte] **glycophorin A** is a [transmembrane (TM]) protein, that is it [completely spans] the membrane. The protein bears [15 o-linked oligosaccharides] (green diamonds) and one that is [N-linked] (dark green hexagon) in its extracellular domain. The predominant sequence of the O-linked oligosaccharides is also shown (NeuNAc= N-acetylneuraminic acid). **The protein's transmembrane portion (brown and purple) consists of 19 sequential predominantly hydrophobic residues**. Its terminal portion, which is located on the membrane's cytoplasmic face, is rich in anionic (pink), cationic (blue), and polar (blue-gray) residues. There are [two] common [genetic variants] of glycophorin A. [Glycophorin A^M^ has Ser and Gly at position 1 and 5, whereas glycophorin A^N^ has Leu and Glu at these positions.] [Nigel and Richard] used [electron crystallography] to [determine] structure of [integral membrane protein] **bacteriorhodopsin**, produced [by archaebacterium] *Halobacterium salinarum*. It consists largely of a bundle [of seven about 25 residues α-helical rod]s that span the lipid bilayer perpendicular to the bilayer plane. It **generates proton concentration gradient across the cell membrane** that powers ATP synthesis. A covalently bound [retinal] (drawn in [stick form]) is the [protein's light absorbing group]. [Transmembrane] proteins [may contain α-helices.] The figure shows the [identification of transmembrane helices by hydropathy plots.]The **average of the hydropathy indices** (hydrophobic and hydrophilic tendencies) **of the amino acid residues** in a sliding window of defined length is **plotted** **against** the **position of the first amino acid** in the window. A [positive value] indicates that the [peptide segment] in the window is h[ydrophobi]c, that is its transfer to water would require an input of energy. [Segments of \>20 hydrophobic residues] are [likely] [to form TM helices]. The plots here are for a) glycophorin A and b) bacteriorhodopsin. The bar above each plot and the square brackets indicate the experimentally determined positions of the TM helices in the corresponding protein. Note [that glycophorin a has only one TM helix], whereas **bacteriorhodopsin has seven TM helices**. [Some] [transmembra]ne proteins [contain β-barrels]. β-barrels occur **in porins**. This is the [X-ray] structure of E. coli [OmpF porin]. This is the [ribbon diagram of 16 stranded monomer. ] X-ray structure of E. coli [OmpF porin]. This is the [ribbon diagram of the trimer] embedded in its semi-transparent surface and viewed along its 3-fold axis of symmetry from cell's exterior, [showing pore through each subunit.] X-ray structure of E. coli OmpF porin. [A space filling model of the trimer viewed perpendicular to its 3-fold axis]. [Trp] and [Tyr] [side] chains are shown in [white]. [These groups delimits about 25 A◦ high hydrophobic band (scale at right)] that is immersed in the nonpolar portion of the bacterial outer membrane. [Some membrane associated proteins contain] **covalently attached lipids** that [anchor the protein to the membrane]. Prenylated proteins have covalently [attached lipids] that are built from [isoprene] units. The [most common isoprenoid groups are] the **C~15~ farnesyl** and **C~20~ geranylgeranyl** residues. (**Prenylation** is a [post-translational modification] where **lipid groups**, like [farnesyl or geranylgeranyl] groups, are **added to proteins**) The [most common prenylation site] in protein is [C terminal tetrapeptide C-X-X-Y], where C is [cysteine] X is often an [aliphatic amino] acid residue. Residue [Y influences the type of prenylation]. The **prenyl group** is **enzymatically** [linked] [to] the [Cys sulfur] atom [via] a **thioether linkage**. [X-X-Y tripeptide ins then proteolytically excised] and the [newly exposed terminal carboxyl group is esterified with methyl group]. Two kinds of fatty acids , myristic acid (myristoylation) and palmitic acid (palmitoylation) are linked to membrane proteins. **Glycosylphosphatidylinositol-linked proteins** (**GPI-linked** proteins) [occur in all eukaryotes]. The [core structure] of the GPI group consists of p[hosphatidylinositol] **glycosidically linked to a linear tetrasaccharide** composed of three mannose residues and one glucosaminyl residue. The [mannose at the nonreducing end] of this assembly [forms] a **phosphodiester bond with a phophoethanolamine** residue [that is amide-linked] to the [protein's C-terminal carboxyl group]. [R1 and R2 represent fatty acid residues] whose identities vary with the protein. The tetra saccharide may have a variety of attached sugar residues whose identities also varies. Role signal transduction, cell adhesion , immuine fuction etc. [Plasma membrane] (membrane that separates and protect the interior of the cell from outside). [Singer and Nicolson] gave "[fluid mosaic model]". In this model, [integral proteins are visualized as "ice bergs" floating in a two-dimensional lipid "sea" in a random or mosaic distribution]. In figure , [Integral proteins (purple]) are embedded in a bilayer composed of [phospholipids (blue] head groups attached to wiggly tails) and [cholesterol (yellow]). The [carbohydrate] components ([green and brown] beads) of glycoproteins and glycolipids occur on only the [external face] of the membrane. Most membranes contain a higher proportion of protein than is depicted here. [Integral proteins] **can diffuse laterally in the lipid matrix**. Membrane fluidity can be [explained] by ; [Fusion of mouse and human cells.] The accompanying photomicrographs (the square boxes) were taken through filters that allowed only red or green light to reach the camera. The [**rates of diffusion of proteins in membranes** can be determined from measurements of **fluorescence recovery after photobleaching** (FRAP]). In this, a [fluorophore is specifically attached to a membrane] component in an immobilized cell or in an artificial membrane system. [An intense laser pulse focused on a very small area destroys (bleaches) the fluorophore there]. **The rate at which the bleached area recovers its fluorescence** , as **monitored** by [fluorescence microscopy] **indicates** the **rate** at which unbleached and bleached fluorophore labeled molecules laterally diffuse into and out of the bleached area. The [membrane skeleton] helps define cell [shape]. [Erythrocyte membrane can be obtained by osmotic lysis], **which causes the cell content to leak out**. The [resulting membranous particle are called erythrocyte ghost], **[because on withdrawal of conditions they reseal to form original shape without cytoplasm]**. The **protein spectrin** was in erythrocyte ghost [accounting] for [75% of erythrocyte membrane skeleton].. Spectrin has [two] polypeptide chains, 280 kDa [α-subunit] and 246 kDa [β subunit]. [Both] of the antiparallel [polypeptide] contains multiple [106 --residue repeats], which are though to [form] flexibly connected [triple-helical bundles.] 2 of these heterodimers join, head to head, to form an (αβ)2 heterotetramer. The [X-ray] structure of three consecutive repeats of chicken [brain α-spectrin]. Each of these repeats consist of an [up-down-up triple-helical bundle] in which the C-terminal helix of one repeat is continuous , via a 5 residue helical linker (purple). An [electron micrograph] of an [erythrocyte membrane skeleton] that has been [stretched] to an area [9 to 10 times greater] than that of the native membrane. Streching makes it possible [to obtain clear images of the membrane skeleton], which in its native state is densely packed and irregularly flexed. A [model of erythrocyte membrane skeleton] with inset showing its relationship to the intact erythrocyte. The [junction between spectrin tetramers include actin and tropomyosin] and [band 4.1] protein (named after its position on SDS PAGE). [Spectrin] is [also associated] with [1880 residue protein ankyrin] **which binds to integral membrane ion channel protein**. Ankyrin's N-terminal 798-residue segment consists almost entirely of 24 tandem , [around 33 residue repeats known as ankyrin repeats]. This is the [X-ray] structure of human ankyrin repeats 13-24. N-terminus is blue, repeat 13 and C-terminus is red, repeat 24. The **interaction of membrane components with the underlying cytoskeleton** helps [explain] **why** integral [membrane proteins exhibit different degrees of mobility within] the [membrane]. Figure shows the **model rationalizing the various mobilities of membrane proteins**. [Protein A,] which [interacts tightly] with underlying cytoskeleton , is *[immobile]*. Protein [B] is [free to rotate] within the confines of the cytoskeletal "fences". Protein [C] [diffuses by travelling through "gates] " in the cytoskeleton. The diffusion of membrane proteins is not affected by the cytoskeleton. [Membrane lipids are distributed asymmetrically]. Figure shows asymmetric distribution of membrane phospholipids in the [human erythrocyte membrane]. The phospholipid is expressed as mol %. In [eukaryotes], the [enzymes] that [synthesizes membrane lipids are] mostly integral membrane [protein] of the endoplasmic reticulum (**ER**). Figure shows [electron micrograph of ER]. This membrane network is contagious with the nuclear membrane. The so-called [smooth ER] is the [site of synthesis membrane lipids], and the **rough E**R, with its **associated ribosomes, is site of synthesis of membrane and secretory proteins.** [In Prokaryotes], [lipids] are [synthesized] by integral membrane proteins in the [plasma membrane]. To demonstrate this Kennedy and Rothmangave [growing bacteria a 1 min pulse of ^32^PO~4~^3-^] in order to [radioactively] label the [phosphoryl] groups of only the [newly synthesized phospholipids]. Immediately afterward, [they added trinitrobenzenesulfonic acid (TNBS]), a membrane impermeable reagent that combines with phophotidylethanolamine (PE). The location of lipid synthesis in a bacterial membrane,. **Newly synthesize PE was labelled by ^32^PO~4~^3-^ (yellow head groups**), and the PE on the cell surface was independently labeld by treatment with the [membrane-impermeable reagent TNBS]. When the TNBS labeled (red circles) occurred immediately after the ^32^P pulse , none of the ^32^P --labelled PE was TNBS labelled (upper right) thereby indicating that the PE is synthesized on the cytoplasmic face of the membrane. If however, there was even a few minutes delay between the two labeling procedures, much of the TNBS-labelled PE in the external face of the membrane was also ^32^P labelled. The [secretory pathway generates] secreted and [transmembrane proteins]. All secreted proteins are synthesized [with leading N terminal] 1336 residue **signal** **peptides**. These signal peptides consist of [6 to 15 residue hydrophobic core] flanked by several relatively hydrophilic residues that usually include one or more basic residues near the N-terminus. Figure shows the --terminus sequences of some eukaryotic proteins, The hydrophobic core (tan) of most signal peptides are **preceded by basic residues (blue**). The [secretory pathway generates] secreted and transmembrane [proteins]. Figure shows the secretory pathway. It depicts ribosomal synthesis, membrane insertion and initial glycosylation. 1) When [signal peptide is synthesized]. 2) **SRP** (signal peptide recognition particle), **binds to signal peptide** and **ribosome** and **polypeptide synthesis is inhibited**. 3) [SRP-ribosome complex diffuse to RER and binds to SRP receptor in complex with translocon.] 4) **SRP** and **SR** **stimulate** each other **to hydrolyze their bound GTP to GDP** resulting in **forming** **ribosome-translocon complex, this will resume polypeptide synthesis**. 5\) When [signal peptide enters ER lumen], it is [cleaved] from polypeptide [by signal peptidase]. 6) **Enzymes in the ER lumen** initiates **posttranslational** modification of polypeptide. 7) When the polypeptide [synthesis] is [completed], it is [released from translocon & ER] and then the [two subunits of ribosome dissociates]. The channel forming component of [translocon] is a [heterotrimeric protein] named **Sec61 in mammals** and **SecY in prokaryotes**. Sec Y is shown here, It has [α, β and γ subunit]. a) [X-ra]y structure of the complex. B) [View] form the cytosol and c) Model for the insertion of TM helix into a membrane. The translocon is viewed as in b). [A polypeptide chain (yellow]) is shown [bound] in the [translocon's pore] during its [translocation] through the membrane, and a [signal-anchor sequence (red) is shown passing through the translocon's lateral gate] and being released into the membrane (arrow) After their synthesis , [partially processed proteins appear in Golgi apparatus]. In this array of flattened golgi apparatus vesicles, newly synthesized transmembrane and secretory proteins [undergo] **processing and sorting**. [Protein transit via two mechanism]. **Anterograde**: forward(cis to trans face) and vice versa called **Retrograd**e. The posttranslational processing of proteins. Membrane, secretory and lysosomal [proteins] are [synthesized] by [RER-associated ribosomes] (gray dots; top). As they are synthesized, the proteins (red dot) are either injected into the lumen of the ER or inserted into its membrane. [After initial processing in the ER] , the proteins are [encapsulated] in [vesicles] that [bud off from the ER] membrane and subsequently [fuse with the cis-golgi network (top]). The proteins are [progressively **processed**] in the [cis] , [medial] and [trans] [cisternae] of the [golgi]. Finally in the trans golgi network (bottom), the [completed glycoproteins] are [**sorted** for **delivery**] to their final destinations, the plasma membrane , secretory vesicles or lysosomes to which they are [transported by yet other vesicles.] Membrane, secretory and lysosomal proteins are [transported in in coated vesicles] (vesicles in which proteins are transported between the RER and different compartments of golgi[). 3 types] of coated vesicles are characterized by their [protein coats.] **Clathrin --coated vesicles** forms a [polyhedral framework around vesicles], that [transport protein from Golgi to plasma membrane]. **COPI coated vesicles**. Forms [fuzzy coating around vesicles] and carry out [both anterograde and retrograde] transport of proteins. (insert show respective vesicles at higher magnification). **COPII coated vesicles**. It [consists] of [two conserved protein heterodimers]. It transport protein [from ER to the golgi]. (insert show respective vesicles at higher magnification). On arriving at its target membrane, a vesicle fuses with it, thereby releasing its contents on the opposite side of the target membrane. Shown here is [fusion of a vesicle with the plasma membrane]. The [inside of the vesicle and the exterior of the exterior of the cell are topologically equivalent]. Fusion of the vesicle with the plasma membrane [preserves the orientation of the integral proteins embedded in the vesicle bilayer] because the same side of the protein is always immersed in the cytosol. Note that soluble proteins packaged inside a secretory vesicle that fuses with the plasma membrane would be released outside the cell. The [clathrin] network is [build from proteins] known as **triskelion**, this is an electron micrograph of triskelion. The variable orientations of their legs are indicative of their flexibility. It is [cryo-EM based image]. It has [three heavy chains] that each [bind] one of the two homologues [light chains]. [Triskeleton assemble to form polyhedral cages]. This is [showing three polyhedral structures] that are [formed] [when] [triskeleton] [assemble] into clathrin cages. [After fusing with target membrane, vesicles may release its content. ] Shown here is [Vesicle fusion at a synapse]. This is an electron micrograph of a frog neurotransmitter synapse. Synaptic vesicles are undergoing fusion (arrows) with the presynapytic plasma membrane (top). This process [discharges] the [neurotransmitter contents] of the [synaptic vesicles] into the [synapytic cleft], the space between the neuron and the muscle cell (the postsynaptic cell). [Protein] **SNAREs** [mediate vesicular fusion]. This is the [X-ray] structure of a [SNARE] [complex] modelled [between two membranes]. The [complex includes an R-SNARE] (blue, contain conserved residues) and [Q-SNARE (red], contain conserved Gln residues). And [SNAP-25 (green], a two Q domain containing Q-SNARE), for a [total of four helices]. The **cleavage** **site** for various costridial **neurotoxins** (**causing tetanus, and botulism**) are indicated by arrows. This a model for SNARE-mediated vesicle fusion. Here R-SNAREs and Q-SNAREs are schematically represented by blue and red worms. Zipping draws 2 membranes together, Hemidiffusion, where bilayer leaflets fuse exposing interior. Tension causes breakdown and forming fusion pore which then expends. The major integral [protein] component of the [membrane] **that envelops influenza virus** is named **hemagglutinin (HA**). HA has [two peptides] designated as **HA1** and **HA2**. Proteolytic removal of [HA's transmembrane helix yield protein named BHA]. This is the X-ray structure of influenza hemagglutinin. a) BHA [monomer] is shown , HA1 is green and HA2 is cyan. B) BHA [trimer] shown. HA undergoes dramatic conformational change , the nature of which was elucidated by X-ray structure of a portion of BHA named TBHA2 that consists of BHA's long helix and some of its flanking regions. This is the [schematic diagram comparing the structures] of BHA and TBHA2. This drawing indicates the positions and heights above the viral membrane surface of TBHA2's various structural elements in the [HA trimer (left)] and [in its low pH form] (right). In the low pH form , the [fusion peptide would protrude well above the receptor --binding heads where it would presumably insert itself into the endosomal membrane.]