生物化學課件：ch8膜脂類固醇和血漿脂蛋白的代謝

1、Chapter 8 Metabolism of membranelipids, steroids and plasma lipoproteins 膜脂、類固醇和血漿脂蛋白的代謝 （脂類代謝）,8.1 甘油磷脂和鞘脂的分解代謝 8.2 甘油磷脂和鞘脂的合成代謝 8.3 膽固醇的代謝 8.4 血漿脂蛋白的代謝,8.1 甘油磷脂和鞘脂的分解代謝 部位：溶酶體 甘油磷脂由特異性磷脂酶（Phospholipase A1、Phospholipase A2、Phospholipase C、Phospholipase D）水解。 溶血磷脂指水解去掉一個脂肪酸的甘油磷脂,即Phospholipase A1、Ph

2、ospholipase A2水解的產物。,磷脂酶的特異性水解,鞘脂的降解是由溶酶體中一套特異性的酶按一定步驟分解極性頭部基團，最后生成神經酰胺。 由于一些水解酶基因變異失去活性，鞘脂降解出現(xiàn)障礙導致鞘脂沉積在溶酶體中。這種遺傳性脂類沉積癥統(tǒng)稱溶酶體病。,8.2 甘油磷脂和鞘脂的合成代謝 8.2.1 甘油磷脂的合成代謝 合成部位在光面內質網(wǎng)或線粒體內膜。 合成以磷脂酸為合成前體。 合成有CDP-甘油二酯和CDP-極性頭部基團兩種策略。 首先，甘油二酯的-OH ( CDP-甘油二酯策略 )或極性頭部基團的-OH ( CDP-極性頭部基團策略 ) ，通過與胞嘧啶核苷酸相連而激活。 然后，由相應的另外

3、一個-OH 基團通過親核反應替換CMP ，從而合成甘油磷脂。,Low activity in adipose tissue,Phosphatidic acid is derived from L-glycerol 3- phosphate and two acyl-CoAs.,Often unsaturated,Often saturated,Phosphatidic acid is the common precursor for both triacylglycerols and glycerophospholipids.,How to synthesize glycerophosphol

4、ipids?,CDP-甘油二酯和CDP-極性頭部基團兩種策略,Bacteria mainly use this strategy,Eukaryotic cells use both strategies,原核細胞主要用CDP-甘油二酯策略合成甘油磷脂。 真核細胞兩種策略都用。通常，酸性(陰離子)甘油磷脂，包括磷脂酰甘油、心磷脂、磷脂酰肌醇用CDP-甘油二酯途徑合成。,原核細胞用CDP-甘油二酯途徑合成甘油磷脂,真核細胞用CDP-甘油二酯途徑合成磷脂酰甘油、心磷脂、磷脂酰肌醇,酵母和哺乳動物合成磷脂酰絲氨酸（PS）、磷脂乙醇胺（PE）、磷脂酰膽堿（PC）的途徑不同。 酵母通過CDP-甘油二酯途徑

5、合成PS，PS通過脫羧或基團交換反應合成PE，PE通過S-腺苷Met ( S-adenosylmethionine, adoMet)甲基化合成PC 。 哺乳動物用CDP-極性基團途徑合成PE和PC。PE通過基團交換反應合成PS。PE通過甲基化合成PC 只在肝臟中發(fā)生。,酵母從磷脂酰絲氨酸合成磷脂酰乙醇胺和磷脂酰膽堿,CDP-甘油二酯途徑合成磷脂酰絲氨酸,哺乳動物CDP-極性基團途徑合成PC和PE,Ethanolamine,Phosphoethanolamine,CDP-ethanolamine,Phosphatidylthanolamine,Summary of the synthesis o

6、f PE, PC, PS,呼吸窘迫綜合癥（Respiratory distress syndrome）是由于二軟脂酰磷脂酰膽堿合成障礙引起的一種病理性癥狀。在健康人中，二軟脂酰磷脂酰膽堿和其他一些磷脂以及一些特異性蛋白質存在于肺泡周圍的細胞外液中，在呼氣時起到減少液體表面張力防止肺塌陷的作用。早產嬰兒由于肺沒有發(fā)育成熟，沒有合成足夠的二軟脂酰磷脂酰膽堿導致發(fā)生呼吸窘迫綜合癥。,8.2.2 醚脂的合成代謝 醚脂如plasmalogens（縮醛磷脂）和 platelet-activating factor（PAF，血小板活化因子）的合成涉及一個脂肪醇基替代脂?；ㄐ纬擅焰I）的反應。,Ether l

7、ipids,Synthesis of the ether lipids,8.2.3 鞘脂的合成代謝 合成部位在光面內質網(wǎng)或線粒體內膜。 鞘脂的鞘氨醇骨架來源于軟脂酰CoA和Ser。,合成過程: Palmitoyl-CoA condenses with serine (PLP is needed for decarboxylate serine) to form b-ketosphinganine, which is then reduced to sphinganine (二氫鞘氨醇). Sphinganine is then acylated and desaturated to form

8、ceramide (containing sphingosine). Addition of sugar(s) or phosphocholine heads leads to the synthesis of cerebroside, gangliosides, or sphingomyelin.,二氫鞘氨醇,Spingolipid synthesis begins with the condensation between palmitoyl-CoA and Ser.,神經酰胺,(not CDP-choline!),鞘脂的來源? 鞘脂的進化除了賦予脂類結構的多樣性，更重要的一點是功能的多樣

9、性。研究發(fā)現(xiàn)，鞘脂具有第二信使的功能。如神經酰胺可啟動一些細胞的程序性死亡過程。,8.3 膽固醇的代謝 8.3.1 膽固醇的合成代謝 8.3.1.1 概述 合成主要在肝臟，少量在腸。腦細胞、紅細胞不能合成。合成部位在胞液和內質網(wǎng)。 膽固醇全部27個C來源于乙酰CoA，這是用同位素標記實驗研究的結果(1940s) 。 The biosynthetic pathway of cholesterol, being the most complex known, was elucidated mainly by Konrad Bloch and Feodor Lynen in the 1950s.,“

10、Cholesterol is the most highly decorated small molecule in biology. Thirteen Nobel Prizes has been awarded to scientists who devoted major parts of their careers to cholesterol. Ever since it was isolated from gallstones in 1784, cholesterol has exerted an almost hypnotic fascination for scientists

11、from the most diverse areas of science and medicine” -Michael Brown and Joseph Goldstein Nobel lectures(1985),The carbon origins of cholesterol as revealed by radioisotope labeling studies.,8.3.1.2 膽固醇的合成過程 膽固醇的生物合成過程可以分為4個階段: Stage I: 3個acetyl-CoA縮合還原生成 1個6C的 mevalonate (甲羥戊酸) 。 Stage II: mevalonat

12、e 轉變?yōu)榛罨?5C isoprene units (異戊二烯單位) 。 Stage III: 6個 isoprene units 縮合生成線性的30C squalene(鯊烯) 。 Stage IV: Squalene 環(huán)化生成1個四環(huán)結構, 通過一系列復雜的反應最后轉變?yōu)?7C的膽固醇。,甲羥戊酸，MVA,異戊二烯,鯊烯,Reactions assembling cholesterol from 18 molecules of acetyl-CoA can be divided into four stages.,(6C),(5C),(30C),(27C),(2C),由乙酰CoA合成甲羥

13、戊酸。 One molecule of b-hydroxy-b-methylglutaryl-CoA (HMG-CoA) is formed from three acetyl-CoA molecules in the cytosol via the same reactions as occurring in mitochondria for ketone body formation. HMG-CoA reductase (an integrated membrane protein in the smooth ER) catalyzes the irreversible reductio

14、n of HMG-CoA to form mevalonate: the committing step of cholesterol synthesis (thus being a major regulation). HMG-CoA還原酶催化的是限速步驟。,羥甲基戊二酸單酰CoA,One mevalonate is synthesized from three acetyl-CoA molecules.,The committing step,HMG-CoA reductase in cytosol,Acetyl-CoA + acetoacetate,HMG-CoA lyase in mi

15、tochondria,甲羥戊酸通過三步磷酸化反應合成兩個活化的異戊二烯單位。 Three phosphate groups are transferred from three ATP molecules to mevalonate to form 3-phospho-5-pyrophosphomevalonate. The leaving of both the carboxyl and the 3-phosphate groups leads to the formation of 3-isopentenyl pyrophosphate. 3-Isopentenyl pyrophospha

16、te is isomerized to form the second activated isoprene: dimethylallyl pyrophosphate.,3-異戊烯焦磷酸酯, IPP,二甲基丙烯焦磷酸酯, DPP,Two activated isoprenes are formed from mavelonate.,6個活化的異戊二烯單位合成30C的線性角鯊烯。 A dimethylallylpyrophosphate is joined to an isopentenylpyrophosphate (head-to-tail) to form the 10-carbon ge

17、ranyl pyrophosphate (牻牛兒焦磷酸酯). A geranyl pyrophosphate is joined to another 3-isopentenyl pyrophosphate (head-to-tail) to form the 15-carbon farnesyl pyrophosphate(法尼基焦磷酸). Two farnesyl pyrophosphate join (head-to-head) to form the 30-carbon squalene.,異戊二烯基轉移酶,牻牛兒焦磷酸酯,法尼基焦磷酸,Squalene is formed from

18、the condensation of two farnesyl pyrophosphates.,由角鯊烯合成膽固醇 Squalene 2,3-epoxide, is first formed in a reaction catalyzed by squalene monooxygenase using O2 and NADPH. Concerted movement of electrons through four double bonds and the migration of two methyl groups generates lanosterol (羊毛固醇). Lanoste

19、rol is converted to cholesterol via about 20 enzymatic reactions including many double bond reduction and demethylations., 20 reactions,Oxygenation induced ring closing converts the linear squalene to lanosterol of four rings, which is converted to cholesterol after going through another 20 or so re

20、actions!,8.3.2 膽固醇酯的合成 由脂酰CoA-膽固醇脂酰轉移酶(Acyl-CoA-Cholesteryl acyl transferase, ACAT)催化。 膽固醇酯在肝內合成，貯存于肝臟或以脂蛋白形式運送到其他組織。,Acyl-CoA-Cholesteryl acyl transferase (ACAT) catalyzes the addition of an acyl group to the hydroxyl group of cholesterol.,8.3.3 膽固醇的轉化 轉化為膽汁酸 Cholesterol can be converted to bile ac

21、ids (salts) ： glycocholate (甘膽酸鹽) and taurocholate (牛黃膽酸鹽).,牛黃膽酸,轉化為類固醇激素 The tail chain of cholesterol is first hydroxylated at C20 and C22, and then cleaved between these two carbons to remove a 6-carbon unit, forming pregnenolone(孕烯醇酮): the common precursor of all steroid hormones. Progesterone (

22、孕酮) is synthesized from pregnenolone by oxidizing the 3-OH group and the isomerization of the double bond (from 5 to 4 position).,( 續(xù) ) Cortisol (a major glucocorticoid) is synthesized from progesterone by hydroxylation at C-17, C-21, and C-11. Aldosterone (a mineralocorticoid) is synthesized from p

23、rogesterone by hydroxylation at C-21, C-11, and oxidation of C-18 to an aldehyde. Testosterone (an androgen, or male hormone) is synthesized from progesterone by the removal of 2-carbon unit and hydroxylation at C-17. Estradiol ( an estrogen, or female hormone) is synthesized from testosterone by th

24、e removal of C-19 and formation of the aromatic A ring.,孕烯醇酮,孕酮,糖皮質激素,雄性激素,雌性激素,鹽皮質激素,碳鏈裂解酶,Prognenolone, the common precursor of steroid hormones, is synthesized from Cholesterol.,孕烯醇酮,cytochrome P450 monooxygenases,Progesterone is synthesized from pregnenolone by oxidizing the 3-OH group and the i

25、somerization of the double bond from 5 to 4 position,Cortisol and aldosterone are synthesized from progesterone by several oxygenation reactions (forming hydroxyl and aldehyde groups).,Testosterone is synthesized from progesterone by the removal of a 2-carbon unit and hydroxylation at C-17,Estradiol

26、 is synthesizedfrom testosterone by the removal of C-19 and formation of the aromatic A ring,轉化為7-脫氫膽固醇 7-脫氫膽固醇是VitD的前體。VitD可以調節(jié)小腸鈣的吸收以及腎和骨鈣的水平。,VitD and rickets（佝僂?。?8.3.4 活化異戊二烯是一大類物質類異戊二烯的合成前體。 These include many pigments (carotenoids, phytol chain of chlorophylls), fragrant principles, vitamines

27、 (A, D, E, K), rubber, dolichols, quinones (ubiquinone, plastoquinone), juvenile hormones of insects. Prenylation of proteins (attaching of geranylgeranyl and farnesyl groups) leads to membrane association.,Some plant pigments are isoprenoids.,Some fragrant molecules are isoprenoids. (called terpene

28、s),Natural rubber is cis-polyisoprene.,lipid-anchored proteins脂錨定蛋白,Prenylation of proteins (attaching of geranylgeranyl and farnesyl groups) leads to membrane association.,Isoprenoid tails function to anchor proteins to membranes,8.3.5 膽固醇代謝的調節(jié) 膽固醇代謝主要受細胞內膽固醇水平和激素兩方面的調節(jié)。細胞內膽固醇水平指的是內源膽固醇和外源膽固醇的總和，因此

29、膽固醇代謝調節(jié)的特點是生物合成途徑和食物攝取的平衡。 HMG-CoA還原酶，催化膽固醇合成的限速步驟，其活性變化可達100倍。其調節(jié)包括: 一個固醇類分子促進HMG-CoA還原酶的酶原降解以及抑制HMG-CoA還原酶基因和LDL 受體基因的轉錄。 激素 (insulin and glucagon)通過可逆的磷酸化/去磷酸化調節(jié)限速酶的活性。,The de novo synthesis of cholesterol is regulated to complement the dietary uptake.,(For storage),8.4 血漿脂蛋白(plasma lipoprotein)的

30、代謝 8.4.1 血漿脂蛋白的一些概念 血脂：血漿所含的脂類。包括TG、CH、CHE、PL、FFA等。 來源：外源性脂類（食物攝?。?內源性脂類（體內合成） 血脂含量不如血糖恒定，受膳食、年齡、性別、職業(yè)以及代謝等的影響，波動范圍較大。,血漿脂蛋白：是脂類在血漿中的運輸形式。以疏水性脂類為核心圍繞著極性脂類和載脂蛋白組成。 不同的脂蛋白脂類和載脂蛋白的種類和數(shù)量不同。大小和密度也不同，可通過超速離心分離。一般分為四種： 乳糜微粒chylomicrons, 極低密度脂蛋白VLDL (very low density lipoproteins),低密度脂蛋白LDL(Low density lipo

31、proteins),高密度脂蛋白HDL (high density lipoproteins) 。,LDL的結構,血漿脂蛋白的電鏡圖,載脂蛋白（apolipoproteins）：脂蛋白中的蛋白質部分。 At least 18 apolipoproteins (named as apo A, B, C, D, E) have been revealed in human, which act as signals to target the lipoprotein particles to various tissues or activating enzymes that will act o

32、n the lipoproteins. （靶向功能和酶激活功能）,8.4.2 血漿脂蛋白的代謝 CM：把外源性TG從小腸運輸?shù)礁鞣N組織。 CM的 apoC-II激活肌肉或脂肪組織毛細血管中的脂蛋白脂肪酶，向這些組織釋放出游離的脂肪酸。 CM的殘余顆粒（含CHE）通過血液循環(huán)運到肝臟細胞進行代謝。,VLDL：把內源性TG從肝臟運輸?shù)狡渌M織。 VLDL的 apoC-II激活肌肉或脂肪組織中的脂蛋白脂肪酶， 向這些組織釋放出游離的脂肪酸。 剩余的VLDL殘粒一部分通過受體介導的內吞作用進入肝細胞代謝，另一部分形成LDL 。,脂蛋白脂肪酶,Lipids are transported as vari

33、ous lipoprotein particles in vertebrate plasma,LDL,HDL,VLDL,CM,LDL：主要把內源性CH運輸?shù)礁瓮饨M織。 這是LDL 受體介導的內吞作用。識別LDL 受體的是LDL 上的apoB-100 蛋白。 血漿中LDL與細胞膜上LDL 受體結合后引起了一系列的代謝過程，稱為LDL受體代謝途徑。,LDL受體代謝途徑,LDL,細胞膜,LDL受體合成,ACAT,HMG CoA還原酶,游離CH,溶酶體水解,內吞,與LDL受體結合,LDL receptors are recycled to the cell surfaces,HDL：逆向運輸CH，清除

34、血中CH。 HDL的前體來源于肝細胞或小腸細胞，從肝外組織收集CH，然后轉化為CHE向肝細胞運輸。 膽固醇的逆向運輸途徑（reverse cholestrol transport pathway）：指膽固醇從肝外組織向肝運輸?shù)倪^程。是機體將外周組織中衰老細胞膜中的膽固醇運輸?shù)礁未x并排出體外的途徑。,LCAT位于nascent HDL 的表面。通過LCAT合成CHE，使disk-shaped nascent HDLs 轉變?yōu)閟pherical mature HDLs。,8.4.3 血漿脂蛋白的代謝異常 高脂血癥：指血脂高于正常人上限。亦稱高脂蛋白血癥。 成人空腹1214小時血中的TG2.26m

35、mol/L，CH6.21mmol/L 兒童 CH4.14mmol/L,動脈粥樣硬化癥（atherosclerosis） 動脈粥樣硬化癥與血清中的CH水平過高相關。引起高血脂癥的遺傳因素主要有兩種： LDL受體基因突變使得LDL的CH不能進入細胞，血液中CH水平升高，而另一方面， 細胞內CH合成繼續(xù)進行，從而引起膽固醇攝取和合成代謝紊亂，造成家族性高膽固醇血癥（familial hypercholesterolemia）和動脈粥樣硬化癥。 Mevalonate analogs (e.g., compactin密實菌素 and lovastatin洛伐他訂) can be used to treat patients with familial hypercholesterolemia.,The mevalonate analogs are used to treat hypercholesterolemia patients.,某些載脂蛋白基因突變引起HDL水平下降。 A negative correlation between blood HDL level and art