生物化学本科课件英文版讲授chapter23glycogen

1、CHAPTER 23 GLYCOGEN METABOLISM n Glycogen is na readily mobilized storage form of Glcna very large, branched polymer of Glc residuesnThe link bonds between Glc residues in glycogen are 1,4 glycosidic bond (mostly) and -1,6 glycosidic bondnenergy content in body glycogen is much more than that in bod

2、y Glcn two major sites of glycogen storage are liver and skeletal musclenGlycogen is present in the cytosol in the form of granules nThe synthesis and degradation of glycogen have important roles in understanding metabolism:nthese processes regulate the blood Glc level and provide a reservior of Glc

3、 for strenuous muscular activitynThey occur by different reaction pathways, which illustrates an important principle of biochemistry (“biosynthetic and degradative pathways in biological systems are almost always distinct”)nmechanisms of the hormonal regulation of glycogen metabolism are of general

4、significance (and c-AMP in the coordinated control, reversible phosphorylation, etc.)nA number of inherited enzyme defects related glycogen metabolism have been characterized Phosphorylase catalyzes the phosphorolytic cleavage of glycogen into G-1-P nPhosphorylase 磷酸化酶catalyzes sequential removal of

5、 glycosyl residues from nonreducing end of glycogen molecule: Glycogenn+PiG-1-P+ glycogenn-1 nPhosphorylase catalyzes sequential removal of glycosyl residues from nonreducing end of glycogen molecule: Glycogenn+PiG-1-P+ glycogenn-1 nG-1-P released can readily be converted into G-6-Pnhigh ratio of Pi

6、/G-1-P in vivo makes phosphorolysis proceed far in the direction of breakdownnphosphorolytic cleavage of glycogen is energetically advantageous because released sugar is phosphorylated nAnother advantage of phosphorolytic cleavage for muscle cells is that G-1-P, ionized under physiologic conditions,

7、 cannot diffuse out of the cell, whereas Glc can.nWe see here an example of the importance of a metabolites being ionized 磷酸解能量上有利(多一个ATP),而且磷酸解产物G-1-P生理条件下离子化不会逸出(肌肉)细胞A debranching enzyme脱支酶 also is needed for the breakdown of glycogen nBesides phosphorylase, two other enzymes are requirednTransfe

8、rase转移酶 shifts a block of 3 glycosyl residues from one out branch to the othern1,6-glucosidase糖苷酶 (or debranching enzyme脱支酶) hydrolyzes the 1,6-glycosidic bondntransferase and the 1,6-glucosidase are present in a single 160-Kd polypeptide chain Phosphoglucomutase converts G-1-P into G-6-P nPhosphogl

9、ucomutase磷酸葡萄糖变位酶 catalyzes phosphoryl shift nG-1-P+ E-Ser-OPG-1.6-BP+ E-Ser-OHG-6-P+ E-Ser-OP nThese reactions are like those of phosphoglyceromutase磷酸甘油酸变位酶, a glycolytic enzyme(3-磷酸甘油酸 2-磷酸甘油酸)nThe role of G-1,6-BP is like that of 2,3-BPGnA phosphoenzyme intermediate participates in both reaction

10、s Liver contains G-6-Pase, a hydrolytic enzyme absent from muscle nliver has ability to release Glc into blood to maintain a relatively constant level of Glc in the blood nA hydrolytic enzyme G-6-Pase located on the lumenal side of smooth ER membrane in liver enables Glc to leave liver内质网膜(腔侧)上的G-6-

11、Pase酶水解G-6-P为葡萄糖(可使之离开肝脏细胞进入血液),此酶为肝脏所特有nThis is essential for GNG. G-6-P+ H2OGlc+ Pi nG-6-P is transported into ER Glc and Pi formed by hydrolysis are then shuttled back into the cytosolnG-6-Pase is also present in the kidneys and intestine, but it is absent from muscle and the brainnThe liver stor

12、es and releases Glc primarily for the benefit of other tissues. Glycogen is synthesized and degraded by different pathways nThe evidence isnthermodynamics of the phosphorylase-catalyzed reaction do not permit synthesis at physiologic Pi/G-1-P 生理条件下此比值很高,不能进行合成(逆)反应nhormones that lead to phosphorylas

13、ealways make breakdown(not synthesis)npatients who lack phosphorylase entirely are able to synthesize muscle glycogen nIt is synthesized by a pathway that utilyzes UDP-Glc rather than G-1-P as the activated Glc donornBiosynthetic and degradative pathways in biological systems are almost always disti

14、nct which afford much greater flexibility, both in energetics and in control 在热力学和调控方面更为有利UDP-Glc is an activated form of Glc nUDP-Glc, the donor in the biosynthesis of glycogen is an activated form of GlcnUDP-Glc is synthesized from G-1-P and UTP: G-1-P+ UTPUDP-Glc+ PPi (2Pi) nhydrolysis of PPi in

15、vivo to 2Pi by an inorganic pyrophosphatase (PPase) drives the synthesis of UDP-Glc焦磷酸被无机焦磷酸酶水解驱动焦磷酸被无机焦磷酸酶水解驱动UDP-Glc的合成的合成nwhich exemplifies a recurring theme in biochemistry: many biosynthetic reactions are driven by the hydrolysis of PPinAnother important aspect is: nucleoside diphosphate sugars

16、 (UDP-Glc) serve as glycosyl donors核苷二磷酸核苷二磷酸糖作为糖作为葡萄糖供体in the biosynthesis of many disaccharides and polysaccharides Glycogen synthase糖原合酶 catalyzes the transfer of Glc from UDP-Glc to a growing chain nNew glucosyl units added to nonreducing terminal of glycogen to form an-1,4-glycosydic linkagenwh

17、ich is catalyzed by glycogen synthase 糖原合酶nThe glycogen synthesis requires a primerglycogenin糖糖原引发素原引发素 (糖原引发蛋白糖原引发蛋白, the core of glycogen)na 37-Kd protein bearing an oligosaccharide of -1,4-Glc unitsnC-1 of the first unit of this chain is covalently attached to the phenolic hydroxyl group of a spe

18、cific Tyr in glycogenin that autocatalyzes the addition of some 8 Glc units 1,4-糖苷键连接糖苷键连接的寡糖中的第一个残基的的寡糖中的第一个残基的C-1与糖原引发素中的特定的与糖原引发素中的特定的Tyr残残基中的酚羟基连接基中的酚羟基连接(自催化使寡糖延伸至约八个残基自催化使寡糖延伸至约八个残基),至此至此, (与与糖原引发素紧密结合的糖原引发素紧密结合的)糖原合酶开始接管糖原合酶开始接管(糖原的进一步合成糖原的进一步合成)n UDP-Glc is the donor in this autoglycosylatio

19、n (自糖基化自糖基化)nAt this point, glycogen synthase, which is tightly bound to glycogenin takes overnGlycogen synthase is catalytically efficient only when it is bound to glycogeninnThe synthase-glycogenin interaction limits the size of glycogen granules 合酶和糖原引发素的相互作用合酶和糖原引发素的相互作用(紧密紧密结合结合)限制了糖原颗粒的大小限制了糖原

20、颗粒的大小A branching enzyme分支酶分支酶 forms -1,6 linkage nGlycogen synthase-1,4 linkage; branching enzyme-1,6 linkage (for making a branched polymer)nBranching is important nincreases the solubility of glycogen ncreates a large number of terminal residues, the site of action of glycogen phosphorylase and sy

21、nthasenthus increases the rate of glycogen synthesis and degradation nBranching reaction is different from debranching one nA block of residues (7 or so) is transferred to a more interior site. 转移7个残基(包括非还原端)nbranching enzyme is quite exactingn7 residues must include nonreducing terminus and come fr

22、om a chain at least 11 residues long来自至少11个残基长度的糖链nIn addition, the new branch point must be at least 4 residues away from a preexisting branch point 将这7个残基转移到新的分枝点,此分枝点距原先存在的分枝点至少4个残基Glycogen is a very efficient storage form of Glc nHow much is the price for G-6-PglycogenG-1-PG-6-P? nThe sum of 5-s

23、tep glycogen synthesis is: G-6-P+ ATP+ glycogenn+ H2Oglycogenn+1+ ADP+ 2Pi 合成合成(从从glycogenn到到glycogenn+1)用掉一个高用掉一个高能键能键degradation of glycogen includes:nabout 90% of the residues: glycogenn+1+ Piglycogenn+ G-1-P (-1,4-bond) G-6-P nabout 10%: glycogenn+1+ H2Oglycogenn+ Glc (residues at branching poin

24、t, -1,6-bond) +ATPG-6-P 降解降解(从从glycogenn+1到到glycogenn)产生一个产生一个G-6-P约消耗约消耗0.1个高能键个高能键(1,6-糖苷键占整个糖苷键约糖苷键占整个糖苷键约1/10)nSo, the storage of Glc in the form of glycogen is very efficient, the overall efficiency of storage: (311.1)/31 x 100%=97% Pyridoxal phosphate participates in the phosphorolytic cleavag

25、e of glycogen nPyridoxal 5-phosphate 5磷酸吡哆醛forms a Schiff base with a Lys residue at the active site of phosphorylase 希夫碱(磷酸化酶+辅酶磷酸吡哆醛; 转醛酶+酮糖底物; 醛缩酶+磷酸二羟基丙酮) Phosphorylase is regulated by allosteric interactions and reversible phosphorylation 磷酸化酶受变构作用和可逆磷酸化的双重调控nGlycogen metabolism is precisely co

26、ntrolled by multiple interlocking mechanisms多重互锁机制nPhosphorylase is regulated by nseveral allosteric effectors that signal the energy state of cell变构作用(能量信号)nreversible phosphorylation, which is responsive to hormones可逆磷酸化(激素)nEb-Ser-OH (inactive)+ ATPEa-Ser-OP (active) +ADP+ H+可逆磷酸化nEa deactivated

27、by hydrolysis of the Ser-OP in each subunit, a reaction catalyzed by protein phosphatase 1(PP1)蛋白磷酸酶蛋白磷酸酶1nEa is fully active, irrespective of level of AMP, ATP and G-6-PnPhosphorylation of b to a puts enzyme almost entirely in R state无活性的无活性的b酶磷酸化后几乎完全处于具活性的酶磷酸化后几乎完全处于具活性的R(松弛松弛)态态, Ea具完全的活性具完全的活性,

28、不管效应不管效应物水平如何物水平如何nEb is active only in AMP, it binds to the nucleotide binding site and alters the conformation of EbnUnder most physiologic conditions, Eb is inactive because of inhibitory effect of ATP and G-6-Pnthe b form by itself is almost entirely in inactive T state; binding of AMP shifts th

29、e allosteric equilibrium to the active R state酶酶b在大部分生理状态在大部分生理状态下下(因因ATP和和G-6-P的抑制作用的抑制作用)几乎完全处于无活性的几乎完全处于无活性的T(紧张紧张)态态,只有当与只有当与AMP结合后将构象由结合后将构象由T态移动至态移动至R态态(从而具有活性从而具有活性).nATP, G-6-P act as a negative allosteric effector (by competing with AMP) and an inhibitor (by binding to the AMP site), respec

30、tively负变构效应物和抑制剂负变构效应物和抑制剂nallosteric regulation of liver phosphorylase differs from that of muscle enzyme in:肝赃磷酸化酶不同于肌肉磷酸化酶肝赃磷酸化酶不同于肌肉磷酸化酶nAMP does not activate liver Eb AMP不能激活不能激活Ebnliver Ea is deactivated by binding of Glc, i.e. Glc shifts allosteric equilibrium of a form from R to T Ea与葡萄糖结合后失

31、活与葡萄糖结合后失活nThis is because the purpose of glycogen degradation in liver is to form Glc for export to other tissue when blood GlcnHence, liver E is responsive to Glc but not to AMP Structural changes at the subunit interface are transmitted to the catalytic sites Phosphorylase kinase is activated by

32、phosphorylation and calcium ion 磷酸化酶激酶受磷酸化和钙离子双重激活磷酸化酶激酶受磷酸化和钙离子双重激活nphosphorylase kinase is under dual control hormones (such as epinephrine肾上腺素肾上腺素)cAMP protein kinase A (PKA) phosphorylase kinasephosphorylasehormones such as vasopressin (加压素加压素) binding to a receptor in plasma membraneG proteinph

33、ospholipase C磷脂酶磷脂酶Cinositol 1,4,5-triphosphaterelease of Ca2+ from ERphosphorylase kinasephosphorylase.nIt is converted from a low-activity form into a high-activity one by phosphorylation (by PKA)nIt can also be partly activated by Ca2+ (1M)nIts subunit is calmodulin (钙调蛋白钙调蛋白), a Ca sensor that s

34、timulates many enzymes in eukaryotes. Glycogen synthase is inactivated by the phosphorylation of a specific Ser residue nThe active a form of the synthase is converted into a usually inactive b form by phosphorylation糖原合酶磷酸化后转变为无活性的糖原合酶磷酸化后转变为无活性的b型型nIts b form requires G-6-Pfor activity, whereas th

35、e dephosphorylated a form is active in its presence or absencenThus, phosphorylation has opposite effects on the enzymatic activities of glycogen synthase and phosphorylase. A cAMP cascade coordinately controls glycogen synthesis and breakdown nEpinephrine肾上腺素adenylate cyclase腺苷酸环化酶cAMPPKA蛋白激酶A phos

36、phorylase kinasephosphorylase (ba)； glycogen synthase (ab)neffects of hormones are highly amplified by the cAMP cascade. nbinding of a small number of hormone molecules to cell-surface receptors leads to release of a very large number of sugar units 少量激素大量葡萄糖释放Glycogen metabolism in the liver regula

37、tes the blood Glc level nGlc in blood is about 80 to 120 mg per 100 ml, Its regulation is mainly through the control of the synthesis and degradation of glycogen in the liver liver senses the Glc in the blood and takes up or releases Glc accordingly nWhen Glc is infused, the amount of liver phosphor

38、ylase a,that of glycogen synthase a(after a lag period)nIn fact, phosphorylase a is the Glc sensor in liver cellsnThe binding of Glc to phosphorylase a shifts its allosteric equilibrium from R to T form葡萄糖与磷酸化酶的结合使构象平衡由葡萄糖与磷酸化酶的结合使构象平衡由R态转向态转向T态态nwhich exposes the phosphoryl group on Ser 14 to hydro

39、lysis by protein phosphatase 1 (PP1) 从而使从而使Ser 14上的磷酸基暴露被上的磷酸基暴露被PP1水解水解 nPP1 plays key roles in regulating glycogen metabolism, it binds tightly to phosphorylase a but acts catalytically only when Glc induces the transition to the T form PP1与磷酸化酶与磷酸化酶a紧密结合紧密结合, 但仅当但仅当Glc诱导磷酸化酶转变为诱导磷酸化酶转变为T态后态后才催化它的水解才催化它的水解nThe conversion of a into b is accompanied by the release of the PP1, which is then free to activate glycogen synthase. (phosphorylase b does not bind the phosphatase)磷酸化酶变为磷酸化酶变为b型后