生物化学原理课件（英文）：Chapter27 Glycogen metabolism

1、杨荣武杨荣武 生物化生物化 学原理学原理 第二版第二版 Chapter27 Glycogen metabolism Outline Glycogen Breakdown Glycogen Synthesis Regulation of Glycogen Metabolism Glycogen Glycogen serves as storage carbohydrate in animals, insects and fungi Heavily branched to allow rapid mobilization of Glc $Muscles cannot mobilize fat as

2、 rapidly as glycogen. $Fatty acid residues of fat cannot be metabolized anaerobically. $Animals cannot convert fatty acids to Glc, so fat metabolism alone cannot adequately maintain essential blood Glc levels. Why use glycogen for energy storage? Glycogen Breakdown Requires Three Enzymes A. Glycogen

3、 phosphorylase to cleave a1,4 linkages Glycogen + Pi - Glycogen + Glc-1-P n residuesn-1 residues B. Glycogen debranching enzyme a(1-4) glycosyl transferase and a(1-6) glucosidase activities - Glc-1-P and Glc C. Phosphoglucomutase to convert to usable form Glc-1-P - Glc-6-P Glc-6-phosphatase is requi

4、red for export from liver as Glc Glycogen Phosphorylase Dimer of identical 97 kDa subunits Structural Domains and Binding Sites Each phosphorylase subunit has N- and C-terminal domains. Allosteric effector binding sites and modification sites are in the N-terminal domain. Cofactor is linked in the C

5、-terminal domain. The catalytic site is located at the interface of glycogen binding subdomain and regulatory domain with the C-terminal domain. Pyridoxal Phosphate is an Essential Cofactor for Phosphorylase Pyridoxal-5-phosphate (PLP), a vitamin B6 derivative, is covalently linked to phosphorylase

6、via a Schiff base to Lys 679. In an unusual role, only the phosphate group participates in the catalytic process. 3D Structure Model of Glycogen Phosphorylase PLP covalently bound to phosphorylase via a Schiff base to K679 Glycogen phosphorylase reaction mechanism 1.Formation of an EPiglycogen terna

7、ry complex 2.Oxonium ion intermediate (I) formation from the a-linked terminal glucosyl residue involving acid catalysis by Pi facilitated by proton transfer from PLP. 3.Reaction of Pi with overall retention of configuration around C1 to form a-D-Glc-1-phosphate. The glycogen, minus one residue, cyc

8、les back to step 1. Reaction mechanism of Glycogen phosphorylase The reactions catalyzed by debranching enzyme Two active sites for two different catalytic activities-a bifunctional enzyme 1. Acts as an a(14) transglycosylase (glycosyl transferase) by transferring an a(14) linked trisaccharide unit

9、from a “limit branch” of glycogen to the nonreducing end of another branch. 2. Also catalyzes the hydrolysis of the a(16) bond of the remaining glycosyl residue to yield Glc. Required for degradation of almost half of glycogen molecule Maximal rate of debranching enzyme much slower than that of glyc

10、ogen phosphorylase Glycogen Debranching Enzyme Phosphoglucomutase Mechanism of action is similar to that of phosphoglycerate mutase except Ser carries phosphoryl group here. If Glc-1,6-bisP dissociates, enzyme inactivated; small amounts are generated by phosphoglucokinase from Glc-1-P to prevent ina

11、ctivation Glycogen Synthesis $Glucose must be activated into UDP-Glc Notice: ADP-Glc for Starch; GDP-Glc or UDP-Glc for Cellulose $The primer (Glycogenin) is required $Proceed from the reducing end to the non-reducing end $Glycogen synthase and Glycogen branching enzyme UDP-Glucose Pyrophosphorylase

12、 converts UTP and Glc-1-P to UDP-Glc and pyrophosphate. Inorganic pyrophosphatase converts PPi to 2Pi, driving reaction UDP-Glc Synthesis UDP-Glc Synthesis UDPGs “high-energy” status permits it to donate glucosyl units to the growing glycogen chain in a thermodynamically favorable reaction. This is

13、a common strategy for carbohydrate addition. Use of UDP-Glc makes synthesis reaction favorable The overall reaction for the formation of UDPG is highly exergonic Glc is added in a 1,4 linkage from UDP-Glc to the non-reducing end of a glycogen chain The reaction mechanism is apparently like that of p

14、hosphorylase Glycogen synthase can only extend a chain- cannot initiate de novo synthesis Glycogen synthase The reaction catalyzed by glycogen synthase Glycogenin $How is glycogen synthesis initiated? $The first step is attachment of a Glc residue to Tyr194-OH group of the glycogenin protein by a ty

15、rosine glucosyltransferase. $Glycogenin autocatalytically extends the glucan chain by up to 7 additional UDP-Glc-supplied residues, forming a primer for the initiation of glycogen synthesis. $Once primer started, glycogen synthase can associate to form ternary complex; upon extension of the chain, g

16、lycogenin dissociates from the complex Role of glycogenin The branching of glycogen Donor from a branch at least 11 residues long New branch at least 4 residues from another branch Regulation of Glycogen Metabolism Glycogen Phosphorylase 1.Allosteric control (AMP,ATP,G-6-P) 2.Covalent modification (

17、phosphorylation , dephosphorylation) Glycogen Synthase 1.Allosteric control (G-6-P) 2.Covalent modification(phosphorylation , dephosphorylation ) Glycogen Phosphorylase Is Activated by Phosphorylation Regulation of Glycogen breakdown T state: active site blocked by loop; Arg569 less available for substrate binding R state: rotation of subunits to achieve more favorable packing; loop disordered s