7 糖代谢的其他途径 0
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Chapter13 Additional Pathways in Carbohydrate Metabolism
Glycogen degradation and synthesis 糖原代谢与分解 Gluconeogenesis 糖异生 The pentose Phosphate pathway 磷酸戊糖途径
糖原的分解和生物合成一、糖原的分解 二、糖原的生物合成 三、糖原代谢的调控
糖原( glycogen ),又称动物淀粉,支链,分子量 数百万以上。主要由葡萄糖以 α(1,4)糖苷键相 连(93%),以少量α(1,6)糖苷键(7%)形成分 支。有肝糖原和肌糖原。
Glycogen Functions 糖原是由多个葡萄糖组成的带分枝的大分子多糖, 分子量一般在106-107道尔顿,可高达108道尔顿,是体内 糖的贮存形式,糖原主要贮存在肌肉和肝脏中,肌肉中糖 原约占肌肉总重量的1-2%约为400克,肝脏中糖原占总量 6-8%约为100克。肌糖原分解为肌肉自身收缩供给能量, 肝糖原分解主要维持血糖浓度。
Liver– Buffer for regulating blood glucose levels
Muscle– Store of glucose as a fuel for exercise high intensity exercise dependent on anaerobic glycolysis
一、糖原的酶促磷酸解 糖原的结构及其连接方式 -1,6糖苷键 非还原性末端
-1,4-糖苷键
磷酸化酶a(催化1.4-糖苷键断裂) 三种酶协同作用: 转移酶(催化寡聚葡萄糖片段转移) 脱枝酶(催化1.6-糖苷键水解断裂)
CH 2OH H H OH OH H CH 2OH H OH H OH O H OH H H O H OH CH 2OH O H OH H H O H H O H
CH 2OH O H OH H H OH H 1 O 6 CH 2 5 H OH 3 H CH 2OH O H 2 OH 1 O H H O H 4 OH H H O OH H H
glycogen
CH 2OH O H OH H H OH OH H
H O
H 4
OH
Glycogen is a polymer of glucose residues linked by (1 4) glycosidic bonds, mainly (1 6) glycosidic bonds, at branch points. Glycogen chains & branches are longer than shown. Glucose is stored as glycogen predominantly in liver and muscle cells.
Glycogen Degradation Glycogen Phosphorylase– Hydrolyzes glucose units from glycogen – Produces glucose-1-P
Removal of branch points– Debranching enzyme complex Glucan transferase Alpha-1,6-glucosidase
Glycogen catabolism (breakdown):Glycogen Phosphorylase catalyzes phosphorolytic cleavage of the (1 4) glycosidic linkages of glycogen, releasing glucose-1-phosphate as reaction product. glycogen(n residues) + Pi glycogen (n–1 residues) + glucose-1-phosphate This phosphorolysis may be compared to hydrolysis: Hydrolysis: R-O-R' + HOH R-OH + R'-OH Phosphorolysis: R-O-R' + HO-PO32- R-OH + R'-O-PO32-
A glycogen storage site on the surface of the Phosphorylase enzyme binds the glycogen particle. Given the distance between storage & active sites, Phosphorylase can cleave (1 4) linkages only to within 4 residues of an (1 6) branch point. This is called a "limit branch“.
Debranching enzyme has 2 independent active sites, consisting of residues in different segments of a single polypeptide chain: The
transferase of the debranching enzyme transfers 3 glucose residues from a 4-residue limit branch to the end of another branch, diminishing the limit branch to a single glucose residue. The (1 6) glucosidase moiety of the debranching enzyme then catalyzes hydrolysis of the (1 6) linkage, yielding free glucose. This is a minor fraction of glucose released from glycogen. The major product of glycogen breakdown is glucose-1-phosphate, from Phosphorylase activity.
Enzyme-Ser-OPO32 CH2OH H OH H OH O H OH H H OPO32
Enzyme-Ser-OH CH2OPO32 O H H H H OH OPO32 OH H OH
Enzyme-Ser-OPO32 CH2OPO32 O H H H H OH OH OH H OH
glucose-1-phosphate
glucose-6-phosphate
Phosphoglucomutase catalyzes the reversible reaction: glucose-1-phosphate glucose-6-phosphate A serine OH at the active site donates & accepts Pi. The bisphosphate is not released. Phosphoglycerate Mutase has a similar mechanism, but instead uses His for Pi transfer.
Glycogen
Glucose-1-P
Glucose Hexokinase or Glucokinase Glucose-6-Pase Glucose-6-P Glucose + Pi Glycolysis Pathway
Pyruvate Glucose metabolism in liver.
Glucose-6-phosphate may enter Glycolysis or (mainly in liver) be dephosphorylated for release to the blood. Liver Glucose-6-phosphatase catalyzes the following, essential to the liver's role in maintaining blood glucose: glucose-6-phosphate + H2O glucose + Pi Most other tissues lack this enzyme.
二、糖原的生物合成1. UDP-葡萄糖焦磷酸化酶(UDP-glucose pytophosphorylase)
—— 催化单糖基活化形成糖核苷二磷酸,为各种聚糖形成 时,提供糖基和能量。动物细胞中糖元合成时需UDPG;植物 细胞中蔗糖合成时需UDPG,淀粉合成时需ADPG,纤维素合成 时需GDPG和UDPG。
2. 糖原合成酶(glycogen synthase)—— 催化 -1,4-糖苷键合成 3.糖原分支酶 ( glycogen branching enzyme)
—— 催化 -1,6-糖苷键合成
Glycogen synthesis
Uridine diphosphate glucose (UDP-glucose) is the immediate precursor for glycogen synthesis.
As glucose residues are added to glycogen, UDP-glucose is the substrate and UDP is released as a reaction product. Nucleotide diphosphate sugars are precursors also for synthesis of other complex carbohydrates, including oligosaccharide chains of glycoproteins, etc.
糖核苷酸的生成(UDP-葡萄糖焦磷酸化酶)
+1-磷酸葡萄糖
UTP
UDPG
+PPi
在UDP-葡萄糖焦磷酸化酶作用下,1-磷酸葡萄糖生成UDP葡萄糖,消耗一个UTP,生成焦磷酸。这是一个磷酸酐交换 反应,Glu的磷酸基团带负电荷,向UTP的a-磷酸基团进攻。
UDP-glucose is formed from glucose-1-phosphate: glucose-1-phosphate + UTP UDP-glucose + PPi PPi + H2O 2 Pi Overall: glucose-1-phosphate + UTP UDP-glucose + 2 Pi Spontaneous hydrolysis of the ~P bond in PPi (P~P) drives the overall reaction. Cleavage of PPi is the only energy cost for glycogen synthesis (one ~P bond per gluc
ose residue).
糖原合成酶将UDP-葡萄糖的糖基加在糖原引物的 非还原端葡萄糖的C4羟基上。引物至少要有4个糖 基,由引发蛋白和糖原起始合成酶合成,将UDP-葡 萄糖加在引发蛋白的酪氨酸羟基上。糖原合酶 UDP+(葡萄糖)n+1
UDPG + 引物
Glycogenin initiates glycogen synthesis. Glycogenin is an enzyme that catalyzes attachment of a glucose molecule to one of its own tyrosine residues. Glycogenin is a dimer, and evidence indicates that the 2 copies of the enzyme glucosylate one another.
Tyr active site
active site Tyr
Glycogenin dimer
6 CH
2OH
H4
5
O H OH H2
UDP-glucoseH O1
tyrosine residue of GlycogeninO C O Uridine HO C CH H2 NH O P O
OH
3
O H OH
P O 6 CH
O
2OH
O-linked H glucose residue 4OH
5
O H OH H2
H C1
O
3
O H OH
C CH H2 NH
+ UDP
CH2OH H O H H
CH2OH O H
H H C O A glycosidic bond is formedH between the anomeric C1 of H OH OH O C CH O OH H the glucose moiety derived OH from UDP-glucose and the NH H H OH hydroxyl oxygen of a tyrosine side-chain of Glycogenin. UDP is released as a product.2
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