细胞科学 el Science 蔡国平
细胞科学 Cell Science （8.5） 蔡国平
7.5 Glycosylation in the Er and gc 1. What is the purpose of glycosy lation? Not only membrane glycolipids but nearly all of the proteins produced on membrane-bound ribosomes -integral membrane proteins, soluble enzymes in the lumens of EMS organelles and secretory proteins include parts of ECM- become glycoproteins that attach carbohydrate chains by either N-linkages(to the nN2 of Asn) or O-linkages(to the OH of Ser or Thr, or and hydroxy lysine in collagen). The sugar sequences of the oligosaccharides of these carbonhydrate conjugates are highl specific. The addition and subsequent processing of carbohydrates are called as glycosylation, which is the principal chemical modification to most such proteins. Some glycosylation reactions occur in the lumen of ER; others in the lumen of GC, sequently, from the cis-, medial- to trans-Golgi cisternae What is the purpose of glycosylation?
7.5Glycosylation in the ER and GC 1. What is the purpose of glycosylation? Not only membrane glycolipids but nearly all of the proteins produced on membrane-bound ribosomes -integral membrane proteins, soluble enzymes in the lumens of EMS organelles and secretory proteins include parts of ECM- become glycoproteins that attach carbohydrate chains by either N-linkages (to the NN2 of Asn) or O-linkages (to the OH of Ser or Thr, or and hydroxylysine in collagen). The sugar sequences of the oligosaccharides of these carborhydrate conjugates are highly specific. The addition and subsequent processing of carbohydrates are called as glycosylation, which is the principal chemical modification to most such proteins. Some glycosylation reactions occur in the lumen of ER; others in the lumen of GC, sequently, from the cis-, medial- to trans-Golgi cisternae. What is the purpose of glycosylation?
Carbohydrate groups have diverse even key roles in the function of many glycoproteins and glycolipids, different from each other, such as recognizing and binding sites in their interactions with other macromolecules, particularly, for the cell-surface proteins, for example, such interaction by cell-adhesion molecules (CAMS) tethers the leukocytes to the endothelium and assists in their movement into tissues during an inflammatory response to infection Thus glycosylation of the some(but not all) nascent made proteins may promote folding of glycoproteins and provide useful marker such as M-6-Pi for lysozyme, for following their movement from ER and through GC cisternae and sorting as well as packing. For example, in the presence of the antibiotic tunicamycin, which blocks the first step in formation the dolichol-linked precursor of N-linked oligosaccharides, the hemagglutinin precusor polypeptide (HAo is synthesized, but it cannot fold properly and form a normal trimer. Maybe, much important, it stability novel proteins, protecting them from enzymatic proteolysis
Carbohydrate groups have diverse even key roles in the function of many glycoproteins and glycolipids, different from each other, such as recognizing and binding sites in their interactions with other macromolecules, particularly, for the cell-surface proteins, for example, such interaction by cell-adhesion molecules (CAMs) tethers the leukocytes to the endothelium and assists in their movement into tissues during an inflammatory response to infection. Thus glycosylation of the some (but not all) nascent made proteins may promote folding of glycoproteins and provide useful marker such as M-6-Piforlysozyme, for following their movement from ER and through GC cisternae and sorting as well as packing. For example, in the presence of the antibiotic tunicamycin, which blocks the first step in formation the dolichol-linked precursor of N-linked oligosaccharides, the hemagglutinin precusor polypeptide (HAo ) is synthesized, but it cannot fold properly and form a normal trimer. Maybe, much important, it stability novel proteins, protecting them from enzymatic proteolysis
CH, OH Polypeptide NHCOCH3 N-Acetylglucosamine Serine X=H) Threonine (X=CH) CH OH C=0 NHCOCH3 N-Acetylgalactosamine
蛋白质糖基化类型 N-连接与O-连接的寡糖比较 特征 N-连接 O-连接 1.合成部位 粗面内质网粗面内质网或主要在高尔 基体 2.合成方式 来自同一个寡一个个单糖加上去 糖前体 3.与之结合的 天冬酰胺 丝氨酸、苏氨酸、 氨基酸残基 羟赖氨酸、羟脯氨酸 4最终长度 至少5个 般1~4个糖残基, 糖残基 但ABO血型抗原较长 5第一个糖残基N-乙酰葡萄糖N-乙酰半乳糖胺等 胺等
特 征 N-连接 O-连接 1. 合成部位 粗面内质网 粗面内质网或 主要在高尔 基体 2. 合成方式 来自同一个寡 糖前体 一个个单糖加上去 3. 与之结合的 氨基酸残基 天冬酰胺 丝氨酸、苏氨酸、 羟赖氨酸、羟脯氨酸 4 最终长度 至少5个 糖残基 一般1～4个糖残基， 但ABO血型抗原较长 5.第一个糖残基 N—乙酰葡萄糖 胺等 N—乙酰半乳糖胺等 N-连接与O-连接的寡糖比较
2. Nontemplet synthesis of glycosylation in the ER and gC The immediate precursors, used in glycosylation, are nucleotide sugars with a high energy ester bond (AG of its hydrolysis -5. kcal/mol), such aS CMP-salic acid GDP-manose, UDP-GICNAc. They are synthesized in the cytosol and can be actively transported actively by different antiports into the Er and GC. They are the donors of oligosaccharides, and the transfer a specific monosaccharide from an appropriate sugar donor to an appropriate sugar receptor, a growing oligosaccharide chain is catalyzed by group of membrane-bound enzyme called glycosyltransferases. Complex carbohydrates require a different enzyme at each step. The sequence of sugars that is assembled into an oligosaccharide depends on both the glycosyltransferases present in differet cells in a particular sequence and the accessibility of the oligosaccharide to the enzymes that participate in its construction. It is a nontemplet
2. Nontemplet synthesis of glycosylation in the ER and GC The immediate precursors, used in glycosylation, are nucleotide sugars with a high energy ester bond (G o’of its hydrolysis ~ -5 kcal/mol), such as CMP-salic acid, GDP-manose, UDP-GlcNAc. They are synthesized in the cytotsol and can be actively transported actively by different antiports into the ER and GC. They are the donors of oligosaccharides, and the transfer a specific monosaccharide from an appropriate sugar donor to an appropriate sugar receptor, a growing oligosaccharide chain is catalyzied by a group of membrane-bound enzyme called glycosyltransferases. Complex carbohydrates require a different enzyme at each step. The sequence of sugars that is assembled into an oligosaccharide depends on both the glycosyltransferases present in differet cells in a particular sequence and the accessibility of the oligosaccharide to the enzymes that participate in its construction. It is a nontemplet synthesis
P716 Dolichol CH3C-CHCH2+CH2C-CHCH2tCHz CH 1518 by tunicamycin UDP- UDP P UDP 5 GDP Dolichol phosphate N-Acetylglucosamine "flip Mannes ER lumen 3UDP→ 3 UDP
3 UDP 3 UDP Nascent Ribosome 4过 emorane 6 12 P Cytos 书 5 GDP 5 GDP-V 1 UMP Dolichol O= Glucose V:Mannose O=NAcetylglucosamine(NAG) MP= Dolichol phosphate
ambane (b) Step-by-step UDP-GlNAC glucose a mannose N-acetylglucosamine ⊙③③ CYTOSOL 4 Dclichol-p ER LUMEN growing polypeptide chain ⊙ cl Transfer to protein oligosaccharide protein transferase pid-linked oligosaccharide Aeparag ne n prate Figure 12-7 Assembly of Core Oligosaccharide tn asparagine during core ghcasyiation, (b)Step-by phospholipid translocator(Flippase). (c) Transfar of the completed core charice from dolichol phosphate to an asparagine of N-adetylglucosam ne NI, mannose MI and glucose
P360 Gytosol Fany acid sile chains COO Ethanolamine NH-CH,-CH- CH :CHa CH, oligosaccharide NH O TR I