The vitamin K-dependent zymogen, protein C, is synthesized in the liver as a single chain polypeptide and is subsequently converted to a disulfide linked heterodimer, by removal of a dipeptide (Lys-146 and Arg-147) from the precursor molecule (1,2). Trace quantities of the single chain form have been observed in plasma. The light chain, which is responsible for the calcium dependent binding of protein C to phospholipid vesicles, contains 11 γ-carboxyglutamic acid (gla) residues, 1 b-hydroxyaspartic acid residue, and 2 epidermal growth factor (EGF) homology domains. The serine protease catalytic triad is located in the heavy chain. Human protein C is susceptible to proteolytic cleavage of a peptide (Mr=3000) from the COOH-terminal end of the heavy chain, yielding an altered form referred to as β-protein C. No functional distinction between α- and β-protein C has been observed. A single cleavage at Arg-12 (Arg-14 in bovine) of the heavy chain of human protein C converts the zymogen into the serine protease, activated protein C. This cleavage is catalyzed by a complex between α-thrombin and the endothelial cell surface protein thrombomodulin. In contrast to the other vitamin K dependent coagulation factors, activated protein C functions as an anticoagulant by catalyzing the proteolytic inactivation of factors Va and VIIIa. APC also contributes to the fibrinolytic response by complex formation with plasminogen activator inhibitors.
Bovine protein C is prepared from fresh citrated bovine plasma by a modification of the Walker procedure (3), as described by Haley et al. (4). Human protein C is prepared from fresh frozen citrated human plasma using a combination of immunoaffinity chromatography (5), and conventional techniques (4,9). Protein C is provided in 50% (vol/vol) glycerol/H2O and should be stored at -20oC. Purity is determined by SDS-PAGE analysis and activity is measured using a chromogenic substrate based assay.
ebiomall.com
>
>
>
>
>
>
>
>
>
>
>
>
请赐教!
若已进入细胞内。则先由巨噬细胞吞噬降解。并形成抗原-MHC复合体传送到表面。被辅助性T淋巴细胞识别和效应T淋巴细胞识别后。效应T淋巴细胞被激活。分解靶细胞。然后再由效应B细胞分泌的抗体进一步消灭病毒。
还有不清楚的么?
荚膜(capsule)是某些细菌在生长繁殖过程中分泌的一层黏液性物质,包围在细胞壁外,通常这种黏液层厚度小于0.2μm,成分是多糖或多肽,只有在营养丰富时或在动物体内,细菌才产生这种半抗原性质的黏液性物质。它具有保护菌体免受巨噬细胞等的捕捉和吞噬,因而具有抗吞噬抗消化、侵袭力强、与致病性关系密切等特点。像肺炎球菌、炭疽杆菌等都有这类荚膜。有些细菌的荚膜层较薄,小于0.2μm,称为微荚。
像链球菌的M蛋白、伤寒杆菌的Vi抗原、大肠杆菌的K抗原等都属于这类微荚膜。
荚膜是某些细菌在细胞壁外包围的一层粘液性物质,一般由糖和多肽组成,是细菌的一种特殊结构。
作用:
①抗吞噬作用:荚膜因其亲水性及其空间占位、屏障作用,可有效抵抗寄主吞噬细胞的吞噬作用。
②黏附作用:荚膜多糖可使细菌彼此间粘连,也可黏附于组织细胞或无生命物体表面,是引起感染的重要因素,具有荚膜的S-型肺炎链球菌毒力强,有助于肺炎链球菌侵染人体;废水生物处理中的细菌荚膜有生物吸附作用,将废水中的有机物、无机物及胶体吸附在细菌体表面上。
③抗有害物质的损伤作用:处于细菌细胞最外层,荚膜犹如盔甲可有效保护菌体免受或少受多种杀菌、抑菌物质的损伤,如溶菌酶、补体等。
④抗干燥作用:荚膜多糖为高度水合分子,含水量在95%以上,可帮助细菌抵抗干燥对生存的威胁。
⑤当缺乏营养时,荚膜可被利用作碳源和能源,有的荚膜还可作氮源。