Prothrombin is a vitamin K-dependent plasma protein which is synthesized in the liver (1). Prior to secretion into plasma, prothrombin undergoes post-translational modification by a vitamin K-dependent carboxylase which converts ten specific glutamic acid residues to γ-carboxyglutamic acid (gla). The ten gla residues are located within the first 40 amino acids of the mature protein and contribute to the ability of prothrombin to bind to negatively charged phospholipid membranes. Prothrombin contains two regions of internal homology which are referred to as "kringle" structures. These regions of conspicuous secondary structure are located between residues 40 and 270 of the mature plasma protein and replace the growth factor domains found in several other plasma serine proteases. Thus far, no function has been ascribed to these regions, but there is suspicion that they may play a role in one of several binary protein interactions involving prothrombin. The mature single chain protein circulates in plasma as a zymogen and, during coagulation, is proteolytically activated to the potent serine protease α-thrombin. This proteolysis is catalyzed by the prothrombinase enzyme complex. During activation, prothrombin is cleaved at Arg271-Thr272 (human) / Arg273-Thr274 (bovine) and at Arg320-Ser321 (human) / Arg323-Ser324 (bovine) to a "pro" fragment (fragment 1.2) and thrombin, the latter of which is composed of two chains covalently linked by a disulfide bond. In the case of human prothrombin/thrombin, there is an additional thrombin feed-back cleavage at Arg284-Thr285 resulting in an additional 13 amino acids being removed from the mature thrombin “A” chain.
Human prothrombin is prepared from fresh frozen human plasma as described by Bajaj and coworkers (2). Bovine prothrombin is prepared from fresh bovine plasma using a modification of the procedure described by Owen and coworkers (3). Purified prothrombin is supplied in 50% (vol/vol) glycerol/H2O and should be stored at -20oC. Purity is determined by SDS-PAGE analysis, and activity is measured by clotting and/or chromogenic substrate assay, following conversion of prothrombin to thrombin.
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请赐教！
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。
颗粒型抗原,除了有细菌、红细胞、螺旋体等天然颗粒型抗原,还有吸附有可溶性抗原的非免疫相关颗粒.
颗粒性抗原光镜下可见，比如细菌性抗原、红细胞抗原等；而可溶性抗原在光镜下不可见，如组织浸出液、细菌毒素、蛋白质分子等。
它们不等同于完全抗原和不完全抗原。完全抗原具有免疫原性和抗原性，而不完全抗原只具有抗原性。完全抗原可以是颗粒性抗原，亦可是可溶性抗原，而不完全抗原一般只能是可溶性抗原，不会是颗粒性抗原。
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？
