维基百科

羧酶体

羧酶体（英语：carboxysome）是一种细菌微区室，为细菌微区室中被研究最多者[2]。羧酶体为多面体的蛋白结构，外为结构蛋白（BMC-H、BMC-P与BMC-T），内为RuBisCO（固碳酵素）与碳酸酐酶两种酵素[3]。此胞器最早于1956年在蓝菌Phormidium uncinatum中发现[4]，后来也在数种其他蓝菌与化学自营细菌（亦进行固碳）中发现，包括盐硫杆状菌、酸硫杆状菌与硝化菌等[3][5][6] 。1973年研究人员首次自Halothiobacillus neapolitanus（法语：Halothiobacillus neapolitanus）纯化羧酶体[7]。

化学自营细菌Halothiobacillus neapolitanus的羧酶体（电子显微镜图像），比例尺为100奈米[1]

羧酶体可能是细菌因应大气中氧气浓度上升演化出的机制，因氧气会与二氧化碳竞争RuBisCO的结合位[8]，羧酶体提供了二氧化碳浓度较高的微环境，碳酸酐酶生成二氧化碳后可马上将其供应给RuBisCO进行固碳，避免发生光呼吸的损耗[9][10]。

结构

 
羧酶体的结构模型，RuBP与碳酸酐酶被结构蛋白包裹与其中

低温电子显微镜显示羧酶体的形状为正二十面体或接近正二十面体[11][12][13]，其外壳为数千个蛋白复合体组成，包裹内部的RuBisCO与碳酸酐酶[11][13]。外壳蛋白大多为组成六聚体的BMC-H，也有少数为组成三聚体的BMC-T与组成五聚体的BMC-P（两者皆为形似六聚体的假六聚体）[14] [15]。BMC-H六聚体中间的孔洞可供固碳作用的受质（碳酸根离子）与产物（3-磷酸甘油酸）经扩散作用进出，此区域带正电的氨基酸可协助扩散进行[14]；BMC-P占据正二十面体的顶点[16]；BMC-T三聚体中间的孔洞较大且可受调控开关，可使固碳作用较大的受质（RuBP）与产物（3-磷酸甘油酸）进出[17][18]。

种类

 
（A）Halothiobacillus neapolitanus的α型羧酶体；（B）细长聚球蓝细菌（英语：Synechococcus elongatus）的β型羧酶体。比例尺为200奈米

羧酶体可分为α与β两型，前者存在α型蓝菌、硝化菌、硫氧化菌与紫细菌中，后者则存在部分蓝菌中[19]，两者外观相似，但组成的蛋白种类有异[20][21][22][23]，其组成细节、组装机制可能也有差异，经分析外壳蛋白的序列显示两型的羧酶体应是独立演化产生的[23][24]。

α型羧酶体

α型羧酶体又称cso型羧酶体，其中的RuBisCO为IA型，为最早被纯化、研究的细菌微区室[25][26]。此类羧酶体的直径约为100至160奈米[27]，BMC-H的种类为CsoS1A、B、C等，BMC-P的种类为CsoS4A、B等，BMC-T的种类则为CsoS1D。

β型羧酶体

β型羧酶体的体积一般大于α型羧酶体，其直径约为200至400奈米[28]，其中的RuBisCO为IB型[2]。此类羧酶体中的蛋白由Ccm基因编码，其BMC-H为CcmK、BMC-P为CcmL，BMC-T则为CcmO，其组装为由内至外，即内部的酵素先组装后，再被外部的结构蛋白包裹[29]。

应用

羧酶体为合成生物学研究所关注[30][31][32]，已有研究透过基因克隆成功在大肠杆菌中表现α型羧酶体[33]，也有生物工程研究透过微调羧酶体外壳蛋白而影响其性质[34]。透过基因克隆将羧酶体转入作物的叶绿体中可能可显著提升其固碳作用的效率而增加产量[35][36]，目前已有相关研究进行中[37][38]。

参考文献

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