Carbonic anhydrase and CO2 concentrating mechanisms in microalgae and cyanobacteria
Carbonic anhydrase and CO2 concentrating mechanisms in microalgae and cyanobacteria
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DOI：
10.1111/j.1574-6968.1986.tb01860.x
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发表时间：
1986-08
影响因子：
2.1
通讯作者：
K. Aizawa;S. Miyachi
中科院分区：
文献类型：
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作者：
K. Aizawa;S. Miyachi
在微生物光养生物中，属于蓝藻的那些利用CO2和HCO−3进行光合作用。一些绿藻主要在光合作用中吸收CO2，而另一些在其细胞表面上具有碳酸酐酶（CA）的绿藻可以利用HCO−3以及CO2。动力学研究表明，大部分HCO−3在通过位于细胞表面的CA转化为CO2后被利用。因此，穿过质膜的实际分子种类主要是游离CO2。其他种类的微藻利用溶解无机碳（DIC）进行光合作用的方式存在明显差异。在普通空气中生长的大多数微藻（低CO2细胞）光合作用的表观Km（CO2）值与陆地C4植物一样低，尽管藻类细胞通过C3途径固定CO2。与此相反，表观Km（CO2）值在细胞生长在富CO2的空气（高CO2细胞）是那些在陆地C3植物一样高。大多数低CO2细胞表现出低光呼吸，低CO2补偿点，低乙醇酸排泄率和无或低O2抑制光合作用。这些结果表明，DIC利用光合作用在低CO2细胞的效率是非常高的。低CO_2细胞CA活性高于高CO_2细胞，而其他光合酶活性在低CO_2和高CO_2细胞之间没有差异。此外，低CO2细胞可以在内部积累大量DIC，表明这些细胞中存在CO2浓缩机制。当CA活性或CO_2富集能力被抑制剂或诱变剂降低时，光合作用和光呼吸速率的表观Km（CO_2）值显著增加。这些结果表明，DIC利用率低CO2细胞的高效率取决于CA和CO2浓缩机制。结论：CA促进DIC从细胞外扩散到羧化反应位点，并通过主动转运蛋白实现DIC的浓缩。
Among the microbial phototrophs, those belonging to the cyanobacteria utilize CO2 and HCO−3 for photosynthesis. Some Chlorophyceae mainly take up CO2 in photosynthesis, and others, which have carbonic anhydrase (CA) on their cell surface can utilize HCO−3 as well as CO2. Kinetic studies revealed that most of the HCO−3 is utilized after this ion is converted to CO2 via CA located on the cell surface. Therefore, the actual molecular species which crosses the plasmalemma is mostly free CO2. There is apparent variation in the mode of utilization of dissolved inorganic carbon (DIC) for photosynthesis in microalgae in other classes. The apparent Km(CO2) values for photosynthesis in most microalgae grown in ordinary air (low-CO2 cells) are as low as in terrestrial C4 plants, although the algal cells fix CO2 via the C3 pathway. In contrast, the apparent Km(CO2) values in cells grown on CO2-enriched air (high-CO2 cells) are as high as those in the terrestrial C3 plants. Most low-CO2 cells show low photorespiration; a low CO2 compensation point, low rates of glycolate excretion and no or low O2 inhibition of photosynthesis. These results indicate that the efficiency of DIC utilization for photosynthesis in low-CO2 cells is very high. The activity of CA in low-CO2 cells is higher than that in high-CO2 cells, while no difference has been confirmed in the activities of other photosynthetic enzymes between low- and high-CO2 cells. In addition, low-CO2 cells can accumulate large amounts of DIC internally, indicating the existence of CO2-concentrating mechanisms in these cells. When CA activity or CO2 concentrating ability is reduced by inhibitors or by mutation, the apparent Km(CO2) values for photosynthesis and the rate of photorespiration increased notably. These results indicate that the high efficiency of DIC utilization in low-CO2 cells depends on both CA and a CO2-concentrating mechanism. It is concluded that CA facilitates the diffusion of DIC from outside the cells to the site(s) of the carboxylation reaction and the concentration of DIC is achieved via an active transporter.