Miyachi Shigetoh, Iwasaki Ikuko, Shiraiwa Yoshihiro
Marine Biotechnology Institute, Kamaishi City, Iwate, 026-0001, Japan,
Photosynth Res. 2003;77(2-3):139-53. doi: 10.1023/A:1025817616865.
Reports in the 1970s from several laboratories revealed that the affinity of photosynthetic machinery for dissolved inorganic carbon (DIC) was greatly increased when unicellular green microalgae were transferred from high to low-CO(2) conditions. This increase was due to the induction of carbonic anhydrase (CA) and the active transport of CO(2) and/or HCO(3) (-) which increased the internal DIC concentration. The feature is referred to as the 'CO(2)-concentrating mechanism (CCM)'. It was revealed that CA facilitates the supply of DIC from outside to inside the algal cells. It was also found that the active species of DIC absorbed by the algal cells and chloroplasts were CO(2) and/or HCO(3) (-), depending on the species. In the 1990s, gene technology started to throw light on the molecular aspects of CCM and identified the genes involved. The identification of the active HCO(3) (-) transporter, of the molecules functioning for the energization of cyanobacteria and of CAs with different cellular localizations in eukaryotes are examples of such successes. The first X-ray structural analysis of CA in a photosynthetic organism was carried out with a red alga. The results showed that the red alga possessed a homodimeric beta-type of CA composed of two internally repeating structures. An increase in the CO(2) concentration to several percent results in the loss of CCM and any further increase is often disadvantageous to cellular growth. It has recently been found that some microalgae and cyanobacteria can grow rapidly even under CO(2) concentrations higher than 40%. Studies on the mechanism underlying the resistance to extremely high CO(2) concentrations have indicated that only algae that can adopt the state transition in favor of PS I could adapt to and survive under such conditions. It was concluded that extra ATP produced by enhanced PS I cyclic electron flow is used as an energy source of H(+)-transport in extremely high-CO(2) conditions. This same state transition has also been observed when high-CO(2) cells were transferred to low CO(2) conditions, indicating that ATP produced by cyclic electron transfer was necessary to accumulate DIC in low-CO(2) conditions.
20世纪70年代，几个实验室的报告显示，当单细胞绿色微藻从高二氧化碳条件转移到低二氧化碳条件时，光合机构对溶解无机碳（DIC）的亲和力大大增加。这种增加是由于碳酸酐酶（CA）的诱导以及二氧化碳和/或碳酸氢根离子（HCO₃⁻）的主动运输，从而增加了细胞内DIC浓度。这一特性被称为“二氧化碳浓缩机制（CCM）”。研究表明，CA促进了DIC从藻类细胞外部向内部的供应。还发现，藻类细胞和叶绿体吸收的DIC活性物种是二氧化碳和/或HCO₃⁻，这取决于物种。在20世纪90年代，基因技术开始揭示CCM的分子层面，并确定了相关基因。鉴定出活性HCO₃⁻转运体、参与蓝细菌能量供应的分子以及真核生物中具有不同细胞定位的CA就是这些成功的例子。对一种红藻进行了光合生物中CA的首次X射线结构分析。结果表明，这种红藻拥有一种由两个内部重复结构组成的同型二聚体β型CA。将二氧化碳浓度提高到百分之几时会导致CCM丧失，进一步提高通常对细胞生长不利。最近发现，一些微藻和蓝细菌即使在高于40%的二氧化碳浓度下也能快速生长。对极高二氧化碳浓度抗性机制的研究表明，只有能够采用有利于光系统I的状态转换的藻类才能在这种条件下适应并存活。得出的结论是，在极高二氧化碳条件下，增强的光系统I循环电子流产生的额外ATP被用作H⁺运输的能量来源。当高二氧化碳细胞转移到低二氧化碳条件时也观察到了同样的状态转换，这表明循环电子传递产生的ATP对于在低二氧化碳条件下积累DIC是必要的。
