Graduate School of Medicine, Dentistry and Pharmaceutical Sciences, Okayama University, 2-5-1 Shikatacho, Kita-ku, Okayama, 700-8558, Japan.
Sci Rep. 2022 Mar 16;12(1):3984. doi: 10.1038/s41598-022-07836-7.
The Calvin-Benson cycle (CB cycle) is quantitatively the most important metabolic pathway for CO fixation. In the canonical CB cycle, fructose 6-phosphate (F6P), fructose 1,6-bisphosphate (FBP), sedoheptulose 7-phosphate (S7P), and sedoheptulose 1,7-bisphosphate (SBP) appear as essential intermediates, where F6P is formed from FBP by the fructose 1,6-bisphosphatase (FBPase) reaction, and S7P is formed from SBP by the sedoheptulose 1,7-bisphosphatase (SBPase) reaction. Although the involvement of SBP and SBPase in the canonical CB cycle is consistent with the reported dependency of photosynthetic carbon metabolism on SBPase, the involvement of FBP and FBPase is not completely consistent with the reported FBP- or FBPase-related findings such as, although with a diminished growth rate, an Arabidopsis mutant lacking FBPase grew photoautotrophically in soil. Here, we show a novel variant of the CB cycle involving SBP, SBPase, and transaldolase, but neither FBP nor FBPase. This novel variant, named the S7P-removing transaldolase variant, bypasses FBP. This variant explains the FBP- or FBPase-related findings more easily than the canonical CB cycle as well as the dependency of photosynthetic carbon metabolism on SBPase and further suggests that co-overexpression of SBPase and transaldolase can be a strategy for enhancing photosynthetic carbon metabolism, which is important for the global environment.
卡尔文-本森循环（CB 循环）是 CO2 固定的最重要的代谢途径，从数量上看。在典型的 CB 循环中，果糖 6-磷酸（F6P）、果糖 1,6-二磷酸（FBP）、景天庚酮糖 7-磷酸（S7P）和景天庚酮糖 1,7-二磷酸（SBP）是必不可少的中间产物，其中 F6P 是由 FBP 通过果糖 1,6-二磷酸酶（FBPase）反应形成的，S7P 是由 SBP 通过景天庚酮糖 1,7-二磷酸酶（SBPase）反应形成的。尽管 SBP 和 SBPase 的参与与光合作用碳代谢对 SBPase 的依赖性一致，但 FBP 和 FBPase 的参与与报道的 FBP 或 FBPase 相关发现并不完全一致，例如，尽管生长速度降低，但缺乏 FBPase 的拟南芥突变体在土壤中仍能进行光自养生长。在这里，我们展示了一种涉及 SBP、SBPase 和转醛醇酶但不涉及 FBP 和 FBPase 的新型 CB 循环变体。这个新变体被命名为 S7P 去除转醛醇酶变体，绕过了 FBP。这种变体比典型的 CB 循环更容易解释 FBP 或 FBPase 相关发现，以及光合作用碳代谢对 SBPase 的依赖性，进一步表明 SBPase 和转醛醇酶的共过表达可以成为增强光合作用碳代谢的策略，这对全球环境很重要。
