Division of Geological and Planetary Sciences, Caltech, Pasadena, CA 91125, USA.
Environmental Genomics and Systems Biology Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
Biomolecules. 2023 Mar 26;13(4):596. doi: 10.3390/biom13040596.
Form I rubiscos evolved in Cyanobacteria ≥ 2.5 billion years ago and are enzymatically unique due to the presence of small subunits (RbcS) capping both ends of an octameric large subunit (RbcL) rubisco assembly to form a hexadecameric (LS) holoenzyme. Although RbcS was previously thought to be integral to Form I rubisco stability, the recent discovery of a closely related sister clade of octameric rubiscos (Form I'; L) demonstrates that the L complex can assemble without small subunits (Banda et al. 2020). Rubisco also displays a kinetic isotope effect (KIE) where the 3PG product is depleted in C relative to C. In Cyanobacteria, only two Form I KIE measurements exist, making interpretation of bacterial carbon isotope data difficult. To aid comparison, we measured in vitro the KIEs of Form I' ( Promineofilum breve) and Form I ( PCC 6301) rubiscos and found the KIE to be smaller in the L rubisco (16.25 ± 1.36‱ vs. 22.42 ± 2.37‱, respectively). Therefore, while small subunits may not be necessary for protein stability, they may affect the KIE. Our findings may provide insight into the function of RbcS and allow more refined interpretation of environmental carbon isotope data.
I 型 Rubisco 起源于 25 亿年前的蓝细菌，由于其小亚基（RbcS）的存在而在酶学上具有独特性，这些小亚基覆盖在八聚体大亚基（RbcL）Rubisco 组装体的两端，形成十六聚体（LS）全酶。尽管以前认为 RbcS 是 I 型 Rubisco 稳定性所必需的，但最近发现了一个与八聚体 Rubisco（I'；L）密切相关的姐妹分支，这表明 L 复合物可以在没有小亚基的情况下组装（Banda 等人，2020 年）。Rubisco 还显示出动力学同位素效应（KIE），其中 3PG 产物相对于 C 被耗尽。在蓝细菌中，只有两种 I 型 KIE 测量值，这使得细菌碳同位素数据的解释变得困难。为了便于比较，我们测量了 I'（Promineofilum breve）和 I 型（PCC 6301）Rubisco 的体外 KIE，发现 L Rubisco 的 KIE 较小（分别为 16.25±1.36‱和 22.42±2.37‱）。因此，虽然小亚基可能不是蛋白质稳定性所必需的，但它们可能会影响 KIE。我们的发现可能提供对 RbcS 功能的深入了解，并允许对环境碳同位素数据进行更精细的解释。
