期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2020
卷号：117
期号：39
页码：24234-24242
DOI：10.1073/pnas.2008824117
出版社：The National Academy of Sciences of the United States of America
摘要：Ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) is the cornerstone of atmospheric CO 2 fixation by the biosphere. It catalyzes the addition of CO 2 onto enolized ribulose 1,5-bisphosphate (RuBP), producing 3-phosphoglycerate which is then converted to sugars. The major problem of this reaction is competitive O 2 addition, which forms a phosphorylated product (2-phosphoglycolate) that must be recycled by a series of biochemical reactions (photorespiratory metabolism). However, the way the enzyme activates O 2 is still unknown. Here, we used isotope effects (with 2 H, 25 Mg, and 18 O) to monitor O 2 activation and assess the influence of outer sphere atoms, in two Rubisco forms of contrasted O 2 /CO 2 selectivity. Neither the Rubisco form nor the use of solvent D 2 O and deuterated RuBP changed the 16 O/ 18 O isotope effect of O 2 addition, in clear contrast with the 12 C/ 13 C isotope effect of CO 2 addition. Furthermore, substitution of light magnesium ( 24 Mg) by heavy, nuclear magnetic 25 Mg had no effect on O 2 addition. Therefore, outer sphere protons have no influence on the reaction and direct radical chemistry (intersystem crossing with triplet O 2 ) does not seem to be involved in O 2 activation. Computations indicate that the reduction potential of enolized RuBP (near 0.49 V) is compatible with superoxide (O 2 •− ) production, must be insensitive to deuteration, and yields a predicted 16 O/ 18 O isotope effect and energy barrier close to observed values. Overall, O 2 undergoes single electron transfer to form short-lived superoxide, which then recombines to form a peroxide intermediate.
