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Radical S‐adenosyl‐L‐methionine (SAM) enzymes couple the reductive cleavage of SAM to radical‐mediated transformations that have proven to be quite broad in scope. DesII is one such enzyme from the biosynthetic pathway of TDP‐desosamine where it catalyzes a radical‐mediated deamination. Previous studies have suggested that this reaction proceeds via direct elimination of ammonia from an α‐hydroxyalkyl radical or its conjugate base (i.e., a ketyl radical) rather than 1,2‐migration of the amino group to form a carbinolamine radical intermediate. However, without a crystal structure, the active site features responsible for this chemistry have remained largely unknown. The crystallographic studies described herein help to fill this gap by providing a structural description of the DesII active site. Computational analyses based on the solved crystal structure are consistent with direct elimination and indicate that an active site glutamate residue likely serves as a general base to promote deprotonation of the α‐hydroxyalkyl radical intermediate and elimination of the ammonia group. [ABSTRACT FROM AUTHOR]

A long pulse ECRH system with a goal of 140GHz 4MW 100~1000s has been developed to meet the requirement of steady-state operation on EAST. Gycom gyrotrons are employed in the No.1 and No.3 systems, CPI gyrotrons are used in the No.2 and No.4 systems. The development of the two Gycom gyrotron systems has been finished. The first short pulse EC wave injection has been demonstrated successfully during the EAST 2015 Spring campaign. In the commissioning and operation towards steady-state operation, 0.4MW 100s has been injected to plasma successfully by using the No.1 system, 4.7keV 102s L-mode and 102s H-mode plasma have been achieved on EAST with the help of ECRH. Recently, a longest pulse of 0.55MW 1000s has been obtained based on calorimetric dummy load measurements on the No.3 gyrotron. The No.2 gyrotron also has been installed and partially tested, 500kW 80s has been demonstrated in the dummy load. The remaining No.4 gyrotron will be ready to test in 2018 or 2019. The whole 4MW system will be completed within two years. The 400s fully non-inductive H-mode operation would be expected in the next four years in the condition of fully tungsten diverter on EAST.

The [4Fe‐4S]‐dependent radical S‐adenosylmethionine (SAM) proteins is one of large families of redox enzymes that are able to carry a panoply of challenging transformations. Despite the extensive studies of structure–function relationships of radical SAM (RS) enzymes, the electronic state‐dependent reactivity of the [4Fe‐4S] cluster in these enzymes remains elusive. Using combined MD simulations and QM/MM calculations, we deciphered the electronic state‐dependent reactivity of the [4Fe‐4S] cluster in Dph2, a key enzyme involved in the biosynthesis of diphthamide. Our calculations show that the reductive cleavage of the S−C(γ) bond is highly dependent on the electronic structure of [4Fe‐4S]. Interestingly, the six electronic states can be classified into a low‐energy and a high‐energy groups, which are correlated with the net spin of Fe4 atom ligated to SAM. Due to the driving force of Fe4−C(γ) bonding, the net spin on the Fe4 moiety dictate the shift of the opposite spin electron from the Fe1‐Fe2‐Fe3 block to SAM. Such spin‐regulated electron transfer results in the exchange‐enhanced reactivity in the lower‐energy group compared with those in the higher‐energy group. This reactivity principle provides fundamental mechanistic insights into reactivities of [4Fe‐4S] cluster in RS enzymes. [ABSTRACT FROM AUTHOR]

Enzymatic carbon fixation is one of the most interesting processes in CO2 sequestration. A number of decarboxylases can catalyze the reversible decarboxylation reaction in vivo, while can strengthen the carboxylation reaction in vitro. The Bio-Kolbe–Schmitt reaction is the carboxylation of phenols and CO2 by hydroxybenzoic acid decarboxylases to generate hydroxybenzoic acids under a mild condition, which requires an alkaline environment. It remains unclear how the carbon fixation via carboxylation can be improved. In this study, the major issues regarding the switching from decarboxylase to carboxylase through thermodynamic calculation and determination of enzyme characteristics, CO2 concentration, and other related species in the reaction system such as HCO3− are described. We identified that the concentration of CO2 in aqueous solution was the main factor for obtaining high CO2 fixation yield. In order to obtain sufficient CO2(aq) to promote the carboxylation by decarboxylase under an alkaline environment, a high concentration of HCO3− is required. The highest CO2 fixation yield of 30% was achieved from the reaction under 0.2 MPa CO2 in 2.7 M KHCO3 buffer for 15 min. Under the optimal reaction conditions, the carboxylation of catechol by 2,3-DHBD_Ao (2,3-dihydroxybenzoic acid decarboxylase from Aspergillus oryzae) achieved a good "turnover frequency" (TOF) of 47 min−1. This study discloses that CO2(aq) concentration is a crucial parameter for increasing CO2 fixation yield in a homogenous reaction under an alkaline condition. [ABSTRACT FROM AUTHOR]