The paper can be accessed at this link:https://onlinelibrary.wiley.com/doi/full/10.1002/pro.3529
QueE is a 7‐Carboxy‐7‐deazaguanine synthase that catalyzes the radical-mediated ring contraction of 6‐carboxy‐5,6,7,8‐tetrahydropterin, forming the pyrrolopyrimidine core of 7‐deazaguanine . QueE is a member of the S‐adenosyl‐L‐methionine (AdoMet), a structurally divergent radical enzyme superfamily, which harnesses the reactivity of radical intermediates to perform its chemical reactions. Members of the AdoMet radical enzyme superfamily utilize a canonical binding motif, a CX3CXϕC motif, to bind a [4Fe‐4S] cluster, and a partial (β/α)6 TIM barrel fold for the arrangement of AdoMet and substrates for catalysis (Grell et al. 2019).
The AdoMet radical enzymes harness the cleavage and reduction of a molecule of AdoMet ligated with a [4Fe‐4S] cluster to initiate radical chemistry, requiring a change in the resting oxidation state from +2 to +1. The intermediate generated, 5′‐deoxyadenosyl radical (5′‐dAdo•), is highly reactive and can abstract a hydrogen‐atom (H+) from many substrates, thus enabling many chemically challenging and reactions which can be generated from the reductive cleavage of SAM.
The biological reductant, flavodoxin, was first shown to be capable of this reduction in studies of pyruvate formate‐lyase activating enzyme. Flavodoxin reduces the AdoMet radical cluster as follows; firstly, radical chemistry is initiated through reductive cleavage of AdoMet since the AdoMet radical cluster needs to be reduced from the resting +2 oxidation state to the +1 oxidation state. Finally, low potentials electrons from NADPH are transferred to the AdoMet radical cluster through the action of Ferredoxin, or flavodoxin, an NADP+ reductase mechanism. The need to understand the protein–protein interactions occurring between AdoMet radical enzymes and flavodoxins are integral for exploring the determinants for activation.
The impact of this paper arises from the breadth of knowledge surrounding the catalytic activity of the enzyme, while the design for interaction with physiological reductants remains unclear. This paper examined structural differences between three 7‐carboxy‐7‐deazaguanine synthases and how their differences may be related to the interaction between these enzymes and their biological reductant, flavodoxin.
As we learned in BCM44, the flavin mononucleotide (FMN) cofactor of flavodoxin must be within electron transfer distance from the AdoMet radical cluster for cluster reduction. Therefore, the authors must consider how changes in protein folding observed in these QueE structures could explain the reductant specificity noted above for QueE enzymes. We have also learned that the N‐ and C‐terminal extensions are important for both substrate binding and dimerization in Bm
The authors present the structure of EcQueE i(n the absence of substrate) and compared this structure with previous QueE structures from Bm and Bs. Interestingly, these three QueEs, which all catalyze the exact same reaction, are farther apart in sequence space than are other AdoMet radical enzymes that catalyze completely different reaction
This paper concludes with not only a discussion but also a conclusion, posing a question that they had at the very beginning. By determining the structural data, they were able to evaluate the relationship between fold variation and AdoMet binding, substrate binding, Mg2+ ion binding, and flavodoxin binding, and propose that the QueE structural variation is most likely in response to flavodoxin variations.