Isotopic replacement has long-proven applications in small molecules. However, applications in proteins are largely limited to biosynthetic strategies or exchangeable (for example, N-H/D) labile sites only. The development of postbiosynthetic, C-H → C-H/D replacement in proteins could enable probing of mechanisms, among other uses. Here we describe a chemical method for selective protein α-carbon deuteration (proceeding from Cys to dehydroalanine (Dha) to deutero-Cys) allowing overall H→H/D exchange at a nonexchangeable backbone site. It is used here to probe mechanisms of reactions used in protein bioconjugation. This analysis suggests, together with quantum mechanical calculations, stepwise deprotonations via on-protein carbanions and unexpected sulfonium ylides in the conversion of Cys to Dha, consistent with a 'carba-Swern' mechanism. The ready application on existing, intact protein constructs (without specialized culture or genetic methods) suggests this C-D labeling strategy as a possible tool in protein mechanism, structure, biotechnology and medicine.

JMJD2A catalyses the demethylation of di- and trimethylated lysine residues in histone tails and is a target for the development of new anticancer medicines. Mechanistic details of demethylation are yet to be elucidated and are important for the understanding of epigenetic processes. We have evaluated the initial step of histone demethylation by JMJD2A and demonstrate the dramatic effect of the protein environment upon oxygen binding using quantum mechanics/molecular mechanics (QM/MM) calculations. The changes in electronic structure have been studied for possible spin states and different conformations of O2 , using a combination of quantum and classical simulations. O2 binding to this histone demethylase is computed to occur preferentially as an end-on superoxo radical bound to a high-spin ferric centre, yielding an overall quintet ground state. The favourability of binding is strongly influenced by the surrounding protein: we have quantified this effect using an energy decomposition scheme into electrostatic and dispersion contributions. His182 and the methylated lysine assist while Glu184 and the oxoglutarate cofactor are deleterious for O2 binding. Charge separation in the superoxo-intermediate benefits from the electrostatic stabilization provided by the surrounding residues, stabilizing the binding process significantly. This work demonstrates the importance of the extended protein environment in oxygen binding, and the role of energy decomposition in understanding the physical origin of binding/recognition.

Invited for the cover of this issue is the group of Robert S. Paton at the University of Oxford and his collaborators from Brazil and the Czech Republic. The image depicts histone–enzyme complexation and the chemical interactions inside the active site that define the mode of action. Read the full text of the article at .

The unique structure of furanocembranoid natural product bielschowskysin has provoked a number of biosynthetic hypotheses: quantum chemical calculations provide a means to assess the feasibility of postulated mechanisms in the construction of this unusual carbon skeleton. Calculations reveal that thermal closure is possible in water via an unusual concerted cyclobutane-forming transition state without the intervention of an enzyme. Photocycloaddition is computed to be extremely efficient, provided enol ether triplet sensitization can be achieved by an appropriate light source. The possible existence of a stable dicarbonyl intermediate presents a challenge for the thermal route, implicating a photochemical pathway in bielschowskysin biosynthesis.

Before trimethylated lysines can be epigenetically “erased”, molecular oxygen must be bound into the active site of JmjC proteins. This work sheds new computational understanding on the role of the wider protein environment in making this process possible. In the foreground of the cover illustration by Dr Karl Harrison an atomistic description of this complex is shown, while in the background these proteins can be seen interacting with the DNA bound histones. For more details see the Full Paper on by R. S. Paton and co‐workers.