Minimalist enzymes designed to catalyze model reactions provide useful starting points for creating catalysts for practically important chemical transformations. We have shown that Kemp eliminases of the AlleyCat family facilitate conversion of leflunomide (an immunosuppressor pro‐drug) to its active form teriflunomide with outstanding rate enhancement (nearly four orders of magnitude) and catalytic proficiency (more than seven orders of magnitude) without any additional optimization. This remarkable activity is achieved by properly positioning the substrate in close proximity to the catalytic glutamate with very high pKa.

A computationally designed, allosterically regulated catalyst (CaM M144H) produced by substituting a single residue in calmodulin, a non-enzymatic protein, is capable of efficient and site selective post-translational acylation of lysines in peptides with highly diverse sequences. Calmodulin's binding partners are involved in regulating a large number of cellular processes; this new chemical-biology tool will help to identify them and provide structural insight into their interactions with calmodulin.

Metalloenzymes often utilize radicals in order to facilitate chemical reactions. Recently, DeGrado and co‐workers have discovered that model proteins can efficiently stabilize semiquinone radical anion produced by oxidation of 3,5‐di‐tert‐butylcatechol (DTBC) in the presence of two zinc ions. Here, we show that the number and the nature of metal ions have relatively minor effect on semiquinone stabilization in model proteins, with a single metal ion being sufficient for radical stabilization. The radical is stabilized by both metal ion, hydrophobic sequestration, and interactions with the hydrophilic residues in the protein interior resulting in a remarkable, nearly 500 mV change in the redox potential of the SQ.−/catechol couple compared to bulk aqueous solution. Moreover, we have created 4G‐UFsc, a single metal ion‐binding protein with pm affinity for zinc that is higher than any other reported model systems and is on par with many natural zinc‐containing proteins. We expect that the robust and easy‐to‐modify DFsc/UFsc family of proteins will become a versatile tool for mechanistic model studies of metalloenzymes.

The cover feature picture shows how a computationally designed allosterically regulated esterase CaM M144H, a derivative of AlleyCatE, can recognize and specifically post‐translationally modify helical domains (highlighted in green) in calmodulin‐binding proteins with an unnatural tag providing structural and functional insight into protein–protein interactions. More information can be found in the communication by O. V. Makhlynets, I. V. Korendovych, et al. on page 1605 in Issue 15, 2018 (DOI: 10.1002/cbic.201800196).

The Front Cover picture shows that AlleyCat2, a member of the AlleyCat family of allosterically regulated Kemp eliminases, is capable of binding leflunomide, an immunosuppressant drug, and converting it into teriflunomide, its active form, with remarkable efficiency. In their Communication, E. A. Caselle, J. H. Yoon et al. show that small libraries of designed catalysts provide fertile ground for discovering new reactivities. AlleyCat2 relies on a high pKa of the active base and proper positioning of the substrate in the hydrophobic cleft of the enzyme to promote catalysis. This work also demonstrates that using pH rate profiles to determine the pKa of the active residue can be quite misleading and NMR studies that can probe specific atoms directly provide invaluable mechanistic information. More information can be found in the Communication by E. A. Caselle, J. H. Yoon et al. on page 1425 in Issue 5, 2019 (DOI: 10.1002/cctc.201801994).

The front cover artwork for Issue 05/2019 is provided by the Korendoych and the Makhlynets Labs at Syracuse University (USA) in collaboration with the VIB Centre for Structural Biology (Belgium), College of Charleston (USA) and Kyiv National University (Ukraine). The image shows AlleyCat2, a minimalist allosterically regulated Kemp eliminase, efficiently promoting ring opening of leflunomide, an immunosuppresant. Small libraries of designed catalysts provide fertile ground for discovering new reactivities. See the Communication itself at https://doi.org/10.1002/cctc.201801994.