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In this study, we expand the repertoire of biological catalysts by showing that a halo- gen bond (X-bond) can functionally replace the magnesium (Mg2+) cofactor in mouse endonuclease G (mEndoG). We mutated the metal coordinating glutamate E136 in mEndoG to a meta-halotyrosine (mXY, X = chlorine or iodine) to form a mXY-mEndoG construct that is both acid and base catalyzed. Under basic conditions, the enzyme is inactivated by the metal chelator ethylene diamine tetraacetic acid (EDTA), indicating that the halogen substituent facilitates deprotonation of the tyrosyl hydroxyl group, allowing recruitment of Mg2+ to restore the metal-dependent catalytic center. At low pHs, we observe that the mXY-mEndoG is resistant to EDTA inactivation and that the iodinated constructed is significantly more active than the chlorinated analogue. These results implicate a hydrogen bond (H-bond) enhanced X-bond as the catalyst in the mXY-mEndoG, with asparagine N103 serving as the H-bond donor that communicates the protonation state of histidine H104 to the halogen. This model is supported by mutation studies and electrostatic potential (ESP) calculations on models for the protonated and unprotonated mXY···N103···H104 system compared to the Mg2+ coor- dination complex of the wild type. Thus, we have designed and engineered an enzyme that utilizes an unnatural catalyst in its active site—a catalytic X-bonding enzyme, or cX-Zyme—by controverting what constitutes a metal catalyst in biochemistry. 

Abstract The structures and associated functions of biological molecules are driven by noncovalent interactions, which have classically been dominated by the hydrogen bond (H‐bond). Introduction of the σ‐hole concept to describe the anisotropic distribution of electrostatic potential of covalently bonded elements from across the periodic table has opened a broad range of nonclassical noncovalent ( nc NC) interactions for applications in chemistry and biochemistry. Here, we review how halogen bonds, chalcogen bonds and tetrel bonds, as they are found naturally or introduced synthetically, affect the structures, assemblies, and potential functions of peptides and proteins. This review intentionally focuses on examples that introduce or support principles of stability, assembly and catalysis that can potentially guide the design of new functional proteins. These three types of nc NC interactions have energies that are comparable to the H‐bond and, therefore, are now significant concepts in molecular recognition and design. However, the recently described H‐bond enhanced X‐bond shows how synergism among nc NC interactions can be exploited as potential means to broaden the range of their applications to affect protein structures and functions.