Enzymatic “tricks”: Carboxylate shift and sulfur shift

We look into two interesting phenomena that occur in enzymes: one has been termed carboxylate shift and the other sulfur shift. Carboxylate shift is an interesting mechanistic phenomenon, which is characterized by a change in the coordination mode of a carboxylate group (monodentate to bidentate or vice versa) with ligand entrance or ligand exit from the metal coordination sphere. We focus on some patterns relating the intrinsic characteristics of a given metal coordination sphere to the occurrence of a carboxylate shift and the corresponding energy stabilization suffered by the enzyme. A structural rearrangement known as sulfur shift has been recognized to occur in some Mo‐containing enzymes of the DMSO reductase family. This mechanism is characterized by the displacement of a coordinating cysteine thiol or selenocysteine thiol from the first to the second shell of the Mo‐coordination sphere metal, allowing for direct coordination of the incoming ligand (substrate or inhibitor) to the hexacoordinated Mo ion found in the enzyme unbound state. The sulfur shift, as well as the carboxylate shift, results in a rearrangement of the enzymatic catalytic center that provides an efficient mechanism to keep a constant coordination number throughout an entire catalytic pathway. © 2014 Wiley Periodicals, Inc.