Catalytic Activity of Electrosprayed Enzyme Ions in the Gas Phase

Most enzyme reactions occur in aqueous solution, but catalytic activity is often retained in nonaqueous solvents. It is an intriguing question whether enzymes can also remain active in the gas phase, without any solvent. Here, we address this question by examining electrosprayed [L·NAG₅ + 8H]⁸⁺ complexes (L = lysozyme, NAG_<i>n</i> = <i>N</i>-acetyl glucosamine oligomer, a lysozyme substrate in solution). Collisional activation triggered cleavage of NAG₅ into NAG₄ and a monomeric NAG unit. The reaction proceeded via a [L·NAG₄ + 8H]⁸⁺ enzyme·product complex, confirming that cleavage took place while NAG₅ was bound to lysozyme. We demonstrate that the reaction exhibits hallmarks of enzyme catalysis, including the participation of specific active site residues (E35 and D52), substrate specificity, and a lowered activation energy. The gas-phase products differ from those in solution, as dictated by the absence of water. Our experimental findings are bolstered by mobile-proton molecular dynamics simulations of gaseous lysozyme complexes with substrates and products. In conclusion, this work for the first time demonstrates that an enzyme can be catalytically active <i>in vacuo</i>. While it is yet to be determined if this finding can be generalized to other enzymes, our study may provide a stepping stone to novel enzyme-catalyzed (bio)molecular transformations in the absence of a solvent.