Examining the Behavior of Surface Tethered Enzymes.

Abstract

Surface-immobilized enzymes are important for a wide range of technological applications, including industrial catalysis, drug delivery, medical diagnosis and biosensors. However, our understanding of how enzymes and proteins interact with abiological surfaces on the molecular level remains extremely limited. We have compared the structure, activity and thermal stability of beta-galactosidase variants attached to a chemically well-defined self assembled monolayer (SAM) surface. Maleimide-terminated ethylene glycol linkers were used to attach beta-galactosidase through a unique cysteinyl residue. These maleimide-terminated linkers were mixed with ethylene glycol linkers terminated with different chemical moieties to engineer surfaces with varying hydrophobicity and electrostatic charge. In collaboration with the Chen Lab, we used SFG and ATR-FTIR to experimentally measure the orientation of the surface tethered enzyme. In collaboration with the Brooks Lab, we conducted coarse grain model simulations to examine the atomic level interactions between the protein and the surface. Through coarse grain modelling, it was shown that the increased range of motion allowed to an enzyme tethered to a flexible loop region increased the number of protein surface interactions relative to the interactions experienced by an enzyme attached to the helix. For mildly hydrophibic surfaces, such as a full maleimide-terminated SAM, these increased interactions are destabilizing. This was confirmed experimentally by the reduction in thermal stability for beta-galactosidase attached by the loop region. Using SFG, it was shown that the distribution of orientations of an enzyme attached to a loop is greater than to an enzyme tethered to a helix. By varying the electrostatic properties of the terminal groups used in the SAMs, it was shown that beta-galactosidase immobilized onto SAMs containing a mixture of positively and negatively terminated residues retained a higher level of specific activity than surfaces terminated with either uncharged hydrophilic or hydrophobic surfaces. Thermal stability was highest on uncharged hydrophilic surfaces. Overall, we were able to develop a molecular level model for the behavior of surface attached enzymes, and a potential approach for predicting approaches for engineering tethered enzyme systems.