Coengineering Vapor-Deposited Polycationic Microenvironments and Immobilization Strategy to Enhance β-Galactosidase Kinetics at Acidic pH

Abstract

The industrial application of enzymes, such as β-galactosidase (LacZ) for processing acid whey, is often constrained by their poor activity at acidic pH. This work addresses this limitation by covalently immobilizing LacZ on copolymer thin films synthesized by initiated chemical vapor deposition (iCVD) containing 2-(dimethylamino)ethyl methacrylate (DMAEMA), which becomes positively charged at acidic pH. Chemically analogous self-assembled monolayer (SAM) supports served as a solution-synthesized benchmark. We systematically investigated immobilization strategies─random immobilization (RI) vs directed immobilization (DI, via SpyCatcher/SpyTag)─and support properties (polycationic content, cross-linking degree, support thickness, and synthesis route: iCVD vs SAM), quantifying their effects on LacZ activity and kinetic parameters. Generally, our results indicate that LacZ immobilized via DI outperformed RI across support properties under wide pHs (4–8), mainly by enhancing the LacZ’s intrinsic turnover number (kcat) rather than by reducing Michaelis constant (Km). For both DI and RI, cationic moieties in the supports were crucial for preserving LacZ activity under stronger acid stress (pH 4); moreover, under DI, kcat scaled with DMAEMA molar content, consistent with a proposed electrostatic shielding mechanism that may reduce local proton exposure that would otherwise depress kcat. Additionally, holding polycationic content constant (25 mol %), the iCVD films (∼200 nm) exhibited an approximately 8-fold higher normalized initial rate relative to SAM films at pH 4, but this enhancement was detectable only for DI, not RI. The optimal formulation depended on the performance metric, with distinct formulations maximizing kcat and catalytic efficiency (kcat/Km). These findings provide mechanistic insights into how immobilization strategy and the local microenvironment jointly affect LacZ kinetics, demonstrating that coengineering both may achieve superior enzyme performance under challenging reaction conditions.