Probing Selective Self-Assembly of Putrescine Oxidase with Controlled Orientation Using a Genetically Engineered Peptide Tag.

Controlling enzyme orientation and location on surfaces is a critical step for their successful deployment in diverse applications from biosensors to lab-on-a-chip devices. Functional activity of the enzymes on the surface will largely depend on the spatial arrangement and orientation. Solid binding peptides have been proven to offer versatility for immobilization of biomolecules on inorganic materials including metals, oxides, and minerals. Previously, we demonstrated the utility of a gold binding peptide genetically incorporated into the enzyme putrescine oxidase (PutOx-AuBP), enabling self-enzyme assembly on gold substrates. PutOx is an attractive biocatalyst among flavin oxidases, using molecular oxygen as an electron acceptor without requiring a dissociable coenzyme. Here, we explore the selective self-assembly of this enzyme on a range of surfaces using atomic force microscopy (AFM) along with the assessment of functional activity. This work probes the differences in surface coverage, distribution, size, shape, and activity of PutOx-AuBP in comparison to those of native putrescine oxidase (PutOx) on multiple surfaces to provide insight for material-selective enzymatic assembly. Surfaces investigated include metal (templated-stripped gold (TSG)), oxide (native SiO2 on Si(111)), minerals (mica and graphite), and self-assembled monolayers (SAMs) with a range of hydrophobicity and charge. Supported by both the coverage and the dimensions of immobilized enzymes, our results indicate that of the surfaces investigated, material-selective binding takes place with orientation control only for PutOx-AuBP onto the TSG substrate. These differences are consistent with the measurements of surface-bound enzymatic activities. Substrate-dependent differences observed indicate significant variations in enzyme-surface interactions ranging from peptide-directed self-assembly to enzyme aggregation. The implications of this study provide insight for the fabrication of enzymatic patterns directed by self-assembling peptide tags onto localized surface regions. Enabling functional enzyme-based nanoscale materials offers a fascinating path for utilization of sustainable biocatalysts integrated into multiscale devices.