Structural mechanisms governing protein-electrode electron transfer: accessibility, redox relays, conformational gating, and quaternary shielding

Direct electron transfer (DET) between redox proteins and electrodes depends on the interplay between molecular architecture and interfacial physicochemical properties. Here, we developed a computationally inexpensive structural framework integrating planar electrode accessibility, internal redox relays, conformational gating, quaternary shielding, and protein-surface chemistry. Experimentally resolved protein structures were analyzed using orientation sampling and redox-center topology. Horseradish peroxidase exhibited favorable electrode accessibility (F20 = 0.392), whereas multicofactor enzymes revealed alternative routes to electronic communication. F20 correlated inversely with molecular weight (Spearman ρ = -0.883) and radius of gyration (ρ = -0.737), but not with size-normalized compactness (ρ = -0.069). Cellobiose dehydrogenase exhibited a pronounced conformational effect, with F20 increasing from 0.0277 to 0.2110 between closed and open states. Quaternary assembly restricted electrode accessibility in selected enzymes, while internal redox relays connected buried catalytic centers to peripheral electron-transfer sites. External evaluation using copper nitrite reductase and [NiFe]‑hydrogenase supported the structural interpretation of relay organization and electrode accessibility. Importantly, comparable geometric accessibility did not necessarily imply equivalent electrochemical competence, emphasizing the influence of surface chemistry, redox energetics, and electrode-interface interactions. The framework provides a reproducible, mechanism-oriented approach for interpreting protein-electrode communication and identifying experimentally testable strategies for bioelectrocatalytic interface design.