Replacing of Cysteine with Selenocysteine: Biochemical Considerations, Computational Modeling, and Protein Engineering Applications.

Selenocysteine (Sec), the 21st amino acid, combines the structural compatibility of cysteine with the unique chemical properties of selenium, enabling exceptional redox and catalytic performance in both natural and engineered systems. Its low pKa, high nucleophilicity, and increased polarizability underpin the activity of selenoproteins involved in antioxidant defence, redox regulation, and metabolic processes. This work integrates biochemical, structural, and evolutionary perspectives on Sec with advances in protein engineering and synthetic biology, where strategic Sec incorporation in place of Cys accelerates folding, enhances stability, and expands catalytic capabilities. Emphasis is placed on computational and quantum chemical studies that elucidate Sec's acid-base behavior, bond dissociation energies, and metal-binding preferences, revealing how subtle atomic substitution drives substantial functional gains. Comparative analysis of diselenide and disulfide bonds highlights Sec's potential to improve protein performance under both oxidative and reductive stress. Finally, this chapter demonstrates the application of computational chemistry tools to calculate key physicochemical parameters, illustrating how such methods can guide rational selenoprotein engineering for therapeutic, industrial, and bioengineering applications.