Recent advances in the covalent-bond-based peptide tagging systems and their applications.

Covalent bond-forming peptide tagging systems have emerged as powerful and versatile tools across a broad spectrum of biological and biotechnological applications. This review systematically summarizes the origins, molecular mechanisms of intramolecular covalent bond formation, major classes, and design strategies of peptide tagging systems. Based on their underlying chemistry, current systems are primarily categorized into isopeptide-bond-based and ester-bond-based platforms, both of which have demonstrated prominent utility in protein cyclization as well as in vivo and in vitro multi-enzyme assembly. Beyond these applications, isopeptide-bond-forming systems have been widely adopted as robust purification tags, whereas ester-bond-based systems offer unique opportunities for pH-responsive modulation of enzyme activity. Collectively, peptide tagging systems based on either isopeptide or ester bond formation represent an expanding and highly efficient toolkit for biotechnology. Continued advances in their design and application are expected to further broaden their functional scope and provide innovative solutions for future developments in protein engineering and related fields.