Stabilization of Proteins by Covalent Cyclization

Abstract

Stabilizing proteins against the thermal stress or proteolytic attacks is an important goal in many protein engineering studies. As the simplest approach to the rational engineering of proteins, cyclizing proteins covalently has attracted a great deal of attention. The idea of stabilizing the folded state of a protein by connecting its loose ends came from the polymer theory and was evidenced by many super-stable cyclic peptides/proteins present in nature. Laboratory methodologies utilizing various tools such as inteins, transpeptidases, transglutaminases and split cellular anchoring proteins have been developed, engineered, and successfully adopted to protein cyclization reactions. Depending on the method used, N- and C-termini could be joined together to yield backbone cyclized proteins, or side chains located near the ends might be crosslinked to yield side chain cyclized ones. As each of the methods has its own pros and cons in its reaction scheme, the outcomes such as an increase in melting temperature of a given protein are different when different methods are applied. In this review, we highlight the stabilizing effects exerted by protein cyclization focusing on not a specific cyclizing method but product proteins.