Transglutaminases, thioredoxins and protein disulphide isomerase: diverse enzymes with a common goal of cross-linking proteins in lower organisms.

Prokaryotes and various eukaryotes have remarkable ability to survive under adverse physiologic conditions and protect themselves from environmental stresses. An important mechanism by which they accomplish this is by synthesizing rigid and biochemically inert structures around them. In general, these structures are highly stable and resistant to mechanical and chemical insults. Biochemically, they are composed of complex carbohydrates, such as chitin and heavily crosslinked scaffold of proteins to form complex structures, such as sheath, cuticle, and epicuticle. Transglutaminases (TGases) are a family of enzymes that share catalytic function with thioredoxin and protein disulphide isomerases (PDI) and catalyze protein crosslink reaction by establishing epsilon-(gamma-glutamyl)lysine isopeptide bonds. The isopeptide bonds thus formed are of great physiologic significance because once formed, they cannot be hydorlysed by any known enzymes of the eukaryote system and exhibit high resistance to reducing agents, detergents, and chaotropic agents. Therefore, it is likely that protective structures viz., sheath, cuticle, epicuticle, and viral core proteins synthesized by microorganisms involve active participation of TGases. In this review, we briefly describe the current knowledge of non-mammalian TGases and their possible role in growth, development, and survival of small organisms. Special reference is made to filarial nematode and bacterial TGases since they are the most well-characterized and studied enzymes among non-mammalian TGases.