Strategies for enhancing extracellular secretion of recombinant cyclodextrin glucanotransferase in E. coli

Cyclodextrin glycosyltransferase (CGTase) is an enzyme that produces cyclodextrins from starch by an intramolecular transglycosylation reaction. Due to the increasing industrial application of cyclodextrins in many fields such as pharmacy, agriculture, biotechnology, food, environment and cosmetics, CGTases have attracted the attention of many scientific researches. Undoubtedly, due to its well-known genetic properties, simplicity and capacity to accommodate many foreign proteins, Escherichia coli remains the most widely used host for recombinant proteins production and thus for CGTases. Like all other proteins, CGTases are originally produced in the cytoplasm, but expressing them out into the periplasm or further to the culture media is preferred due to several advantages such as simplified downstream processing and high expression level which otherwise would be costly, complicated and time consuming. Since E. coli, other than some of its degradative enzymes and toxins, does not normally secrete proteins extracellularly, many strategies have been tried to overcome this drawback using the recombinant technologies. Unfortunately, oversecretion of the recombinant proteins most of the time results in the formation of inactive protein aggregates, called inclusion bodies, which result as a consequence of the burden caused by the methods meant to enhance the secretion. Thus, in this mini-review, the few but most commonly used strategies which offered a solution to the enhancement of extracellular secretion of CGTase in its native state are discussed.