Microbial synthesis of poly-γ-glutamic acid: current progress, challenges, and future perspectives

Abstract

Poly-γ-glutamic acid (γ-PGA) is a naturally occurring biopolymer made from repeating units of l-glutamic acid, d-glutamic acid, or both. Since some bacteria are capable of vigorous γ-PGA biosynthesis from renewable biomass, γ-PGA is considered a promising bio-based chemical and is already widely used in the food, medical, and wastewater industries due to its biodegradable, non-toxic, and non-immunogenic properties. In this review, we consider the properties, biosynthetic pathway, production strategies, and applications of γ-PGA. Microbial biosynthesis of γ-PGA and the molecular mechanisms regulating production are covered in particular detail. Genetic engineering and optimization of the growth medium, process control, and downstream processing have proved to be effective strategies for lowering the cost of production, as well as manipulating the molecular mass and conformational/enantiomeric properties that facilitate screening of competitive γ-PGA producers. Finally, future prospects of microbial γ-PGA production are discussed in light of recent progress, challenges, and trends in this field.

Keywords: Industrial applications; Metabolic regulation; Microbial fermentation; Poly-γ-glutamic acid; Process optimization; Strain development.

Fig. 1

Microbial biosynthesis of γ-PGA [8, 10]. Types of substrates in the culture medium were mostly a variety of biomass materials, cane molasses, agro-industrial wastes, which could be degraded into C6 and C5 compound, entering into the main carbon metabolism via glycolysis and pentose phosphate pathway. In addition, glycerol as well as metabolic intermediates of citrate cycle was also used as candidate substrate [79]. The main byproducts were acetoin and 2,3-butanediol; other byproducts with little production were lactate, ethanol, and acetate [80]. PPP pentose phosphate pathway, G3P glyceraldehyde 3-phosphate, E1 glutamate dehydrogenase (GD), E2 glutamate 2-oxoglutarate aminotransferase, E3 glutamine synthetase (GS), E4 l-glutamic acid: pyruvate aminotransferase, E5 alanine racemase, E6 d-glutamic acid: pyruvate aminotransferase, E7 direction conversion, E8 PGA synthetase

Fig. 2

Arrangement of genes encoding γ-PGA synthetase and γ-PGA peptidase complexes in various species. All components of γ-PGA synthetase are essentially membrane associated) [8]