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