Microbial fermentation usually takes glycerol, glucose and other biomass carbon sources as raw materials, and accumulates L-tyrosine through fermentation of excellent microbial strains under appropriate conditions. Early research often used artificial mutagenesis to select L-tyrosine high-yield strains, such as screening L-phenylalanine or L-tryptophan defective or anti feedback inhibition strains. However, the ability of most microorganisms to accumulate aromatic amino acids is very low, and the regulatory mechanism of their metabolic pathways is very complex. Traditional mutation breeding methods can only act on local metabolic pathways or key enzymes, and it is difficult to have a great impact on the overall L-tyrosine metabolic flow. In recent years, with the rapid development of metabolic engineering and various advanced biotechnology, it has become a research hotspot to redesign the metabolic pathway of microorganisms to better realize the fermentation of L-tyrosine. The L-tyrosine metabolic engineering bacteria studied more mainly include Escherichia coli, Corynebacterium glutamicum, Brevibacterium flavum and Bacillus subtilis. Among them, the synthesis pathway and regulation mechanism of L-tyrosine in Escherichia coli and Bacillus glutamicum have been studied most and explained most clearly.
L-Tyrosine biosynthesis pathway belongs to aromatic amino acid biosynthesis pathway. The precursor of its synthesis, Erythrose-4-phosphate (E4P), and phosphoenolpyruvate (PEP), are condensed under the catalysis of DAHP synthetase (DS) to produce 3-deoxy-D-arabinogeptanosyl-7-phosphate (DAHP), which is also the first rate limiting step of L-tyrosine biosynthesis pathway. In Escherichia coli, DAHP synthetase contains three isoenzymes: AroG, AroF and AroH. Its expression and activity are inhibited and suppressed by the feedback of the products L-phenylalanine, L-tyrosine and L-tryptophan, respectively. The 7-step reaction from DAHP to branched acids is a common route for all aromatic amino acids. Branched acid is the branch point of aromatic amino acid synthesis pathway. One branch pathway is used to synthesize L-tryptophan, and the other part generates 4-hydroxyphenylpyruvate (4HPP) under the action of branched acid mutase (CM) and prephenate dehydrate (PD) bifunctional enzyme TyrA. The latter generates L-tyrosine through transamination with L-glutamate, The expression and activity of TyrA were also inhibited by the feedback of L-tyrosine.