L-Tyrosine [G-BSA]

Histidine decarboxylase (HDC; EC 4.1.1.22), an enzyme that catalyzes histamine synthesis with high substrate specificity, is a member of the group II pyridoxal 5'-phosphate (PLP) -dependent decarboxylase family. Tyrosine is a conserved residue among group II PLP-dependent decarboxylases. Human HDC has a Y334 located on a catalytically important loop at the active site. In this study, we demonstrated that a HDC Y334F mutant is capable of catalyzing the decarboxylation-dependent oxidative deamination of histidine to yield imidazole acetaldehyde. Replacement of the active-site Tyr with Phe in group II PLP-dependent decarboxylases, including mammalian aromatic amino acid decarboxylase, plant tyrosine/DOPA decarboxylase, and plant tryptophan decarboxylase, is expected to result in the same functional change, given that a Y-to-F substitution at the corresponding residue (number 260) in the HDC of Morganella morganii, another group II PLP-dependent decarboxylase, yielded the same effect. Thus, it was suggested that the loss of the OH moiety from the active-site Tyr residue of decarboxylase uniquely converts the enzyme to an aldehyde synthase.

l-Tyrosine is a versatile compound used in the fine chemical, pharmaceutical, and functional food industries. Here, we report a bi-enzymatic cascade involving alanine racemase (ALR) and d-amino acid oxidase (DAAO) to produce pyruvate, as co-substrate for l-tyrosine production, from the cheap substrate l-alanine. The BpALR (ALR from Bacillus pseudofirmus) was used as a whole-cell biocatalyst, converting l-alanine to d, l-alanine. The FsDAAO (DAAO from Fusarium solani) was immobilized to oxidize the d-alanine generated in the first step to pyruvate. Both systems were combined as a continuous-flow reactor for maximized l-alanine-to-pyruvate conversion rates. The optimal parameters and appropriate conditions for FsDAAO immobilization were investigated. The pyruvate concentration of 86.6 g/L was achieved within 17 h. Subsequently, a whole-cell biocatalyst system for l-tyrosine production, catalyzed by the tyrosine phenol-lyase (TPL) from Erwinia herbicola (EhTPL), was developed, and a fed-batch approach was applied with phenol and the pyruvate produced with the ALR/DAAO system mentioned above. The concentration of phenol and pyruvate in the reactor should not exceed 7.5 g/L and 10 g/L, respectively. Significantly, the l-tyrosine concentration of 152.5 g/L was achieved within 10 h, demonstrating the great potential for high-efficiency production of l-tyrosine through the approach we established in this paper.