Two routes for tyrosol production by metabolic engineering of Corynebacterium glutamicum

The phenolic compound tyrosol is widely used in the pharmaceutical industry, owing to its beneficial effects on human health and its use as a precursor for key pharmaceuticals, including β1 receptor blockers. Tyrosol can be found in olive oil, but despite its natural synthesis in plants, low extraction efficiencies render microbial production a more viable alternative.

Here, we engineered the ʟ-tyrosine overproducing Corynebacterium glutamicum strain AROM3 for the de novo production of tyrosol. Two routes were compared: one via 4-OH-phenylpyruvate as intermediate and the other via tyramine. Contrary to our expectation for the first route, the heterologous expression of the prephenate dehydrogenase gene from E. coli was not required for 4-OH-phenylpyruvate synthesis, although C. glutamicum lacks this enzymatic function. Instead, our findings revealed that 4-OH-phenylpyruvate is released by native aminotransferases upon deamination of ʟ-tyrosine. Its further decarboxylation was achieved by heterologous expression of ARO10 from Saccharomyces cerevisiae encoding a pyruvate decarboxylase. Given the instability of 4-OH-phenylpyruvate, the synthesis of tyrosol via the stable intermediate tyramine was pursued via the second route. Decarboxylation of ʟ-tyrosine followed by deamination was accomplished by overexpression of the ʟ-tyrosine decarboxylase gene tdc from Levilactobacillus brevis and the tyramine oxidase gene tyo from Kocuria rhizophila. Both routes converge by the synthesis of 4-OH-phenylacetaldehyde, which is subsequently reduced to tyrosol by native alcohol dehydrogenases. We identified the furfural dehydrogenase FudC to be one of the enzymes involved in this process, as its gene deletion reduced tyrosol production by 75 %. Via both routes, strain AROM3 overexpressing either ARO10Sc or tdcLb together with tyoKr produced more than 9 mM (1.2 g/L) of tyrosol from 40 g/L glucose in shake flask experiments. This proof-of-concept of fermentative tyrosol production will be followed up further.