Characterization of two key enzymes for aromatic amino acid biosynthesis in symbiotic archaea

Biochemical characterization of TyrA dehydrogenases from Saccharomyces cerevisiae (Ascomycota) and Pleurotus ostreatus (Basidiomycota)

L-Tyrosine is an aromatic amino acid necessary for protein synthesis in all living organisms and a precursor of secondary (specialized) metabolites. In fungi, tyrosine-derived compounds are associated with virulence and defense (i.e. melanin production). However, how tyrosine is produced in fungi is not fully understood. Generally, tyrosine can be synthesized via two pathways: by prephenate dehydrogenase (TyrA_p/PDH), a pathway found in most bacteria, or by arogenate dehydrogenase (TyrA_a/ADH), a pathway found mainly in plants. Both enzymes require the cofactor NAD⁺ or NADP⁺ and typically are strongly feedback inhibited by tyrosine. Here, we biochemically characterized two TyrA enzymes from two distantly related fungi in the Ascomycota and Basidiomycota, Saccharomyces cerevisiae (ScTyrA/TYR1) and Pleurotus ostreatus (PoTyrA), respectively. We found that both enzymes favor the prephenate substrate and NAD⁺ cofactor in vitro. Interestingly, while PoTyrA was strongly inhibited by tyrosine, ScTyrA exhibited relaxed sensitivity to tyrosine inhibition. We further mutated ScTyrA at the amino acid residue that was previously shown to be involved in the substrate specificity of plant TyrAs; however, no changes in its substrate specificity were observed, suggesting that a different mechanism is involved in the TyrA substrate specificity of fungal TyrAs. The current findings provide foundational knowledge to further understand and engineer tyrosine-derived specialized pathways in fungi.

Biochemical characterization of TyrA enzymes from Ignicoccus hospitalis and Haemophilus influenzae: A comparative study of the bifunctional and monofunctional dehydrogenase forms

Biosynthesis of l-tyrosine (l-Tyr) is directed by the interplay of two enzymes. Chorismate mutase (CM) catalyzes the rearrangement of chorismate to prephenate, which is then converted to hydroxyphenylpyruvate by prephenate dehydrogenase (PD). This work reports the first characterization of the independently expressed PD domain of bifunctional CM-PD from the crenarchaeon Ignicoccus hospitalis and the first functional studies of both full-length CM-PD and the PD domain from the bacterium Haemophilus influenzae. All proteins were hexa-histidine tagged, expressed in Escherichia coli and purified. Expression and purification of I. hospitalis CM-PD generated a degradation product identified as a PD fragment lacking the protein's first 80 residues, Δ80CM-PD. A comparable stable PD domain could also be generated by limited tryptic digestion of this bifunctional enzyme. Thus, Δ80CM-PD constructs were prepared in both organisms. CM-PD and Δ80CM-PD from both organisms were dimeric and displayed the predicted enzymatic activities and thermal stabilities in accord with their hyperthermophilic and mesophilic origins. In contrast with H. influenzae PD activity which was NAD⁺-specific and displayed >75% inhibition with 50μM l-Tyr, I. hospitalis PD demonstrated dual cofactor specificity with a preference for NADP⁺ and an insensitivity to l-Tyr. These properties are consistent with a model of the I. hospitalis PD domain based on the previously reported structure of the H. influenzae homolog. Our results highlight the similarities and differences between the archaeal and bacterial TyrA proteins and reveal that the PD activity of both prokaryotes can be successfully mapped to a functionally independent unit.