Exploiting Substrate Promiscuity of Ectoine Hydroxylase for Regio- and Stereoselective Modification of Homoectoine

Extant enzymes are not only highly efficient biocatalysts for a single, or a group of chemically closely related substrates but often have retained, as a mark of their evolutionary history, a certain degree of substrate ambiguity. We have exploited the substrate ambiguity of the ectoine hydroxylase (EctD), a member of the non-heme Fe(II)-containing and 2-oxoglutarate-dependent dioxygenase superfamily, for such a task. Naturally, the EctD enzyme performs a precise regio- and stereoselective hydroxylation of the ubiquitous stress protectant and chemical chaperone ectoine (possessing a six-membered pyrimidine ring structure) to yield trans-5-hydroxyectoine. Using a synthetic ectoine derivative, homoectoine, which possesses an expanded seven-membered diazepine ring structure, we were able to selectively generate, both in vitro and in vivo, trans-5-hydroxyhomoectoine. For this transformation, we specifically used the EctD enzyme from Pseudomonas stutzeri in a whole cell biocatalyst approach, as this enzyme exhibits high catalytic efficiency not only for its natural substrate ectoine but also for homoectoine. Molecular docking approaches with the crystal structure of the Sphingopyxis alaskensis EctD protein predicted the formation of trans-5-hydroxyhomoectoine, a stereochemical configuration that we experimentally verified by nuclear-magnetic resonance spectroscopy. An Escherichia coli cell factory expressing the P. stutzeri ectD gene from a synthetic promoter imported homoectoine via the ProU and ProP compatible solute transporters, hydroxylated it, and secreted the formed trans-5-hydroxyhomoectoine, independent from all currently known mechanosensitive channels, into the growth medium from which it could be purified by high-pressure liquid chromatography.

Overexpression, crystallization and preliminary X-ray crystallographic analysis of the ectoine hydroxylase from Sphingopyxis alaskensis

The ectoine hydroxylase (EctD) is a member of the non-haem-containing iron(II)- and 2-oxoglutarate-dependent dioxygenase superfamily. Its mononuclear iron centre is a prerequisite for the activity of this enzyme and promotes the O2-dependent oxidative decarboxylation of 2-oxoglutarate, which is coupled to a two-electron oxidation of the substrate ectoine to yield 5-hydroxyectoine. An expression and purification protocol for the EctD enzyme from Sphingopyxis alaskensis was developed and the protein was crystallized using the sitting-drop vapour-diffusion method. This resulted in two different crystal forms, representing the apo and iron-bound forms of the enzyme.

Synthesis and release of the bacterial compatible solute 5-hydroxyectoine in Hansenula polymorpha

Ectoine and 5-hydroxyectoine belong to the family of compatible solutes which are known to mainly contribute to the adaptation of the cell to osmotic stress by mediation of a constant turgor. In addition the cell's essential functions are maintained under stress conditions like high salinity, heat or aridity stress. Hansenula polymorpha was engineered to catalyze the transformation of monomeric substrates to 5-hydroxyectoine. For this purpose four genes encoding the enzymes of the 5-hydroxyectoine biosynthesis pathway of Halomonas elongata, EctA, EctB, EctC, and EctD, were inserted into the genome of H. polymorpha. Subsequently the syntheses of ectoine and 5-hydroxyectoine were analyzed and optimized. We showed that H. polymorpha is a suitable system for recombinant 5-hydroxyectoine synthesis in gram per liter scale (2.8 g L⁻¹ culture supernatant, 365 μmol/g dcw) in which almost 100% conversion of ectoine to 5-hydroxyectoine without necessity of high salinity were achieved.