Fig 1.
Biosynthetic routes for ectoine and 5-hydroxyectoine.
Scheme of the ectoine and 5-hydroxyectoine biosynthetic pathway [20, 21, 25].
Table 1.
Conversion of N-γ-ADABA into ectoine by (Sa)EctC mutant derivatives and their iron-content.
Fig 2.
Abbreviated alignment of EctC-type proteins.
The amino acid sequences of 20 selected EctC-type proteins are compared. Strictly conserved amino acid residues are shown in yellow. Dots shown above the (Sa)EctC protein sequence indicate residues likely to be involved in iron-binding (red), ligand-binding (green) and stabilization of the loop-architecture (blue). The conserved residue Tyr-52 with so-far undefined functions is indicated by a green dot circled in red. Secondary structural elements (α-helices and β-sheets) found in the (Sa)EctC crystal structure are projected onto the amino acid sequences of EctC-type proteins.
Fig 3.
Dependency of the ectoine synthase activity on metals.
(a) Impact of the iron-chelator EDTA on the enzyme activity of the purified (Sa)EctC protein. Metal depletion and reconstitution experiments with (b) stoichiometric and (c) excess amounts of metals. The (Sa)EctC protein was present at a concentration of 10 μM. The level of enzyme activity given in (b) is benchmarked relative to that of ectoine synthase enzyme assays in which 1 mM FeCl2 was added.
Fig 4.
Overall structure of the “open” and “semi-closed” crystal structures of (Sa)EctC.
(a) The overall structure of the “semi-closed” (Sa)EctC resolved at 2.0 Å is depicted in green in a cartoon (upper panel) and surface (lower panel) representation. The β-strands are numbered β1-β11 and the helices α-I to α-II. (b) The overall structure of the “open” (Sa)EctC was resolved at 1.2 Å and is depicted in yellow in a cartoon (upper panel) and surface (lower panel) representation. The entrance to the active site of the ectoine synthase is marked. (c) Overlay of the “semi-closed” and “open” (Sa)EctC structures.
Fig 5.
Crystal structure of (Sa)EctC.
(a) Top-view of the dimer of the (Sa)EctC protein. The position of the water molecule, described in detail in the text, is shown in one of the monomers as an orange sphere. (b) Side-view of a (Sa)EctC dimer allowing an assessment of the dimer interface formed by two β-strands of each monomer. (c) Close-up representation of the dimer interface mediated by beta-strand β1 and β6.
Fig 6.
Architecture of the presumed metal-binding site of the (Sa)EctC protein and its flexible carboxy-terminus.
(a) The described water molecule (depicted as orange sphere) is bound via interactions with the side chains of Glu-57, Tyr-85, and His-93. The position occupied by this water molecule represents probably the position of the Fe2+ cofactor in the active side of the ectoine synthase. His-55 interacts with the double proline motif (Pro-109 and Pro-110). It is further stabilized via an interaction with the side chain of Glu-115 which is localized in the flexible carboxy-terminus (colored in orange) of (Sa)EctC that is visible in the “semi-closed” (Sa)EctC structure. (b) An overlay of the “open” (colored in light blue) and the “semi-closed” (colored in green) structure of the (Sa)EctC protein.
Fig 7.
A chemically undefined ligand is captured in the active site of the “semi-closed” (Sa)EctC crystal structure.
(a) The observed electron density in the active site of the “semi-closed” structure of (Sa)EctC is modeled as a hexane-1,6-diol molecule and compared with the electron density of the N-γ-ADABA substrate of the ectoine synthase to emphasize the similarity in size of these compounds. (b) The presumable binding site of the iron co-factor and of the modeled hexane-1,6-diol molecule is depicted. The amino acid side chains involved in iron-ligand binding are colored in blue and those involved in the binding of the chemically undefined ligand are colored in green using a ball and stick representation. The flexible carboxy-terminal loop of (Sa)EctC is highlighted in orange. The electron density was calculated as an omit map and contoured at 1.0 σ.