The Fold of MetE and Similarities between the Two Barrels

<p>(A) MetE folds into two (βα)₈ barrels. The N-terminal barrel (aqua) is joined to the C-terminal barrel (yellow) by a 35-residue inter-domain linker (gray) that spans 65 Å. Except for the α1 helix (at the right), the linker residues are in extended conformations. This view is along the approximate 2-fold axis that relates the two barrels. The drawing is based on coordinates for zinc-replete MetE in complex with CH₃-H₄folate (<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0030031#pbio-0030031-t001" target="_blank">Table 1</a>). The zinc ligands, zinc, and CH₃-H₄folate are shown in ball-and-stick representation. This figure and all subsequent figures were prepared using RIBBONS [<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0030031#pbio-0030031-b43" target="_blank">43</a>]. See <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0030031#pbio-0030031-g001" target="_blank">Figure 1</a> for the nomenclature used to describe secondary structures.</p> <p>(B) A side-by-side view of the barrels of MetE, arranged to show the similarities of the β–α loop extensions. Major extensions that follow the first four β strands of the barrels are shown in cyan and gold for the N-terminal and C-terminal barrels, respectively. The drawing is based on coordinates for the zinc-replete binary complex with folate (not shown). The active site is located in the C-terminal barrel (on the right) between the extra-barrel β hairpin of the β2–α2 loop and the C-termini of the barrel strands. Zinc is gray and the zinc ligands, His618, Cys620, Glu642, and Cys704, are shown in ball-and-stick mode.