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Figure 4
Hexamer stabilization through ion binding and potential sulfilimine-bond sites. (a) The chloride ion ClA (red) interacts with the main-chain amino groups of Lβ7β8 (ClA motif) and a water molecule (blue) in two opposing monomers. (b) The chloride ion ClB (green) binds to residues of flexible regions from three opposing monomers in the ClB motif (Ala186 in Lβ7′β8′ and Tyr189 in β8′) and Asn66 in SM2. (c) Semi-transparent surface representation (enclosing a cartoon representation) of the equatorial view of the protomer in the canonical α1NC1homo hexamer indicating the six ClA ions (red) and six ClB ions (green) at the interface between the protomers in the hexamer. In addition, the six interfacial Met93 residues (yellow) and the six Lys211 residues (blue), three from the shown protomer and the other three from the top protomer (not shown), are also indicated to stress their proximity, which allows easy sulfilimine-bond formation. These residues and the ClA and ClB ions are also shown in the view of the complete α1NC1homo hexamer illustrated on the right. (d) Zoom on the flexible Lβ8β9 (SM3) and β9′ (SM3′) regions of opposing monomers, highlighting the closeness of the Met93 and Lys211 residues of each region, respectively. A mere rotamer change of Lys211 would place it in position for the formation of a sulfilimine bridge with Met93.

IUCrJ
Volume 5| Part 6| November 2018| Pages 765-779
ISSN: 2052-2525
https://doi.org/10.1107/S2052252518012459