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![[Figure 7]](cb5132fig7mag.jpg)
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Figure 7
Proposed reaction mechanism for hydrolysis of choline-O-sulfate by SmeCOSe based on structural models. Residues participating in the catalytic center of SmeCOSe from a different subunit appear in red. (1) Substrate binding is likely to occur concomitantly with closure of the Asp500–Asn146 hydrogen bond (*), producing the rearrangement of Leu499 towards the catalytic site. The sulfate moiety is bound to Asn75, His201 and Lys309, whereas the choline moiety is `clamped' between His145 and Trp129. (2) Activation of the diol of FGly54 by His201 would allow an SN2 attack, and cleavage of choline-sulfate, followed by protonation of the choline alkoxylate moiety by Lys309. (3) Lys309 can be reprotonated, probably by the same proton bound to His201. Rearrangement of choline into the catalytic cleft by direct or water-mediated contacts with Asp386 (**) is suggested by our structures, which might unleash the aperture of gating residues (***), allowing choline release. (4) Deprotonation of the covalent FGly sulfate intermediate by His104 allows a rearrangement producing the FGly aldehyde and release of sulfate, accompanied by the entrance of water into the catalytic cleft. (5) Rehydration of the FGly aldehyde is expected to occur by this metal-activated water molecule as proposed previously (Hanson et al., 2004  ). This water molecule is also at a binding distance from His201/Lys309. By comparison with other sulfatase structures, the residue suggested to deprotonate the water molecule would be Asp396 (van Loo, Berry et al., 2019). However, His297 (also one of the metal-bound residues) is at a shorter binding distance from the water molecule in our structures (#). His201 is also at a binding distance from this water molecule in the choline-bound C54S mutant structure. |
![[Figure 7]](cb5132fig7.jpg)
 | STRUCTURAL BIOLOGY |
ISSN: 2059-7983
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