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![[Figure 5]](cb5050fig5mag.jpg)
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Figure 5
Proposed reaction mechanism in SGSH (schematic). The active-site formylglycine (FGly70), which is intrinsically reactive, undergoes hydration to form the resting state of the enzyme with a gem-diol group (step 1). Coordination of one of the hydroxyl groups of the gem-diol to a Ca2+ ion facilitates the development of a negative charge on the O atom as its proton is lost to a base. The negatively charged O atom nucleophilically attacks the sulfur centre of the N-linked sulfate group on the glucosamine substrate (step 2), resulting in a covalently bound enzyme–substrate complex with a pentavalent sulfur transition state. An acid (possibly His181) facilitates the cleavage of the S—N bond by protonating the bridging N atom to form an amine leaving group on the N-desulfated substrate, which diffuses away, leaving an O-sulfated enzyme (step 3). Finally, in a step that underlines the importance of the formylglycine residue, another base (His125) deprotonates the second hydroxyl group, resulting in a negatively charged O atom (step 4) that forms a double bond with the Cβ atom as the C—O bond between it and the bridging O atom of the sulfate group breaks, eliminating the sulfate ion and regenerating the formylglycine residue (step 5). |
![[Figure 5]](cb5050fig5.jpg)
 | STRUCTURAL BIOLOGY |
ISSN: 2059-7983
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