Enzymatic Specificity
HS enzymatic specificity of QSulf1 was first analyzed. QSulf1 enzymatic specificity on 6-O sulfates was linked to the trisulfated disaccharides (HexA,2SGlcNS,6S) in S domains of HS (HS regions where most of the GlcNS residues are in contiguous sequences) and not NA/NS domains (regions of alternating N-acetylated and N-sulfated units; transition zones). Sulf1 and 2 null murine embryonic fibroblasts were generated to test the HS specificity of mammalian Sulf as opposed to avian Sulf (QSulf). Investigators found mSulf1−/−;mSulf2−/− HS showed overall large increases in all 6S disaccharides. Cooperativity between mSulf1/2 was found because a 2-fold increase in S-domain-associated disaccharides (UA–GlcNS(6S) and UA(2S)–GlcNS(6S)) was observed in double knock-out HS as compared with either single knock-out HS alone. However, one difference from mSulf1 is that mSulf2−/− HS shows an increase in 6S almost exclusively within the non-sulfated and transition zones. This sulfation effect on non-sulfated and transition zones is also different from QSulfs, which catalyze desulfation exclusively in S-domains. Although 6S changes were dominant, other small changes in NS and 2S sulfation do occur in the Sulf knock out MEFs, which may be a compensatory mechanism. Further biochemical studies elucidated specificity and localization of human Sulfs 1 and 2. Sulf1 and 2 hydrophilic domains associate with the cell membrane components through electrostatic interactions and not by integration with into the lipid bilayer. In addition to cell membrane association, Sulfs also secreted freely into the media, which contrasts the findings with QSulf1 and 2. Biochemical analysis of HSPGs in Sulf 1 and 2 knockout MEFS reveal enzyme specificities to disulfated and, primarily, trisulfated 6S disaccharide units UA-GlcNS(6S) and UA(2S)-GlcNS(6S) within the HS chain, with specific exclusion of monosulfated disaccharide units. In vivo studies, however, demonstrate that loss of Sulf1 and Sulf2 result in sulfation changes of nonsubstrates (UA-GlcNAc(6S), N and 2-O Sulfate), indicating Sulf modulates HS biosynthetic machinery. This was further demonstrated by PCR analysis, showing dynamic changes in HS biosynthesis enzymes after Sulf1 and 2 loss. Also, the authors showed in an MEF model system, that Sulf1 and Sulf2 definitively and differentially modify HS proteoglycan fractions including cell surface, GPI-anchored (glypican), shed, and ECM-associated proteoglycans.
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