= Chondroitin 4-sulfotransferase
<Chondroitin 4-sulfotransferase>, EC 2.8.2.5, transfers a group from <PAPS> to the 4-position of <N-acetylgalactosamine> residues in <chondroitin>, producing <chondroitin 4-sulfate> and <adenosine 3',5'-bisphosphate>. The official account distinguishes this activity from <chondroitin 6-sulfotransferase>, which modifies the 6-position.<Chondroitin 4-sulfotransferase in IUBMB Enzyme Nomenclature>[Reaction, Systematic name and Comments]{ref}
The number in the name is structural information. Both positions belong to the sugar-containing backbone, but attachment at different positions can produce the same overall number of added sulfate groups. A bulk sulfur measurement therefore cannot by itself identify which activity acted.
An invented composition table makes that limitation quantitative. Of one hundred <N-acetylgalactosamine> positions, suppose thirty are modified only at position 4, twenty only at position 6, ten at both, and forty at neither. Then forty positions carry a 4-substitution, thirty carry a 6-substitution, and sixty positions carry at least one substitution. The total number of sulfate substituents is $30+20+2(10)=70$. Counting modified residues and counting attached groups are different operations.
Experimental separation also established that the two positional activities could be distinguished. A 1982 study by <O. Habuchi> and <N. Miyashita> separated activities from chick embryo cartilage using <size-exclusion chromatography>. The resulting preparations also differed in their response to the absence of sulfhydryl compounds: the 4-sulfotransferase activity became very low under the reported conditions, while the 6-sulfotransferase activity was essentially unaffected.<Separation and characterization of chondroitin 6-sulfotransferase and chondroitin 4-sulfotransferase from chick embryo cartilage by O. Habuchi and N. Miyashita>[Abstract, chromatographic separation and differential sulfhydryl dependence]{ref} The comparison connects a positional reaction definition with experimentally separable biochemical behavior.
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