Chondroitin 4-sulfate

Chondroitin 4-sulfate is a positional form of chondroitin sulfate with sulfate substitution at the 4-position of N-acetylgalactosamine residues. The modification belongs to a particular position on a sugar residue within the chondroitin chain. It is distinct from changing the chain length or carrying out N-sulfation of glucosamine in a different carbohydrate framework.

Chondroitin 4-sulfotransferase uses PAPS to perform the corresponding positional transfer. The official reaction distinguishes this enzyme from chondroitin 6-sulfotransferase; a shared donor does not imply a shared attachment position.^[ref]

An original structural comparison starts with two equal segments, each containing eight complete repeats and sixteen sugar residues. Put one sulfate group at the 4-position of each of the eight eligible amino-sugar residues in the first segment, and instead at the 6-position in the second. Both segments have eight sulfate groups, but their local arrangements differ. Counting groups establishes composition under this model, not the modified position.

This explains why a preparation's name, backbone analysis and positional characterization contribute different information. A proteoglycan containing a chondroitin 4-sulfate chain is also a larger protein–carbohydrate assembly, rather than another name for the isolated sugar chain.^[ref]