FIGURE 6.

An overview of the HC transfer mechanism. The schematic illustrates the reaction mechanism for the HC transfer and summarizes the relation between the different cross-links mediated by the ester bond from the C-terminal Asp residue of the HCs. In the bikunin proteins the HCs are cross-linked to a C-6 of an internal N-acetylgalactosamine in the CS chain originating from Ser¹⁰ of bikunin (top). In the presence of TSG-6 and HC2 this ester bond is cleaved and a covalent intermediate is formed, where the C-terminal of the HCs are esterified to Ser²⁸ of TSG-6 (middle). The HC from this intermediate is subsequently transferred to HA and TSG-6 released. In the last step a new ester bond is formed between the C-terminal of the HC and C-6 of an internal N-acetylglucosamine in HA (bottom). In addition to HA, chondroitin also acts as HC acceptor. The reaction mechanism contains two transesterification steps; both of these steps are divalent cation-dependent and require the presence of HC2. R₁ represents the N-terminal sequence of the HCs and R₂ represents the C-terminal sequence of TSG-6.