Figure 8.

A proposed model for HG synthesis by GAUT1:GAUT7. Distributive elongation of a DP11 acceptor by the GAUT1:GAUT7 complex is shown as a series of five steps, with GAUT1 in red and GAUT7 in green. A patch of positively charged residues proposed to be necessary for the binding and orientation of growing acceptor chains is indicated in blue. In the center images, individual GalA units are numbered 1–11 to visualize binding of the acceptor along an extended acceptor-binding groove. Numbered steps (circled) are outlined under “Discussion.” A, slow phase of HG synthesis, representing the initiation of new polysaccharide chains and the elongation of short-chain acceptors. Short-chain acceptors are inefficiently elongated and cannot fill subsites along the extended acceptor-binding groove. B, fast phase of HG synthesis, representing the elongation of HG acceptors with chain lengths longer than the critical DP, estimated to be DP11.