Key Role of the Carboxyl Terminus of Hyaluronan Synthase in Processive Synthesis and Size Control of Hyaluronic Acid Polymers

The essential pathophysiological roles of hyaluronic acid (HA) strongly depend on HA binding and HA size. Here we deployed the atomic vision of molecular dynamics (MD) simulation to experimentally investigate the influence of C-terminal mutations of Streptococcus equisimilis hyaluronan synthase (SeHAS) on HA product synthesis in Escherichia coli. R413 was vital for HA production, as the removal or mutation of R413 led to inactivation of SeHAS. MD simulations indicated that R406-R413 constituted an HA-binding pattern that stabilized the HA-SeHAS complex. We further increased HA product size via site-directed mutation of the SeHAS C-terminal residues 414-417 based on the hypothesis that higher binding affinity between the SeHAS C-terminus and HA would lead to larger HA size, underlying the important role of the HA-SeHAS interaction in HA size control. W410A and T412A mutations also completely deactivated SeHAS. Moreover, a catalysis-transformation-translocation model was proposed for the HA synthesis and translocation processes.