eLife Assessment
This work provides a fundamental molecular mechanism of how a single enzyme can coordinate the ordered assembly of hyaluronan, a complex polysaccharide, from two different building blocks in an alternating pattern. The authors present compelling evidence by combining high-resolution structural data with rigorous biochemical validation to define the underlying process. Major strengths of the study include the clarity and coherence of the mechanistic insights and the complementary use of structural and functional approaches to address the research question.