Heparan sulfate (HS), a class of polysaccharides that includes the well-known drug heparin, plays essential roles in cell growth and development. The many structural variants of HS observed in mammals have been hypothesized to differentially modulate growth factor-mediated signaling and regulate hemostasis during health and disease. Our project will explore how the HS structural variation is generated during biosynthesis/catabolism as well as map routes to prepare more defined molecules having greater selectivity and more potent desired bioactivities. A favored hypothesis is that the pattern of HS domains (comprised of both N-sulfo (NS) and N-acetyl (NA) domains) encodes information that modulates HS interaction with proteins like growth factors, cytokines, and clotting factors. Currently, it is very difficult to understand how HS biosynthetic modification an domain placement is controlled and which HS domain structures possess the highest activities in different biological systems. We will apply our newly developed synthetic methodology to answer key questions in the field. A small library of HS polymers having size-defined and placement-defined NS and NA domains will be generated. These HS polymers will then be modified by biosynthetic enzymes, including O-sulfotransferases, and C5-epimerase and/or catabolic enzymes including endo-6- Oendosulfatases and heparanase. A focused combinatorial approach will be used to produce defined HS polymers to test two opposing models of growth factor signaling, a key event in cell proliferation, embryonic development, and cancer. In this project our specific aims are: Aim 1: To chemoenzymatically synthesize a small library of HS polymers having defined NS and NA domains. Aim 2. To analyze the modification patterns generated through the action of various enzymes on the HS polysaccharide library. Aim 3: To characterize the structure/function relationship of HS activity as the co-receptor fibroblast growth factor receptor signaling.