Mechanisms of coordinating hyaluronan and glycosaminoglycan production by nucleotide sugars

Abstract

Hyaluronan is a versatile macromolecule capable of an exceptional range of functions from cushioning and hydration to dynamic signaling in development and disease. Because of its critical roles, hyaluronan production is regulated at multiple levels including epigenetic, transcriptional, and posttranslational control of the three hyaluronan synthase (HAS) enzymes. Precursor availability can dictate the rate and amount of hyaluronan synthesized and shed by the cells producing it. However, the nucleotide-activated sugar substrates for hyaluronan synthesis by HAS also participate in exquisitely fine-tuned cross-talking pathways that intersect with glycosaminoglycan production and central carbohydrate metabolism. Multiple UDP-sugars have alternative metabolic fates and exhibit coordinated and reciprocal allosteric control of enzymes within their biosynthetic pathways to preserve appropriate precursor ratios for accurate partitioning among downstream products, while also sensing and maintaining energy homeostasis. Since the dysregulation of nucleotide sugar and hyaluronan synthesis is associated with multiple pathologies, these pathways offer opportunities for therapeutic intervention. Recent structures of several key rate-limiting enzymes in the UDP-sugar synthesis pathways have offered new insights to the overall regulation of hyaluronan production by precursor fate decisions. The details of UDP-sugar control and the structural basis for underlying mechanisms are discussed in this review.

Keywords: UDP-N-acetylglucosamine; UDP-glucuronate; hexosamine biosynthesis pathway; hyaluronan; nucleotide sugars.

Figure 1.

HA precursor biosynthesis pathways. UDP-GlcA and UDP-GlcNAc are produced through a series of enzymatic reactions downstream of glucose metabolism. UDP-GlcA synthesis begins with Glu-6-P and ends with oxidation of UDP-Glc catalyzed by UDP-glucose dehydrogenase (UGDH). UDP-GlcNAc is produced from Fru-6-P and the first reaction of this pathway, the production of d-glucosamine-6-phosphate, is considered the rate-limiting step. Both nucleotide sugars have three downstream competing fates, but both are utilized at the Golgi for proteoglycan (PG) and glycosaminoglycan (GAG) production and at the plasma membrane for HA synthesis via HA synthase (HAS1-3) enzymes. In addition, UDP-GlcA is utilized in the ER for glucuronidation via UDP-glucuronosyltransferase (UGT) enzymes and UDP-GlcNAc is used in the cytosol for serine O-linked GlcNAcylation. ER, endoplasmic reticulum; HA, hyaluronan; UDP-GlcA, UDP-glucuronate; UDP-GlcNAc, UDP-N-acetylglucosamine.

Figure 2.

Ribbon representations of the active (E) and inhibited (E^Ω) substate conformations of hexameric human UGDH. The abortive ternary complex of the substrate, UDP-glucose, and the reduced cofactor, NADH [2Q3E (26); A] and the feedback inhibited complex of UDP-xylose and NAD⁺ [3PTZ (27); B] are shown in ribbon representation. The solvent accessible surface representation is superimposed. Individual dimers are illustrated in dark/light pairs of red, green, and blue. The allosteric switch, which includes the Thr 131 loop and the adjacent α6 helix, is labeled and highlighted in yellow; the start of the intrinsically disordered C-terminal tail is labeled and colored in pink. Ligands are shown in space-filling representation with oxygen colored in red, nitrogen in blue, carbon in gray, and phosphorus in orange. UGDH, UDP-glucose dehydrogenase.

Figure 3.

Ribbon representation of the structure of human GFAT1. The structure of human GFAT1 in complex with glutamate and Glc6P was determined by x-ray crystallography (6R4E). Subunit A (darker shade) and subunit B (lighter shade) each contain a glutaminase domain (green) and a synthase/isomerase domain (purple), with β-strands colored in gold. Glutamate and Glc6P are rendered in a space filling representation and relevant side chains in ball and stick format. The superimposed position of UDP-GlcNAc from the feedback inhibited structure (6R4G) is presented in stick format. Atoms are colored as follows: oxygen—red, nitrogen—blue, phosphate—orange, and sulfur—yellow. Sites of gain-of-function mutants are colored pale blue in the ribbon representation (41). UDP-GlcNAc, UDP-N-acetylglucosamine.