Tunicamycin

Tunicamycin is a mixture of homologous nucleoside antibiotics that inhibits the UDP-HexNAc: polyprenol-P HexNAc-1-P family of enzymes. In eukaryotes, this includes the enzyme GlcNAc phosphotransferase (GPT), which catalyzes the transfer of N-acetylglucosamine-1-phosphate from UDP-N-acetylglucosamine to dolichol phosphate in the first step of glycoprotein synthesis. Tunicamycin blocks N-linked glycosylation (N-glycans) and treatment of cultured human cells with tunicamycin causes cell cycle arrest in G1 phase. It is used as an experimental tool in biology, e.g. to induce unfolded protein response. Tunicamycin is produced by several bacteria, including Streptomyces clavuligerus and Streptomyces lysosuperificus.

Tunicamycin homologues have varying molecular weights owing to the variability in fatty acid side chain conjugates.

Biosynthesis
The biosynthesis of tunicamycins was studied in Streptomyces chartreusis and a proposed biosynthetic pathway was characterized. The bacteria utilize the enzymes in the tun gene cluster (TunA-N) to make tunicamycins.

TunA uses the starter unit uridine diphosphate-N-acetyl-glucosamine (UDP-GlcNAc) and catalyzes the dehydration of the 6’ hydroxyl group. First, a Tyr residue in TunA abstracts a proton from the 4’ hydroxyl group, forming a ketone at that position. A hydride is subsequently abstracted from the 4’ carbon by NAD+, forming NADH. The ketone is stabilized by hydrogen bonding from the Tyr residue, and a nearby Thr residue. A glutamate residue then abstracts a proton from the 5’ carbon, pushing the electrons up to form a double bond between the 5’ and 6’ carbon. A nearby cysteine donates a proton to the hydroxyl group as it leaves as water. NADH donates a hydride to the 4’ carbon, reforming a hydroxide in that position and forming UDP-6’-deoxy-5-6-ene-GlcNAc. TunF then catalyzes the epimerization of the intermediate to UDP-6’-deoxy-5-6-ene-GalNAc, changing the 4’ hydroxyl from the equatorial to axial position.