Tunicamycin

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.{{cite journal |last1=Wyszynski |first1=Filip |last2=Hesketh |first2=Andrew |last3=Bibb |first3=Mervyn |last4=Davis |first4=Benjamin |date=2010 |title=Dissecting tunicamycin biosynthesis by genome mining: cloning and heterologous expression of a minimal gene cluster |journal=Chemical Science |volume=1 |issue=5 |pages=581 |doi=10.1039/c0sc00325e}}

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.{{cite journal |last1=Wyszynski |first1=Filip |last2=Lee |first2=Seung |last3=Yabe |first3=Tomoaki |last4=Wang |first4=Hua |last5=Gomez-Escribano |first5=Juan Pablo |last6=Bibb |first6=Mervyn |date=July 2012 |title=Biosynthesis of the tunicamycin antibiotics proceeds via unique exo-glycal intermediates |journal=Nature Chemistry |volume=4 |issue=7 |pages=539–546 |doi=10.1038/nchem.1351 |pmid=22717438|bibcode=2012NatCh...4..539W }}

Natural products scheme 1.png

The other starter unit for tunicamycin is uridine, which is produced from uridine triphosphate (UTP). TunN is a nucleotide diphosphatase, and catalyzes the removal of pyrophosphate from UTP to form uridine monophosphate. The last phosphate is removed by the putative monophosphatase, TunG.

Scheme 2.png

Once uridine and UDP-6’-deoxy-5-6-ene-GalNAc are produced, TunB catalyzes their linkage at the 6’ carbon of UDP-6’-deoxy-5-6-ene-GalNAc. TunB uses S-adenyslmethionine (SAM) to form a radical on the 5’ carbon of the ribose on uracil. TunM is thought to catalyze the formation of a new bond between the 5’ carbon of uridine and the 6’ carbon of UDP-6’-deoxy-5-6-ene-GalNAc using the electron from the uridine radical and one of the electrons from the double bond of UDP-6’-deoxy-5-6-ene-GalNAc. The radical on UDP-6’-deoxy-5-6-ene-GalNAc is then quenched by abstracting a hydrogen from SAM.{{cite journal |last1=Giese |first1=Bernd |date=August 1989 |title=The Stereoselectivity of Intermolecular Free Radical Reactions [New Synthetic Methods (78)] |journal=Angewandte Chemie International Edition in English |volume=28 |issue=8 |pages=969–980 |doi=10.1002/anie.198909693}} The resulting molecule is UDP-N-acetyl-tunicamine. TunH then catalyzes the hydrolysis of UDP from UDP-N-acetyl-tunicamine. Another molecule of UDP-GlcNAc is introduced, and a β-1,1 glycosidic bond is subsequently formed, catalyzed by TunD. The resulting molecule is deacetylated by TunE. TunL and a fatty acyl-ACP ligase are used to load metabolic fatty acids onto the acyl carrier protein, TunK. TunC then attaches the fatty acid to the free amine, producing tunicamycin.

tunicamycin scheme 3.png

• Glycosylation - tunicamycin blocks all N-glycosylation of proteins
• Glycoprotein
• Streptomyces

{{reflist}}

• Book section of Essentials in Glycobiology (1999) [https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=glyco.section.3051 Tunicamycin: Inhibition of DOL-PP-GlcNAc Assembly]

Category:Antibiotics