The synthesis of core polysaccharide starts from lipid Ⅳ a. After adding KDO (core oligosaccharide) at the C-6 'position of non-reducing glucosamine, heptose, and hexose are gradually added

1. Synthesis and adhesion of KDO

The synthesis of KDO includes three continuous reactions: ① 5-phosphate D-ribose ↔ 5-Phospho-D-arabinose. ② 5-phosphate - D-arabinose+phosphoenolpyruvate → 8-phosphate - KDO+phosphate. ③ 8-phosphate - KDO+alkanoic acid. These three reactions were carried out under the catalysis of 5-phospho-D-nucleate isomerase, 8-phosphor-KDO synthetase, and 8-phosphatase respectively. The kid's gene located at 27 minutes of chromosome encodes 8-phosphor-KDO synthetase.

KDO generates CMP-KDO activated form under the catalysis of CMP-KDO synthetase. The enzyme that catalyzes this reaction is encoded by kds B gene located at 85 minutes of a chromosome. CMP-KDO binds KDO to C-6 'of lipid Ⅳ a under the catalysis of dual-function or tri-function protein WaaA, and then adds the second KDO molecule to the first KDO molecule.

2. Synthesis of heptose and hexose regions

Heptose is added to the core polysaccharide one by one in the form of ADP activation and hexose in the form of UDP activation. The glycosyltransferases that catalyze nucleotide monosaccharides belong to a series of membrane-related glycosyltransferases, which transfer sugar groups to mature lipid A molecules on the cytoplasmic surface of the cell membrane. The synthesized core molecule of lipid A may be transferred from the cytoplasmic surface of the cell membrane to the periplasmic surface of the cell membrane with the participation of the ABC transport device [ATP-binding cassette (ABC) transporter], and complete the connection reaction with the O antigen polysaccharide chain at the periplasmic surface to generate a complete LPS molecule. In addition, lipid A core can also be used as the receptor of enterobacterial common antigen (ECA) and polysaccharide chain of capsular K antigen of the Escherichia coli group.

The glycosyltransferases or modifying enzymes involved in the synthesis of core polysaccharides are encoded by a class of genes called wa ※ ※, which are located at 81-82 minutes of the chromosome, between cysE and pyrE genes, and these genes are distributed in three operons. For the five core structures of Escherichia coli, the gene composition and genetic distribution of different core syntheses are different.

The waaA operon contains the structural gene waaA encoding a bifunctional or trifunctional KDO transferase and a polypeptide gene with an unknown function and a relative molecular weight of 18000. The products of gmhD, waaF, and waaC genes in the gmhD operon are related to the synthesis of the core heptose region, while the waaQ operon is related to the synthesis of the outer hexose region and the chemical modification of the core region. In Escherichia coli K-12, the transcription of the gmhD operon is regulated by a heat shock promoter and the transcription of what operon is positively regulated by a non-translated region upstream of the operon and the transcription elongation factor RfaH.