Discovery and Functional Characterization of a Clandestine ATP-Dependent Amidoligase in the Biosynthesis of the Capsular Polysaccharide from Campylobacter jejuni

Identification and Functional Characterization of the Polymerizing Glycosyltransferase Required for the Transfer of d-Ribose to the d-GalfNAc Moiety of the Capsular Polysaccharide of Campylobacter jejuni

Campylobacter jejuni is the leading cause of food poisoning in the United States. The exterior surface of this bacterium is coated with a capsular polysaccharide (CPS) that helps protect the organism from the host immune system. In the HS:2 serotype of strain C. jejuni NCTC 11168, the minimal repeating trisaccharide consist of d-ribose, N-acetyl-d-galactosamine (GalNAc) and the serinol amide of d-glucuronic acid. Here we demonstrate that the C-terminal domain of Cj1432 (residues 574-914) is responsible for the transfer of d-ribose-5-P from phosphoribosyl pyrophosphate (PRPP) to C5 of the d-GalfNAc moiety of the growing polysaccharide chain. In the next step the middle domain of Cj1432 (residues 357-573) catalyzes the hydrolysis of phosphate from this product. The N-terminal domain of Cj1432 (residues 1-356) catalyzes the transfer of d-GlcA from UDP-d-GlcA to C2 of the d-ribose moiety and thus Cj1432 catalyzes three consecutive reactions during the biosynthesis of the capsular polysaccharide of C. jejuni. We have previously shown that the remaining three reactions required for the polymerization of the CPS are catalyzed by the bifunctional enzyme Cj1438 and Cj1435. We have now demonstrated that the minimal repeating trisaccharide of the CPS of C. jejuni NCTC 11168 requires six enzyme-catalyzed reactions with six intermediate structures. This accomplishment will now enable the large-scale cell-free enzyme-catalyzed synthesis of well-defined oligomers of the CPS that can potentially be used in the production of glycoconjugate vaccines for the prevention of infections by C. jejuni.

Identification of the Polymerizing Glycosyltransferase Required for the Addition of d-Glucuronic Acid to the Capsular Polysaccharide of Campylobacter jejuni

Campylobacter jejuni is the leading cause of food poisoning in Europe and North America. The exterior surface of this bacterium is encased by a capsular polysaccharide that is attached to a diacyl glycerol phosphate anchor via a poly-Kdo (3-deoxy-d-manno-oct-2-ulosinic acid) linker. In the HS:2 serotype of C. jejuni NCTC 11168, the repeating trisaccharide consists of d-ribose, N-acetyl-d-glucosamine, and d-glucuronate. Here, we show that the N-terminal domain of Cj1432 (residues 1-356) is responsible for the reaction of the C2 hydroxyl group from the terminal d-ribose moiety of the growing polysaccharide chain with UDP-d-glucuronate as the donor substrate. This discovery represents the first biochemical identification and functional characterization of a glycosyltransferase responsible for the polymerization of the capsular polysaccharide of C. jejuni. The product of the reaction catalyzed by the N-terminal domain of Cj1432 is the substrate for the reaction catalyzed by the C-terminal domain of Cj1438 (residues 453-776). This enzyme catalyzes amide bond formation using the C6 carboxylate of the terminal d-glucuronate moiety and (S)-serinol phosphate as substrates. It is also shown that Cj1435 catalyzes the hydrolysis of phosphate from the product catalyzed by the C-terminal domain of Cj1438. These results demonstrate that amide decoration of the d-glucuronate moiety occurs after the incorporation of this sugar into the growing polysaccharide chain.

Functional Characterization of Two Polymerizing Glycosyltransferases for the Addition of N-Acetyl-d-galactosamine to the Capsular Polysaccharide of Campylobacter jejuni

The exterior surface of the human pathogen Campylobacter jejuni is coated with a capsular polysaccharide (CPS) that consists of a repeating sequence of 2-5 different sugars that can be modified with various molecular decorations. In the HS:2 serotype from strain NCTC 11168, the repeating unit within the CPS is composed of d-ribose, N-acetyl-d-galactosamine, and a d-glucuronic acid that is further amidated with either serinol or ethanolamine. The d-glucuronic acid moiety is also decorated with d-glycero-l-gluco-heptose. Here, we show that two different GT2 glycosyltransferases catalyze the transfer of N-acetyl-d-galactosamine from UDP-NAc-d-galactosamine furanoside to the C4-hydroxyl group of the d-glucuronamide moiety at the growing end of the capsular polysaccharide chain. Catalytic activity was not observed with glycosides of d-glucuronic acid, and thus, the C6-carboxylate of the d-glucuronic acid moiety must be amidated prior to chain elongation. One of these enzymes comprises the N-terminal domain of Cj1438 (residues 1-325) and the other is from the N-terminal domain of Cj1434 (residues 1-327). These two glycosyltransferases are ∼87% identical in sequence, but it is not clear why there are two glycosyltransferases from the same gene cluster that apparently catalyze the same reaction. This discovery represents the second polymerizing glycosyltransferase that has been isolated and functionally characterized for the biosynthesis of the capsular polysaccharide in the HS:2 serotype of C. jejuni.

Biosynthesis of UDP-α-N-Acetyl-d-mannosaminuronic Acid and CMP-β-N-Acetyl-d-neuraminic Acid for the Capsular Polysaccharides of Campylobacter jejuni

Campylobacter jejuni is a human pathogen and a leading cause of food poisoning in North America and Europe. The exterior surface of the bacterial cell wall is attached to a polymeric coat of sugar molecules known as the capsular polysaccharide (CPS) that helps protect the organism from the host immune response. The CPS is composed of a repeating sequence of common and unusual sugar residues. In the HS:11 serotype of C. jejuni, we identified two enzymes in the gene cluster for CPS formation that are utilized for the biosynthesis of UDP-α-N-acetyl-d-mannosaminuronic acid (UDP-ManNAcA). In the first step, UDP-α-N-acetyl-d-glucosamine (UDP-GlcNAc) is epimerized at C2 to form UDP-α-N-acetyl-d-mannosamine (UDP-ManNAc). This product is then oxidized by a NAD+-dependent C6-dehydrogenase to form UDP-ManNAcA. In the HS:6 serotype (C. jejuni strain 81116), we identified three enzymes that are required for the biosynthesis of CMP-β-N-acetyl-d-neuraminic acid (CMP-Neu5Ac). In the first step, UDP-GlcNAc is epimerized at C2 and subsequently hydrolyzed to form N-acetyl-d-mannosamine (ManNAc) with the release of UDP. This product is then condensed with PEP by N-acetyl-d-neuraminate synthase to form N-acetyl-d-neuraminic acid (Neu5Ac). In the final step, CMP-N-acetyl-d-neuraminic acid synthase utilizes CTP to convert this product into CMP-Neu5Ac. A bioinformatic analysis of these five enzymes from C. jejuni serotypes HS:11 and HS:6 identified other bacterial species that can produce UDP-ManNAcA or CMP-Neu5Ac for CPS formation.

Functional Characterization of a HAD Phosphatase Involved in Capsular Polysaccharide Biosynthesis in Campylobacter jejuni

Campylobacter jejuni is a Gram-negative, pathogenic bacterium found in the intestinal tracts of chickens and many other farm animals. C. jejuni infection results in campylobacteriosis, which can cause nausea, diarrhea, fever, cramps, and death. The surface of the bacterium is coated with a thick layer of sugar known as the capsular polysaccharide. This highly modified polysaccharide contains an unusual d-glucuronamide moiety in serotypes HS:2 and HS:19. Previously, we have demonstrated that a phosphorylated glucuronamide intermediate is synthesized in C. jejuni NCTC 11168 (serotype HS:2) by cumulative reactions of three enzymes: Cj1441, Cj1436/Cj1437, and Cj1438. Cj1441 functions as a UDP-d-glucose dehydrogenase to make UDP-d-glucuronate; then Cj1436 or Cj1437 catalyzes the formation of ethanolamine phosphate or S-serinol phosphate, respectively, and finally Cj1438 catalyzes amide bond formation using d-glucuronate and either ethanolamine phosphate or S-serinol phosphate. Here, we investigated the final d-glucuronamide-modifying enzyme, Cj1435. Cj1435 was shown to catalyze the hydrolysis of the phosphate esters from either the d-glucuronamide of ethanolamine phosphate or S-serinol phosphate. Kinetic constants for a range of substrates were determined, and the stereoselectivity of the enzyme for the hydrolysis of glucuronamide of S-serinol phosphate was established using 31P nuclear magnetic resonance spectroscopy. A bioinformatic analysis of Cj1435 reveals it to be a member of the HAD phosphatase superfamily with a unique DXXE catalytic motif.