Structural and functional analysis of an l-serine O-phosphate decarboxylase involved in norcobamide biosynthesis

Methylations in vitamin B12 biosynthesis and catalysis

Vitamin B₁₂ is an essential biomolecule that assists in the catalysis of methyl transfer and radical-based reactions in cellular metabolism. The structure of B₁₂ is characterized by a tetrapyrrolic corrin ring with a central cobalt ion coordinated with an upper ligand, and a lower ligand anchored via a nucleotide loop. Multiple methyl groups decorate B₁₂, and their presence (or absence) have structural and functional consequences. In this minireview, we focus on the methyl groups that distinguish vitamin B₁₂ from other tetrapyrrolic biomolecules and from its own naturally occurring analogues called cobamides. We draw information from recent advances in the field to understand the origins of these methyl groups and the enzymes that incorporate them, and discuss their biological significance.

Functional Characterization of Two PLP-Dependent Enzymes Involved in Capsular Polysaccharide Biosynthesis from Campylobacter jejuni

Campylobacter jejuni is a Gram-negative, pathogenic bacterium that causes campylobacteriosis, a form of gastroenteritis. C. jejuni is the most frequent cause of food-borne illness in the world, surpassing Salmonella and E. coli. Coating the surface of C. jejuni is a layer of sugar molecules known as the capsular polysaccharide that, in C. jejuni NCTC 11168, is composed of a repeating unit of d-glycero-l-gluco-heptose, d-glucuronic acid, d-N-acetyl-galactosamine, and d-ribose. The d-glucuronic acid moiety is further amidated with either serinol or ethanolamine. It is unknown how these modifications are synthesized and attached to the polysaccharide. Here, we report the catalytic activities of two previously uncharacterized, pyridoxal phosphate (PLP)-dependent enzymes, Cj1436 and Cj1437, from C. jejuni NCTC 11168. Using a combination of mass spectrometry and nuclear magnetic resonance, we determined that Cj1436 catalyzes the decarboxylation of l-serine phosphate to ethanolamine phosphate. Cj1437 was shown to catalyze the transamination of dihydroxyacetone phosphate to (S)-serinol phosphate in the presence of l-glutamate. The probable routes to the ultimate formation of the glucuronamide substructures in the capsular polysaccharides of C. jejuni are discussed.

Functional and Structural Characterization of the UDP-Glucose Dehydrogenase Involved in Capsular Polysaccharide Biosynthesis from Campylobacter jejuni

Campylobacter jejuni is a pathogenic organism that can cause campylobacteriosis in children and adults. Most commonly, campylobacter infection is brought on by consumption of raw or undercooked poultry, unsanitary drinking water, or pet feces. Surrounding the C. jejuni bacterium is a coat of sugar molecules known as the capsular polysaccharide (CPS). The capsular polysaccharide can be very diverse among the different strains of C. jejuni, and this diversity is considered important for evading the host immune system. Modifications to the CPS of C. jejuni NCTC 11168 include O-methylation, phosphoramidylation, and amidation of glucuronate with either serinol or ethanolamine. The enzymes responsible for amidation of glucuronate are currently unknown. In this study, Cj1441, an enzyme expressed from the CPS biosynthetic gene cluster in C. jejuni NCTC 11168, was shown to catalyze the oxidation of UDP-α-d-glucose into UDP-α-d-glucuronic acid with NAD⁺ as the cofactor. No amide products were found in an attempt to determine whether the putative thioester intermediate formed during the oxidation of UDP-glucose by Cj1441 could be captured in the presence of added amines. The three-dimensional crystal structure of Cj1441 was determined in the presence of NAD⁺ and UDP-glucose bound in the active site of the enzyme (Protein Data Bank entry 7KWS). A more thorough bioinformatic analysis of the CPS gene cluster suggests that the amidation activity is localized to the t-terminal half of Cj1438, a bifunctional enzyme that is currently annotated as a sugar transferase.

Cobamide remodeling in the freshwater microalga Chlamydomonas reinhardtii

Microalgae are not able to produce cobamides (Cbas, B12 vitamers) de novo. Hence, the production of catalytically active Cba-containing methionine synthase (MetH), which is present in selected representatives, is dependent on the availability of exogenous B12 vitamers. Preferences in the utilization of exogenous Cbas equipped with either adenine or 5,6-dimethylbenzimidazole as lower base have been reported for some microalgae. Here, we investigated the utilization of norcobamides (NorCbas) for growth by the Cba-dependent Chlamydomonas reinhardtii mutant strain (ΔmetE). The growth yields in the presence of NorCbas were lower in comparison to those achieved with Cbas. NorCbas lack a methyl group in the linker moiety of the nucleotide loop. C. reinhardtii was also tested for the remodeling of NorCbas (e.g. adeninyl-norcobamide) in the presence of different benzimidazoles. Extraction of the NorCbas from C. reinhardtii, their purification, and identification confirmed the exchange of the lower base of the vitamers. However, the linker moiety of the NorCbas nucleotide loop was not exchanged. This observation strongly indicates the presence of an alternative mode of Cba deconstruction in C. reinhardtii that differs from the amidohydrolase (CbiZ)-dependent pathway described in Cba-remodeling bacteria and archaea.