Stereochemistry of methyl group insertion in corrinoid biosynthesis. Determination of carbon isotope chirality by 13C nuclear magnetic resonance

Aflatoxin and deconstruction of type I, iterative polyketide synthase function

In this viewpoint highlights are drawn from a deep analysis of the multifaceted problem of aflatoxin biosynthesis, one of the most highly rearranged polyketide natural products known. Fundamental chemical insights have emerged into how cytochrome P450-mediated skeletal rearrangements occur through probable cationic intermediates and oxidative dearomatizations, which are applicable more widely in natural product catabolism. So to where current experimental methods have failed in our hands, bioinformatic tools and fresh experimental strategies have been developed to identify linker regions in large, polydomain proteins and guide the dissection and reassembly of their component parts. It has been possible to deduce individual catalytic roles, how overall synthesis is coordinated and how these enzymes can be re-engineered in a rational manner to prepare non-natural products. These insights and innovations were often not planned or anticipated, but sprung from the inability to answer fundamental questions. Advances in science can take place by chance favoring the prepared mind, other times by refusing to give up and devising new solutions to address hard questions. Both ways forward played important roles in the investigation of aflatoxin biosynthesis. For these contributions I am pleased to share this special issue of NPR with John Vederas and Tom Simpson, who have been leaders in this field for the last third of a century.

Biosynthesis of vitamin B12: factor IV, a new intermediate in the anaerobic pathway

The structure of a novel tetradehydrocorrin, factor IV, isolated from Propionibacterium shermanii has been established by multidimensional NMR spectroscopy. Incorporation of radiolabeled factor IV into cobyrinic acid established the biointermediacy of this cobalt complex, whose structure has implications for the mechanisms of the anaerobic pathway to B12.

Biosynthesis of vitamin B12: structure of precorrin-6x octamethyl ester

13C-labeled precorrin-6x is biosynthesized by cell-free protein preparations from Pseudomonas denitrificans in separate experiments using delta-amino[5-13C]levulinic acid and the corresponding delta-amino[4-13C]- and delta-amino[3-13C]levulinic acid-labeled forms in conjunction with S-[methyl-13C]adenosylmethionine for the latter two experiments. These labeled precorrin-6x samples, as their octamethyl esters, are studied by a range of NMR techniques. In addition, nuclear Overhauser effect difference measurements are made on unlabeled precorrin-6x ester to determine connectivities. The structure 6a so established for precorrin-6x ester (i) confirms the results reported in the preceding paper that precorrin-6x has a ring-contracted macrocycle, still carries the C-12 acetate residue, and stands at the oxidation level of a dehydrocorrin; (ii) reveals the unexpected methylation at C-11 not C-12, leading to a structure with separated chromophores; and (iii) implies that methyl migration from C-11 to C-12 occurs when precorrin-6x is converted into hydrogenobyrinic acid. Proposals for the biosynthesis of the corrin macrocycle of hydrogenobyrinic acid and vitamin B12 are made.

Biosynthesis of vitamin B12: concerning the origin of the methine protons of the corrin nucleus

13C NMR spectroscopy has been used to locate six deuterium atoms incorporated biosynthetically on the periphery of the corrin nucleus of vitamin B12 (cyanocobalamin) derived from cells of Propionibacterium shermanii grown in a medium containing 50% 2H2O and 13C-enriched delta-aminolevulinic acid. The implications of these results for the mechanism of vitamin B12 biosynthesis are discussed, and it is concluded that the same oxidation level of the intermediates is maintained throughout the biosynthetic pathway, from delta-aminolevulinic acid to corrin.

Biosynthesis of vitamin B12. Experiments with the anaerobe Eubacterium limosum and some labelled substrates

1. The anaerobic microorganism Eubacterium limosum (DSM 20 402), grown in the presence of L-[methyl-14C]methionine, produces vitamin B12 not only labeled in the corrin ring, but also in the 5,6-dimethylbenzimidazole moiety. The vitamin B12 obtained in a similar experiment with L-[methyl-13C]methionine showed nine signals in the 13CV NMR spectrum. Seven of these signals are due to the seven 'extra' methyl groups of the corrin ring, two signals originate from the two methyl groups of the 5,6-dimethylbenzimidazole moiety. This result is in contrast to previous experiments with aerobic and aerotolerant organisms, where L-methionine is only the precursor of the seven corrin methyl groups, and the 5,6-dimethylbenzimidazole is formed from riboflavin. 2. E. limosum transforms exogenously added benzimidazole into benzimidazolylcobamide, but with [2-14C]benzimidazole it was shown that benzimidazole is not a precursor of the 5,6-dimethylbenzimidazole moiety of vitamin B12. 3. Radioactivity from [2-14C]glycine is almost exclusively incorporated into the 5,6-dimethylbenzimidazole moiety of vitamin B12, but not into the corrin moiety. This demonstrates that in E. limosum delta-aminolevulinic acid, the precursor of the corrin moiety, is not formed as in many other organisms from succinyl-CoA and glycine.