Amino Acid Bio Synthesis | Biosynthesis | Alanine
Considerable structural diversity among the sugar moieties in secondary metabolites is well known despite the majority of these sugars being derived from a relatively small collection of common precursors such as nucleotide diphosphate activated glucose (NDP-glucose) [,]. This process of “glycodiversification” starting from a common sugar precursor is achieved via complex biosynthetic transformations that often involve a few key steps of unique enzymatic chemistry to introduce the distinguishing structural feature(s). It has been demonstrated that enzyme bound radical intermediates are involved in some of the more novel and difficult transformations converting NDP-glucose to the final modified sugar species. More recently and for the purposes of this review, some of these diversification reactions have been established to involve radical S-adenosylmethionine (1,SAM) chemistry.
Non-Essential Amino Acid Biosynthesis Glutamate and ..
BtrN is thus one of the few enzymes known to catalyze radical mediated dehydrogenation. It is in fact one of the only two dehydrogenases utilizing radical SAM chemistry in a biologically relevant context. Together with the anaerobic sulfatase maturating enzymes[,], BtrN offers an excellent system for the study of this novel radical chemistry and its potential applicability in the biosynthesis of secondary metabolites. It is also of some interest as to why a unique mechanism of DOIA dehydrogenation has evolved only in the butirosin biosynthetic pathway.
In this review, a summary of the discovery and current mechanistic understanding of BtrN and DesII is presented. Consideration of the biosynthetic roles of these enzymes highlights the importance of radical mediated dehydrogenation reactions as well as the means by which sugars are regioselectively deoxygenated during processing to their final biologically active forms. Likewise, study of the mechanisms by which these enzymes employ radical SAM chemistry to effect their transformations provides general insight into how radical SAM enzymes are able to generate and control the highly reactive radical intermediates. Our discussion will focus on not only the comparison of the current biosynthetic and catalytic models but also the key questions which remain open to future investigation. Finally, the review is concluded with a selection of additional proteins hypothesized to function as radical SAM enzymes in other biosynthetic pathways of carbohydrate secondary metabolism.
Deregulation of S -adenosylmethionine biosynthesis …
AB - The radical S-adenosyl-l-methionine (SAM) superfamily is a widely distributed group of iron-sulfur containing proteins that exploit the reactivity of the high energy intermediate, 5′-deoxyadenosyl radical, which is produced by the reductive cleavage of SAM, to carry-out complex radical-mediated transformations. The reactions catalyzed by radical SAM enzymes range from simple group migrations to complex reactions in protein and RNA modification. This review will highlight three radical SAM enzymes that catalyze reactions involving modified guanosines in the biosynthesis pathways of the hypermodified tRNA base wybutosine; secondary metabolites of 7-deazapurine structure, including the hypermodified tRNA base queuosine; and the redox cofactor F 420. This article is part of a Special Issue entitled: Radical SAM enzymes and Radical Enzymology.
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The production of antibiotics in different Streptomyces strains has been reported to be stimulated by the external addition of S-adenosylmethionine (SAM) and by overexpression of the SAM synthetase gene metK. We investigated the influence of SAM addition, and of the expression of SAM biosynthetic genes, on the production of the aminocoumarin antibiotic novobiocin in the heterologous producer strain Streptomyces coelicolor M512 (nov-BG1). External addition of SAM did not influence novobiocin accumulation. However, overexpression of a SAM synthase gene stimulated novobiocin formation, concomitant with an increase of the intracellular SAM concentration. Streptomyces genomes contain orthologs of all genes required for the SAM cycle known from mammals. In contrast, most other bacteria use a different cycle for SAM regeneration. Three secondary metabolic gene clusters, coding for the biosynthesis of structurally very different antibiotics in different Streptomyces strains, were found to contain an operon comprising all five putative genes of the SAM cycle. We cloned one of these operons into an expression plasmid, under control of a strong constitutive promoter. However, transformation of the heterologous novobiocin producer strain with this plasmid did not stimulate novobiocin production, but rather showed a detrimental effect on cell viability in the stationary phase and strongly reduced novobiocin accumulation.
Biosynthesis of the salinosporamide A polyketide …
Schramma determined that in order to function properly, the StrB enzyme required some key components: the pre-crosslinked substrate, which she prepared synthetically, cofactor SAM, reductant, and two iron-sulfur (Fe-S) clusters carefully assembled in the protein interior. The team then showed that one of the FeS clusters reductively activated one molecule of SAM, kicking off a chain of one-electron (radical) reactions that gave rise to the novel carbon-carbon bond.