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Loss of allostery and coenzyme B12 delivery by a pathogenic mutation in adenosyltransferase.

ABSTRACT: ATP-dependent cob(I)alamin adenosyltransferase (ATR) is a bifunctional protein: an enzyme that catalyzes the adenosylation of cob(I)alamin and an escort that delivers the product, adenosylcobalamin (AdoCbl or coenzyme B(12)), to methylmalonyl-CoA mutase (MCM), resulting in holoenzyme formation. Failure to assemble holo-MCM leads to methylmalonic aciduria. We have previously demonstrated that only 2 equiv of AdoCbl bind per homotrimer of ATR and that binding of ATP to the vacant active site triggers ejection of 1 equiv of AdoCbl from an adjacent site. In this study, we have mimicked in the Methylobacterium extorquens ATR, a C-terminal truncation mutation, D180X, described in a patient with methylmalonic aciduria, and characterized the associated biochemical penalties. We demonstrate that while k(cat) and K(M)(Cob(I)) for D180X ATR are only modestly decreased (by 3- and 2-fold, respectively), affinity for the product, AdoCbl, is significantly diminished (400-fold), and the negative cooperativity associated with its binding is lost. We also demonstrate that the D180X mutation corrupts ATP-dependent cofactor ejection, which leads to transfer of AdoCbl from wild-type ATR to MCM. These results suggest that the pathogenicity of the corresponding human truncation mutant results from its inability to sequester AdoCbl for direct transfer to MCM. Instead, cofactor release into solution is predicted to reduce the capacity for holo-MCM formation, leading to disease.

SUBMITTER: Lofgren M

PROVIDER: S-EPMC3119721 | biostudies-literature | 2011 Jun

REPOSITORIES: biostudies-literature

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Loss of allostery and coenzyme B12 delivery by a pathogenic mutation in adenosyltransferase.

Lofgren Michael M Banerjee Ruma R

Biochemistry 20110602 25

ATP-dependent cob(I)alamin adenosyltransferase (ATR) is a bifunctional protein: an enzyme that catalyzes the adenosylation of cob(I)alamin and an escort that delivers the product, adenosylcobalamin (AdoCbl or coenzyme B(12)), to methylmalonyl-CoA mutase (MCM), resulting in holoenzyme formation. Failure to assemble holo-MCM leads to methylmalonic aciduria. We have previously demonstrated that only 2 equiv of AdoCbl bind per homotrimer of ATR and that binding of ATP to the vacant active site trigg ...[more]

PMID: 21604717

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Project description:The ratio of the major monensin analogs produced by Streptomyces cinnamonensis is dependent upon the relative levels of the biosynthetic precursors methylmalonyl-coenzyme A (CoA) (monensin A and monensin B) and ethylmalonyl-CoA (monensin A). The meaA gene of this organism was cloned and sequenced and was shown to encode a putative 74-kDa protein with significant amino acid sequence identity to methylmalonyl-CoA mutase (MCM) (40%) and isobutyryl-CoA mutase (ICM) large subunit (36%) and small subunit (52%) from the same organism. The predicted C terminus of MeaA contains structural features highly conserved in all coenzyme B12-dependent mutases. Plasmid-based expression of meaA from the ermE* promoter in the S. cinnamonensis C730.1 strain resulted in a decreased ratio of monensin A to monensin B, from 1:1 to 1:3. Conversely, this ratio increased to 4:1 in a meaA mutant, S. cinnamonensis WM2 (generated from the C730.1 strain by insertional inactivation of meaA by using the erythromycin resistance gene). In both of these experiments, the overall monensin titers were not significantly affected. Monensin titers, however, did decrease over 90% in an S. cinnamonensis WD2 strain (an icm meaA mutant). Monensin titers in the WD2 strain were restored to at least wild-type levels by plasmid-based expression of the meaA gene or the Amycolatopsis mediterranei mutAB genes (encoding MCM). In contrast, growth of the WD2 strain in the presence of 0.8 M valine led only to a partial restoration (<25%) of monensin titers. These results demonstrate that the meaA gene product is significantly involved in methylmalonyl-CoA production in S. cinnamonensis and that under the tested conditions the presence of both MeaA and ICM is crucial for monensin production in the WD2 strain. These results also indicate that valine degradation, implicated in providing methylmalonyl-CoA precursors for many polyketide biosynthetic processes, does not do so to a significant degree for monensin biosynthesis in the WD2 mutant.

Project description:Adenosylcobalamin-dependent isomerases catalyze carbon skeleton rearrangements using radical chemistry. We have recently demonstrated that an isobutyryl-CoA mutase variant, IcmF, a member of this enzyme family that catalyzes the interconversion of isobutyryl-CoA and n-butyryl-CoA also catalyzes the interconversion between isovaleryl-CoA and pivalyl-CoA, albeit with low efficiency and high susceptibility to inactivation. Given the biotechnological potential of the isovaleryl-CoA/pivalyl-CoA mutase (PCM) reaction, we initially attempted to engineer IcmF to be a more proficient PCM by targeting two active site residues predicted based on sequence alignments and crystal structures, to be key to substrate selectivity. Of the eight mutants tested, the F598A mutation was the most robust, resulting in an ∼17-fold increase in the catalytic efficiency of the PCM activity and a concomitant ∼240-fold decrease in the isobutyryl-CoA mutase activity compared with wild-type IcmF. Hence, mutation of a single residue in IcmF tuned substrate specificity yielding an ∼4000-fold increase in the specificity for an unnatural substrate. However, the F598A mutant was even more susceptible to inactivation than wild-type IcmF. To circumvent this limitation, we used bioinformatics analysis to identify an authentic PCM in genomic databases. Cloning and expression of the putative AdoCbl-dependent PCM with an α2β2 heterotetrameric organization similar to that of isobutyryl-CoA mutase and a recently characterized archaeal methylmalonyl-CoA mutase, allowed demonstration of its robust PCM activity. To simplify kinetic analysis and handling, a variant PCM-F was generated in which the αβ subunits were fused into a single polypeptide via a short 11-amino acid linker. The fusion protein, PCM-F, retained high PCM activity and like PCM, was resistant to inactivation. Neither PCM nor PCM-F displayed detectable isobutyryl-CoA mutase activity, demonstrating that PCM represents a novel 5'-deoxyadenosylcobalamin-dependent acyl-CoA mutase. The newly discovered PCM and the derivative PCM-F, have potential applications in bioremediation of pivalic acid found in sludge, in stereospecific synthesis of C5 carboxylic acids and alcohols, and in the production of potential commodity and specialty chemicals.

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Loss of allostery and coenzyme B12 delivery by a pathogenic mutation in adenosyltransferase.

Publications

Loss of allostery and coenzyme B12 delivery by a pathogenic mutation in adenosyltransferase.

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