Substitution of norleucine for methionine residues in a crustacean pigment-dispersing hormone

Norvaline is accumulated after a down-shift of oxygen in Escherichia coli W3110

Background

Norvaline is an unusual non-proteinogenic branched-chain amino acid which has been of interest especially during the early enzymological studies on regulatory mutants of the branched-chain amino acid pathway in Serratia marcescens. Only recently norvaline and other modified amino acids of the branched-chain amino acid synthesis pathway got attention again when they were found to be incorporated in minor amounts in heterologous proteins with a high leucine or methionine content. Earlier experiments have convincingly shown that norvaline and norleucine are formed from pyruvate being an alternative substrate of α-isopropylmalate synthase, however so far norvaline accumulation was not shown to occur in non-recombinant strains of E. coli.

Results

Here we show that oxygen limitation causes norvaline accumulation in E. coli K-12 W3110 during grow in glucose-based mineral salt medium. Norvaline accumulates immediately after a shift to oxygen limitation at high glucose concentration. On the contrary free norvaline is not accumulated in E. coli W3110 in aerobic cultures. The analysis of medium components, supported by transcriptomic studies proposes a purely metabolic overflow mechanism from pyruvate into the branched chain amino acid synthesis pathway, which is further supported by the significant accumulation of pyruvate after the oxygen downshift. The results indicate overflow metabolism from pyruvate as necessary and sufficient, but deregulation of the branched chain amino acid pathway may be an additional modulating parameter.

Conclusion

Norvaline synthesis has been so far mainly related to an imbalance of the synthesis of the branched chain amino acids under conditions were pyruvate level is high. Here we show that simply a downshift of oxygen is sufficient to cause norvaline accumulation at a high glucose concentration as a consequence of the accumulation of pyruvate and its direct chain elongation over α-ketobutyrate and α-ketovalerate.

Although the flux to norvaline is low, millimolar concentrations are accumulated in the cultivation broth, which is far above the level which has been discussed for being relevant for misincorporation of norvaline into recombinant proteins. Therefore we believe that our finding is relevant for recombinant protein production but also may even have implications for the physiology of E. coli under oxygen limitation in general.

Incorporation of norvaline at leucine positions in recombinant human hemoglobin expressed in Escherichia coli

We report here a novel finding that norvaline can be incorporated in place of leucine in recombinant human hemoglobin expressed in Escherichia coli. The presence of the norvaline was confirmed by several analytical methods such as amino acid analysis, peptide mapping, electrospray mass spectrometry, and Edman protein sequencing. It appears that substitution is distributed across both the beta- and di-alpha-globins in purified recombinant hemoglobin. The level of misincorporation correlated with the ratio of the free norvaline/leucine pool available in the cell culture. This suggests that the incorporation of norvaline for leucine occurs through misaminoacylation of tRNALeu, similar to the misincorporation of norleucine for methionine found in many recombinant proteins expressed in E. coli.

Pigment-dispersing hormones: a novel family of neuropeptides from arthropods

This report summarizes recent work on the chemistry and structure-activity relationships of crustacean chromatophorotropic pigment-dispersing hormones (PDHs) and the identification of structurally related peptides from insects. So far, the primary structures of crustacean alpha-PDH and beta-PDH and of a pigment-dispersing factor (PDF) from the grasshopper Romalea microptera have been deduced. Additionally, 17 of the 18 residues of a PDF from the cricket Acheta domesticus were identified. In the PDH/PDF family, the chain length (18 residues), terminal residues (N-terminal Asn and C-terminal Ala-NH2), and at least 50% of the amino acid sequence appear to be conserved. The functions of these peptides in insects are unknown. Detailed studies of structure-activity relationships of crustacean PDHs have been conducted, leading to the tentative identification of the message sequence, preparation of hyperpotent oxidation-resistant analogs, and synthesis of bioactive tyrosinated analogs for radioiodination. Antisera were raised against PDHs, and immunoreactive soma and fibers have been localized in the crustacean eyestalk neurosecretory system. This progress is expected to stimulate more intensive research on the PDH family.

C-terminal deletion analogs of a crustacean pigment-dispersing hormone

This study deals with the effect of deamidation and C-terminal truncation on the potency of an octadecapeptide pigment-dispersing hormone (PDH: Asn-Ser-Gly-Met-Ile-Asn-Ser-Ile-Leu-Gly-Ile-Pro-Arg-Val-Met-Thr-Glu-Ala- NH2), first described as light-adapting distal retinal pigment hormone (DRPH) from Pandalus borealis. Bioassay of synthetic analogs for melanophore pigment dispersion in destalked fiddler crabs (Uca pugilator) showed that deamidation causes a 300-fold decrease in potency. The analogs 1-17 NH2 and 1-16 NH2 were about 3 times more potent than 1-18-OH. Further truncation led to decreases in potency, with the peptide 1-9-NH2 being the smallest C-terminal deletion analog to display activity (0.001% potency). Smaller analogs (1-8-NH2, 1-6-NH2 and 1-4-NH2) were inactive when tested in doses as high as 500 nmoles/crab. On the basis of our earlier work on N-terminal deletion analogs and the present findings the residues 6 to 9 seem to be important for PDH action.