Capsaicin Synthesis Requires in Situ Phenylalanine and Valine Formation in in Vitro Maintained Placentas from Capsicum chinense.

Capsaicinoids (CAP) are nitrogenous metabolites formed from valine (Val) and phenylalanine (Phe) in the placentas of hot Capsicum genotypes. Placentas of Habanero peppers can incorporate inorganic nitrogen into amino acids and have the ability to secure the availability of the required amino acids for CAP biosynthesis. In order to determine the participation of the placental tissue as a supplier of these amino acids, the effects of blocking the synthesis of Val and Phe by using specific enzyme inhibitors were analyzed. Isolated placentas maintained in vitro were used to rule out external sources' participation. Blocking Phe synthesis, through the inhibition of arogenate dehydratase, significantly decreased CAP accumulation suggesting that at least part of Phe required in this process has to be produced in situ. Chlorsulfuron inhibition of acetolactate synthase, involved in Val synthesis, decreased not only Val accumulation but also that of CAP, pointing out that the requirement for this amino acid can also be fulfilled by this tissue. The presented data demonstrates that CAP accumulation in in vitro maintained placentas can be accomplished through the in situ availability of Val and Phe and suggests that the synthesis of the fatty acid chain moiety may be a limiting factor in the biosynthesis of these alkaloids.

A reporter system that discriminates EF-hand-sensor motifs from signal-modulators at the single-motif level.

The T-protein is a single-polypeptide bi-functional enzyme composed of a chorismate mutase domain fused to a prephenate dehydrogenase domain (TyrA). We replaced the chorismate mutase domain with canonical or pseudo-Ca(2+)-binding motifs (EF-hand). Canonical-EF-hand-motifs differentiate from pseudo-EF-hand-motifs by experimenting a Ca(2+)-dependent conformational change. The Ca(2+)-free EF-hand-TyrA fusion-proteins showed TyrA activity at the T-protein level. Canonical-EF-hand-TyrA fusions showed a Ca(2+)-dependent loss of TyrA activity, but a pseudo-EF-hand-TyrA fusion showed high TyrA activity level in excess-Ca(2+) conditions. Because TyrA activity exhibits robust changes in response to Ca(2+)-dependent-EF-hand conformational alterations, TyrA could be a good Ca(2+)-reporter enzyme. A chimeric canonical/pseudo-EF-hand strategy is proposed to confer pseudo-EF-hand motifs with a Ca(2+)-dependent conformational change.

The ß1 domain of protein G can replace the chorismate mutase domain of the T-protein.

T-protein is composed of chorismate mutase (AroQ(T)) fused to the N-terminus of prephenate dehydrogenase (TyrA). Here, we report the replacement of AroQ(T) with the ß1-domain of protein G (Gß1). The TyrA domain shows a strong dehydrogenase activity within the context of this fusion, and our data indicate that Gß1-TyrA folds into a dimeric conformation. Amino acid substitutions in the Gß1 domain of Gß1-TyrA identified residues involved in stabilizing the TyrA dimeric conformation. Gß1 substitutions in the N-terminal ß-hairpin eliminated Gß1-TyrA expression, whereas Gß1-TyrA tolerated Gß1 substitutions in the C-terminal ß-hairpin and in the α-helix. All of the characterized variants folded into a dimeric conformation. The importance of the ß2-strand in forming a Gß1 homo-dimerization interface explains the relevance of the first-ß-hairpin in stabilizing the dimeric TyrA protein.

Metabolic engineering of Escherichia coli for improving L-3,4-dihydroxyphenylalanine (L-DOPA) synthesis from glucose.

L-3,4-dihydroxyphenylalanine (L-DOPA) is an aromatic compound employed for the treatment of Parkinson's disease. Metabolic engineering was applied to generate Escherichia coli strains for the production of L-DOPA from glucose by modifying the phosphoenolpyruvate:sugar phosphotransferase system (PTS) and aromatic biosynthetic pathways. Carbon flow was directed to the biosynthesis of L-tyrosine (L-Tyr), an L-DOPA precursor, by transforming strains with compatible plasmids carrying genes encoding a feedback-inhibition resistant version of 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase, transketolase, the chorismate mutase domain from chorismate mutase-prephenate dehydratase from E. coli and cyclohexadienyl dehydrogenase from Zymomonas mobilis. The effects on L-Tyr production of PTS inactivation (PTS(-) gluc(+) phenotype), as well as inactivation of the regulatory protein TyrR, were evaluated. PTS inactivation caused a threefold increase in the specific rate of L-Tyr production (q( L-Tyr)), whereas inactivation of TyrR caused 1.7- and 1.9-fold increases in q( L-Tyr) in the PTS(+) and the PTS(-) gluc(+) strains, respectively. An 8.6-fold increase in L-Tyr yield from glucose was observed in the PTS(-) gluc(+) tyrR (-) strain. Expression of hpaBC genes encoding the enzyme 4-hydroxyphenylacetate 3-hydroxylase from E. coli W in the strains modified for L-Tyr production caused the synthesis of L-DOPA. One of such strains, having the PTS(-) gluc(+) tyrR (-) phenotype, displayed the best production parameters in minimal medium, with a specific rate of L-DOPA production of 13.6 mg/g/h, L-DOPA yield from glucose of 51.7 mg/g and a final L-DOPA titer of 320 mg/l. In a batch fermentor culture in rich medium this strain produced 1.51 g/l of L-DOPA in 50 h.