Strong Feedback Inhibition of Key Enzymes in the Morphine Biosynthetic Pathway from Opium Poppy Detectable in Engineered Yeast

Systematic screening of morphine pathway intermediates in engineered yeast revealed key biosynthetic enzymes displaying potent feedback inhibition: 3'-hydroxy-N-methylcoclaurine 4'-methyltransferase (4'OMT), which yields (S)-reticuline, and the coupled salutaridinol-7-O-acetyltransferase (SalAT) and thebaine synthase (THS2) enzyme system that produces thebaine. The addition of deuterated reticuline-d1 to a yeast strain able to convert (S)-norcoclaurine to (S)-reticuline showed reduced product accumulation in response to the feeding of all four successive pathway intermediates. Similarly, the addition of deuterated thebaine-d3 to a yeast strain able to convert salutaridine to thebaine showed reduced product accumulation from exogenous salutaridine or salutaridinol. In vitro analysis showed that reticuline is a noncompetitive inhibitor of 4'OMT, whereas thebaine exerts mixed inhibition on SalAT/THS2. In a yeast strain capable of de novo morphine biosynthesis, the addition of reticuline and thebaine resulted in the accumulation of several pathway intermediates. In contrast, morphine had no effect, suggesting that circumventing the interaction of reticuline and thebaine with 4'OMT and SalAT/THS2, respectively, could substantially increase opiate alkaloid titers in engineered yeast.

Alkaloid binding to opium poppy major latex proteins triggers structural modification and functional aggregation

Opium poppy accumulates copious amounts of several benzylisoquinoline alkaloids including morphine, noscapine, and papaverine, in the specialized cytoplasm of laticifers, which compose an internal secretory system associated with phloem throughout the plant. The contiguous latex includes an abundance of related proteins belonging to the pathogenesis-related (PR)10 family known collectively as major latex proteins (MLPs) and representing at least 35% of the total cellular protein content. Two latex MLP/PR10 proteins, thebaine synthase and neopione isomerase, have recently been shown to catalyze late steps in morphine biosynthesis previously assigned as spontaneous reactions. Using a combination of sucrose density-gradient fractionation-coupled proteomics, differential scanning fluorimetry, isothermal titration calorimetry, and X-ray crystallography, we show that the major latex proteins are a family of alkaloid-binding proteins that display altered conformation in the presence of certain ligands. Addition of MLP/PR10 proteins to yeast strains engineered with morphine biosynthetic genes from the plant significantly enhanced the conversion of salutaridine to morphinan alkaloids.

Phloem-specific localization of benzylisoquinoline alkaloid metabolism in opium poppy

Opium poppy is the only commercial source of the narcotic analgesics morphine and codeine, and semi-synthetic derivatives of the natural opiate precursor thebaine, including oxycodone and the opioid antagonist naloxone. The plant also accumulates the vasodilator and antitussive agents papaverine and noscapine, respectively, which together with morphine, codeine and thebaine comprise the major benzylisoquinoline alkaloids (BIAs) in opium poppy. A majority of enzymes involved in the highly branched BIA metabolism in opium poppy have now been discovered, with many specifically localized to sieve elements of the phloem based on immunofluorescence labeling techniques. Transcripts corresponding to sieve element-localized biosynthetic enzymes were detected in companion cells, as expected. The more recent application of shotgun proteomics has shown that several enzymes operating late in the morphine and noscapine biosynthetic pathways occur primarily in laticifers that are adjacent or proximal to sieve elements. BIA biosynthesis and accumulation in opium poppy involves three phloem cell types and implicates the translocation of key pathway intermediates between sieve elements and laticifers. The recent isolation of uptake transporters associated with laticifers supports an apoplastic rather than a symplastic route for translocation. In spite of the extensive elucidation of BIA biosynthetic enzymes in opium poppy, additional transporters and other auxiliary proteins are clearly necessary to support the complex spatial organization and dynamics involved in product formation and sequestration. In this review, we provide an update of BIA metabolism in opium poppy with a focus on the role of phloem in the biosynthesis of the major alkaloids.

Identification of two amino acids in the C-terminal domain of mouse CRY2 essential for PER2 interaction

BACKGROUND

Cryptochromes (CRYs) are a class of flavoprotein blue-light signaling receptors found in plants and animals, and they control plant development and the entrainment of circadian rhythms. They also act as integral parts of the central circadian oscillator in humans and other animals. In mammals, the CLOCK-BMAL1 heterodimer activates transcription of the Per and Cry genes as well as clock-regulated genes. The PER2 proteins interact with CRY and CKIε, and the resulting ternary complexes translocate into the nucleus, where they negatively regulate the transcription of Per and Cry core clock genes and other clock-regulated output genes. Recent studies have indicated that the extended C-termini of the mammalian CRYs, as compared to photolyase proteins, interact with PER proteins.

RESULTS

We identified a region on mCRY2 (between residues 493 and 512) responsible for direct physical interaction with mPER2 by mammalian two-hybrid and co-immunoprecipitation assays. Moreover, using oligonucleotide-based degenerate PCR, we discovered that mutation of Arg-501 and Lys-503 of mCRY2 within this C-terminal region totally abolishes interaction with PER2.

CONCLUSIONS

Our results identify mCRY2 amino acid residues that interact with the mPER2 binding region and suggest the potential for rational drug design to inhibit CRYs for specific therapeutic approaches.

The Interaction of Cationic Polymers and Their Bisphosphonate Derivatives with Hydroxyapatite

Conjugating proteins with bisphosphonates (BPs), a class of molecules with exceptional affinity to hydroxyapatite (HA), is a feasible means to impart bone affinity to protein-based therapeutic agents. To increase the targeting effectiveness while minimizing protein modification, a polymeric linker containing multiple copies of BPs could be constructed for protein conjugation and targeting to bone. Towards this goal, poly(L-lysine) (PLL) and poly(ethylenimine) (PEI) were utilized as the polymeric backbones to incorporate a BP, namely 2-(3-mercaptopropylsulfanyl)-ethyl-1,1-bisphosphonic acid (thiolBP), by using N-hydroxysuccinimidyl polyethylene glycol maleimide and succinimidyl-4-(N-maleimidomethyl)-cyclohexane-1-carboxylate, respectively. In vitro and in vivo mineral affinity of the polymer-BP conjugates were determined in comparison with the unmodified polymers. The in vitro results indicated strong binding of the cationic polymers to HA in their unmodified form. BP conjugation did not enhance the inherent mineral affinity of the polymers; in contrast, certain modifications negatively affected the polymers' binding to the HA. In vivo results from a subcutaneous implant model in rats also showed no significant difference in mineral affinity of the BP modified and unmodified PEI. We conclude that thiolBP conjugation to the cationic polymers PLL and PEI was not beneficial for increasing the mineral affinity of the polymeric molecules. The strong interaction between the cationic polymers and HA may make the polymers suitable for imparting mineral affinity to bone-acting therapeutics.