Studies on the stereochemistry of biliprotein chromophores and related model compounds

The Cyanobacterial "Nutraceutical" Phycocyanobilin Inhibits Cysteine Protease Legumain

The blue biliprotein phycocyanin, produced by photo-autotrophic cyanobacteria including spirulina (Arthrospira) and marketed as a natural food supplement or "nutraceutical," is reported to have anti-inflammatory, antioxidant, immunomodulatory, and anticancer activity. These diverse biological activities have been specifically attributed to the phycocyanin chromophore, phycocyanobilin (PCB). However, the mechanism of action of PCB and the molecular targets responsible for the beneficial properties of PCB are not well understood. We have developed a procedure to rapidly cleave the PCB pigment from phycocyanin by ethanolysis and then characterized it as an electrophilic natural product that interacts covalently with thiol nucleophiles but lacks any appreciable cytotoxicity or antibacterial activity against common pathogens and gut microbes. We then designed alkyne-bearing PCB probes for use in chemical proteomics target deconvolution studies. Target identification and validation revealed the cysteine protease legumain (also known as asparaginyl endopeptidase, AEP) to be a target of PCB. Inhibition of this target may account for PCB's diverse reported biological activities.

Structure and characterization of methyl (±)-1-(2-aminophenyl)-2-hydroxy-4-(4-methoxybenzoyl)-5-(4-methoxyphenyl)-3-oxo-2,3-dihydro-1H-pyrrole-2-acetate

The stoichiometry of 1 was derived from the results of elemental analysis, IR, 1H, 13C-NMR, UV-Vis and mass spectroscopic techniques. The structure derived from physico-chemical methods is consistent with that of the X-ray diffraction. The compound crystallizes in the triclinic system, space group P1̅, with a = 9.3895(12), b = 11.3759(11), c = 12.8077(14)Å, α = 103.290(3)° β = 98.110(4)°, γ = 105.545(3)°, V = 1252.4(2)Å3, Z = 2. The molecule has a non-planar configuration. The methoxy carbonylmethyl group exhibits an E-configuration. The crystal structure is stabilized by intermolecular C—H…O, N—H…O and O—H…O contacts.

New strategies for the synthesis of biologically important tetrapyrroles. The "B,C + D + A" approach to linear tetrapyrroles

Linear tetrapyrroles related to phytochrome (1) were prepared in enantiospecific fashion by a new strategy beginning with ring-B,C synthons of type 19 (bis-iododipyrrins). Rings A and D were elaborated by Pd(0)-mediated coupling of 19a with the appropriate alkyne acid or amide derivatives 9 and 20, followed by intramolecular cyclization (method C: BC + D + A --> ABCD).

(3Z)- and (3E)-phytochromobilin are intermediates in the biosynthesis of the phytochrome chromophore

Using a high performance liquid chromatography (HPLC)-based assay, we have demonstrated that isolated oat etioplasts convert the linear tetrapyrrole biliverdin IX alpha to (3E)-phytochromobilin, the proposed precursor of the chromophore of the plant photoreceptor phytochrome. In addition to (3E)-phytochromobilin, the synthesis of a second phytochromobilin was detected by its ability to functionally assemble with recombinant oat apophytochrome A. The structure of this new pigment has been determined to be the 3Z isomer of phytochromobilin by absorption and 1H NMR spectroscopy. Like (3E)-phytochromobilin, assembly of HPLC-purified (3Z)-phytochromobilin with apophytochrome yielded a holoprotein that is spectrally indistinguishable from native oat phytochrome A. However, the postchromatographic conversion of (3Z)- to (3E)-phytochromobilin appears to be responsible for this result. Kinetic HPLC analyses have demonstrated that (3Z)-phytochromobilin is synthesized prior to the 3E isomer by oat etioplasts. We therefore propose that (3Z)-phytochromobilin is the immediate product of biliverdin IX alpha reduction by the enzyme phytochromobilin synthase. This implicates the presence of an isomerase that catalyzes the conversion of (3Z)- to (3E)-phytochromobilin, the immediate precursor of the phytochrome A chromophore.