The ubiquitin-proteasome proteolytic pathway plays a major role in selective protein degradation and regulates various cellular events including cell cycle, signal transduction, stress response, antigen presentation, and protein quality control. Ubiquitin, a highly conserved small protein in eukaryotes, attaches to a target protein prior to degradation. The polyubiquitin chain tagged to the target protein is recognized by the 26 S proteasome, a high-molecular-mass protease subunit complex, and the protein portion is degraded by the proteolytic active sites in a cavity of the 26 S proteasome. The potential of specific proteasome inhibitors, which act as anti-cancer agents, is now under intensive investigation. In addition, various inhibitors of a ubiquitin-activating enzyme, ubiquitin ligases, and deubiquitinating enzymes have been isolated from natural resources. Recently, we found that girolline, an antitumor compound, inhibits the recruitment of polyubiquitinated proteins to the proteasome. In this review, we summarize the structures and biological activities of natural products that inhibit various factors involved in the ubiquitin-proteasome proteolytic pathway.
This review summarizes the recent advances in catalytic reactions involving alkylidene-, vinylidene-, and allenylidene complexes in situ generated from functionalized alkynes. Reactions of alkylidene complex intermediates are classified by their generation pathways : 1) alkyne-metathesis with carbene complexes as catalysts, 2) cyclization of π-alkyne complexes with intramolecular alkene moieties, 3) cyclization of π-alkyne complexes with intramolecular carbonyl or iminyl moieties, 4) migration in π-alkyne complexes, 5) tautomerism of metalacyclopentadiene generated from ω-diynes. Catalytic reactions via vinylidene and allenylidene complexes are presented with an emphasis on the type of reaction mechanism involved; 1) nucleophilic addition, 2) metallacycle formation ([2+2] cycloaddition), 3) migratory insertion of ligand moieties, 4) cycloaromatization, 5) pericyclic reaction.
Three epimeric analogues at D-Ala-1, Ala-2, and Ala-4 of RA-VII, an antitumor bicyclic hexapeptide from the plants of the genus Rubia, were prepared from its thioamides through oxazole formation on the peptide backbone and subsequent partial hydrolysis. Several other RA-VII analogues, in which D-Ala-1, Ala-2 or Ala-4 was replaced by glycine or D-Ala-1 by D-2-aminobutyric acid or D-norvaline, or in which a 1, 2, 4-triazole cis-amide bond surrogate was incorporated at residues 2 and 3, were prepared by connection of the cycloisodityrosine unit with corresponding tetrapeptides and subsequent macrocyclization. The cycloisodityrosine unit was obtained by degradation of the bis (thioamide) of RA-VII. Studies on conformation and cytotoxicities indicated that in RA-VII series peptides, the original D, L, L-configurations at residues 1, 2 and 4 are essential for the bioactive conformation, and that the minor conformer of RA-VII having cis Ala-2/Tyr-3 and Tyr-5/Tyr-6, and trans Ala-4/Tyr-5 takes little, if any, part in expressing the activity.
One of the great challenges in the post-genome era is to clarify the biological significance of intracellular molecules directly in living cells. If we can visualize a molecule in action, it is possible to acquire biological information, which is unavailable if we deal with cell homogenates. One possible approach is to design and synthesize sensor molecules that can convert biological information to chemical reactions that are easily monitored. For this purpose, fluorescence sensor molecules for intracellular messengers have been developed and successfully applied to living cells. Ratiometric measurement is a technique to reduce artifacts by minimizing the influence of extraneous factors on the fluorescence of a sensor molecule. FRET is one mechanism that is applicable for ratiometric measurement. We have designed two FRET sensor molecules, based on changing donor-acceptor distance and changing overlap integral. Protein tyrosine phosphatase activity was successfully monitored by designed sensor. Design, synthesis and neurological application of Zn2+ specific sensor molecules are also introduced.
Synthesis, thermal and photochemical reactions of vinylidenecyclopropanes are described. Vinylidenecyclopropanes have been prepared by the reaction of 1, 1-disubstituted 2, 2-dibromocyclopropanes with alkenes under phase transfer conditions in a stereospecific manner. Cis-trans photoisomerization of C 2, C 3-disubstituted vinylidenecyclopropanes proceeded via their excited singlet and triplet states and via their radical cations. Direct photoreaction of C 2, C 2, C 3, C 3-tetrasubstituted vinylidenecyclopropanes afforded rearranged products, 1, 2, 3-butatriene derivatives. Triplet-sensitized photoreaction of vinylidenecyclopropanes with electron-deficient alkenes gave vinylidenecyclopentanes. 9, 10-Dicyanoanthracene-sensitized photoreaction of vinylidenecyclopropanes with organic carbonitriles gave vinylidene-l-pyrrolines via photoinduced electron transfer. Oxidation of vinylidenecyclopropanes by oxone or m-CPBA gave cyclobutanone derivatives via methylenecyclopropanes with elimination of carbon monoxide. Thermal rearrangement from 1-diarylvinylidene- 2 -vinylcyclopropanes occurred at relatively low temperature to give vinylidenecyclopentenes. Addition of dihalocarbenes to vinylidenecyclopropanes proceeded regioselectively to give cyclopropylidenecyclopropanes which rearranges to 1- (dihalomethylene) - spiropentanes. These reactions are in sharp contrast with those of methylenecyclopropanes. It has been demonstrated that the thermal and photochemical bond cleavage of vinylidenecyclopropanes occurred mainly at their C 1-C 2 positions.
Triplex-forming oligonucleotides (TFOs) are potential DNA-targeting molecules, and would become powerful tools for genomic research. However, the problem that the stable triplexes form only with homopurine : homopyrimidine sequences has not been generally solved in spite of extensive studies. In this study we have developed new base analogs (WNA) constructed of three parts, a benzene ring, a heterocyclic ring, and a bicyclic skeleton to hold these two parts. Among a number of WNA analogs, we have determined two useful WNA analogs, WNA-/βT and WNA-/βC, for selective stabilization of triplexes at a TA and a CG interrupting site with higher stability than the natural-type triplexes, respectively. The results of this study will provide useful information for the design of new WNA analogs to overcome inherent problems for further expansion of triplex recognition codes.