Lactacystine

Both the benzylic C-O and the C-N bonds were cleaved during the synthesis of (4)-lactacystin. The hydrogenolysis of the oxazoline moiety on Pd/C, Pd(OH2)/C, or Pd black in MeOH proved unsuccessful, but catalytic transfer... 

Whereas standard proteases use serine, cysteine, aspartate, or metals to cleave peptide bonds, the proteasome employs an unusual catalytic mechanism. N-terminal threonine residues are generated by self-removal of short peptide extensions from the active yS-subunits and act as nucleophiles during peptide-bond hydrolysis [23]. Given its unusual catalytic mechanism, it is not surprising that there are highly specific inhibitors of the proteasome. The fungal metabolite lactacystin and the bacterial product epoxomicin covalently modify the active-site threonines and in-... 

Fenteany, G. et al. Inhibition of proteasome activities and subunit-specific amino-terminal threonine modification by lactacystin. Science 1995, 268, 726-731. 

Besides the synthetic inhibitors, a variety of natural compounds is known to inhibit the CP. One of these natural inhibitors, lactacystin, was discovered by its ability to induce neurite outgrowth in a murine neuroblastoma cell line. Incubation of cells in the presence of radioactive lactacystin leads to the labelling of the yS5 subunit (Fenteany et al. 1995) and to irreversible inhibition of the CP. As shown by X-ray analysis, the inhibitor is covalently attached to subunit fS5 by an ester bond with the N-terminal ThrlO (Groll et al. 1997) (see Figure 10.7A). The subunit selectivity of lactacystin can be attributed to its dimethyl group, which mimics a valine or a leucine side chain and closely interacts with Met45 in the hydrophobic SI pocket of subunit j85. 

Fig. 10.7. Inh ibitor binding to individual active sites of the yeast 20S proteasome. The inhibitors lactacystin (A), epoxomicin (B) and TMC95A (C) are colored green and are shown in stereo mode together with their unbiased electron densities. The active-site Thrl is highlighted in black. (A) Covalent binding of the Streptomyces metabolite lactacystin to the active site of 5. The SI pockets of the active subunits and differ from that of 5 and are not suitably constructed to bind the inhibitor. As discussed in the text, Met45 (black), which is located at the bottom of the 5-Sl pocket, makes the difference for inhibitor...

Fenteany, G., Standaert, R. F., Lane, W. S., Choi, S., Corey, E. J., and Schreiber, S. L. Inhibition of proteasome activities and subunit-spedfic amino-terminal threonine modification by lactacystin. Science 1995, 268, 726-731. 

Moreover, an UCH (Ap-uch) that interacts with the proteasome was found to be induced by 5-HT, the neurotransmitter that induced long-term facilitation. Ap-uch was found to be critical for the induction of longterm facilitation. Subsequently, Chain et al showed that at sensory-motor neuron synapses, injection of lactacystin, a specific proteasome inhibitor blocked induction of long-term facilitation. Since R subunit inhibits the activity of C subunits of PKA, the results were interpreted to suggest that the ubiquitin-proteasome pathway operates to remove inhibitory constraints on the formation of long-term memory. This has been... 

Proteasome inhibition by lactacystin and Bz-LLL-COCHO (benzol-Leu-Leu-Leu-glyoxal) causes a significant increase of ABP and cell death by altering APP processing at the y-secretase site (406). Resveratrol does not inhibit ABP production because it has no effect on 3-, or y-secretases, but promotes instead intracellular degradation of ABP via a mechanism that involves the proteasome. The resveratrol-induced decrease of ABP can be effectively prevented by several selective proteasome inhibitors and by small interfering RNA-directed silencing on the proteasome subunit P5 (407). 

Several groups employed this strategy for the synthesis of lactacystin analogues, an important class of nonprotein neurotrophic factors. For example, in their total... 

Synthesis of the Proteasome Inhibitors Salinosporamide A, Omuralide and Lactacystin... 

The structurally-related 7-lactams salinosporamide A 1, omuralide 2 and lactacystin 3, of bacterial origin, inhibit proteasome activity, and so are of interest as lead compounds for the development of anticancer agents. Barbara . M. Potts of Nereus Pharmaceuticals in San Diego has reported (J. Med. Chem. 2005,48,3684) a detailed structure-activity studies in this series, and E.J. Corey of Harvard University has prepared (J. Am. Chem. Soc. 2005,127, 8974, 15386) several interesting structural analogues. Susumi Hatakeyama of Nagasaki University, building on previous work in this area, has reported (J. Org. Chem. 2004, 69,7765) a synthesis of 2 and 3 from Tris. 

Lactacystin synthesis 196 Lasubine synthesis 134 Lepadin synthesis 142... 

From a biosynthetic perspective, naturally occurring tetramic acids can be regarded to arise from the assembly of an amino acid and an activated acyl entity derived from an acetyl group or a more complex activated ester, Fig. (1). Alternatively, the simple tetramic acid formed can undergo substitution at C-3 with a second acyl group. On this basis, compounds such as lactacystin (5) have not been considered [11]. The carboxylic acid... 

There are several examples of metabolites where the tetramic acid domain is teasingly disguised. In these cases, it is difficult to be definitive about the nature of the apparent modification unless this is substantiated by biosynthetic studies. The case of lactacystin (5), Fig. (2), has already been considered. Another example is presented by the oxazolomycin group of antibiotics, e.g. (Ill), found in strains of Streptomyces [179]. Biosynthetic studies indicate that the carboxylic acid of the amino acid required to form tetramic acids contributes to the formation of the 3-lactone [180]. In this section, metabolites that probably have a tetramic acid origin are presented. 

A specific inhibitor of the major proteasomal activities is lactacystin, a compound formed by Streptomyces. Lactacystin is converted reversibly, by loss of N-acetyl-cysteine, into a P-lactone known as c/asto-lactacystin. The N-terminal amino group attacks the reactive four-membered ring of the lactone (Eq. 12-22)358/359... 

Lac repressor 240 Laccase 887 Lactacystin 620, 620s Lactate 506s... 

Shibasaki M, Kanai M, Fukuda N (2007) Total Synthesis of Lactacystin and Salinosporamide A. Chem Asian J 2 20... 

Neurodegeneration excitotoxicity Neurodegeneration lactacystin Neurodegeneration neurite outgrowth... 

S Omura, T Fujimoto, K Otoguro, K Matsuzaki, R Moriguchi, H Tanaka, Y Sasaki. Lactacystin, a novel microbial metabolite, induces neuritogenesis of neuroblastoma cells. J Antibiot 44 113-116, 1991. 

Ohfune and co-workers have developed several methodologies involving an asymmetric version of the Strecker synthesis called asymmetric transferring Strecker synthesis (ATS) which has been successfully applied to the synthesis of optically active / -hydroxy-a-substituted a-amino acids [20]. This technique was further applied toward the synthesis of the Corey intermediate of lactacystin [21]. 

Hydroxyalkanoic acids are easily dehydrated to either spiro or fused /3-lactones. Sulfonyl chlorides have been the traditional reagents employed for this transformation <1996S586, 2001CC753>. However, as illustrated in Equation (38) for a cyclization used as part of a synthesis of lactacystin/omuralide (see also Section 2.06.12.4), bis(oxazolidi-none) phosphinyl chloride (BOP-C1) has proven an effective agent for application in sensitive structures note also the selectivity for introduction of the fused (vs. spiro) lactone <2006JOC1220>. 

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