Channeling of misfolded proteins into repair, assembly or degradation pathways is often mediated by complex and multifunctional cellular factors. Despite detailed structural information, the underlying regulatory mechanisms governing these factors are not well understood. The extracytoplasmic heat-shock factor DegP (HtrA) is a well-suited model for addressing mechanistic issues, as it is regulated by the common mechanisms of allostery and activation by oligomerization. Site-directed mutagenesis combined with refolding and oligomerization studies of chemically denatured DegP revealed how substrates trigger the conversion of the resting conformation into the active conformation. Binding of specific peptides to PDZ domain-1 causes a local rearrangement that is allosterically transmitted to the substrate-binding pocket of the protease domain. This activated state readily assembles into larger oligomeric particles, thus stabilizing the catalytically active form and providing a degradation cavity for protein substrates. The implications of these data for the mechanism of protein quality control are discussed.

Allostery is a basic principle of control of enzymatic activities based on the interaction of a protein or small molecule at a site distinct from an enzyme's active center. Allosteric modulators represent an alternative approach to the design and synthesis of small-molecule activators or inhibitors of proteases and are therefore of wide interest for medicinal chemistry. The structural bases of some proteinaceous and small-molecule allosteric protease regulators have already been elucidated, indicating a general mechanism that might be exploitable for future rational design of small-molecule effectors.

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Unleashing the proteolytic activity of DegP: A study with synthetic mimics of cellular stress signals reveals an allosteric (chemical) activation mechanism of the bacterial HtrA protease DegP, leading to a fine‐tuned amplification of protein degradation during bacterial protein quality control (see scheme: binding of a peptide sequence (circles) increases activation. The nature of the penultimate residue (red) is shown to be decisive).

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The solid phase total synthesis of the marine cyanobacterial Ahp-cyclodepsipeptide Symplocamide A is reported as a model for a general route for the synthesis of tailor-made non-covalent serine protease inhibitors.

The S1 serine protease family is one of the largest and most biologically important protease families.D espite their biomedical significance,g eneric approaches to generate potent, class-specific, bioactive non-covalent inhibitors for these enzymes are still limited. In this work, we demonstrate that Ahp-cyclodepsipeptides represent as uitable scaffold for generating target-tailored inhibitors of serine proteases.F or efficient synthetic access,wedeveloped apractical mixed solidand solution-phase synthesis that we validated through performing the first chemical synthesis of the two natural products Tasipeptin Aand B. The suitability of the Ahp-cyclodepsipeptide scaffold for tailored inhibitor synthesis is showcased by the generation of the most potent human HTRA protease inhibitors to date.W ea nticipate that our approach maya lso be applied to other serine proteases,thus opening new avenues for asystematic discovery of serine protease inhibitors.Serine proteases of the S1 family (trypsin/chymotrypsinlike) are one of the largest and biomedically relevant protease families.[1] In contrast to the design of covalent inhibitors or activity-based probes for this enzyme class, [2] generic approaches for designing potent, enzyme-class-specific,c ellactive,and non-covalent small-molecule inhibitors are highly limited and have been achieved only for distinct serine proteases.[3] Consequently,a lternative approaches for the systematic generation of such chemical probes are urgently required.Ahp-cyclodepsipeptides (also termed cyanopeptolins or peptolides) are aclass of over 100 peptidic natural products of non-ribosomal origin that display potent inhibitory properties against serine proteases.[4] All Ahp-cyclodepsipeptides are containing an Ahp (3-amino-6-hydroxy-2-piperidone) unit and an N-methyl aromatic amino acid at conserved positions in their 19-membered ring structure,while other residues are much less conserved (Figure 1a). Crystal structures of Ahpcyclodepsipeptides in complex with serine proteases indicate that inhibition is based on as ubstrate-like binding mode in which distinct amino acid residues occupy the S-and S'-pockets,h owever,p roteolytic cleavage does not occur (Figure 1b,S chechter and Berger nomenclature is used).[5] Accordingly,t hey act in as imilar manner to proteinaceous canonical serine protease inhibitors,w ith the conserved residues stabilizing the inhibitory fold, while the other less conserved residues define serine protease selectivity through optimal accommodation to the specific Sa nd S'-sites.T his suggests that Ahp-cyclodepsipeptides may represent as uit- Figure 1. Structural features and determinants of Ahp-cyclodepsipeptides for serine protease inhibition. a) Chemicals tructures of the natural product Ahp-cyclodepsipeptides TasA (1)a nd TasB (2). The red color indicates strictly conserved residues found in all so far known Ahp-cyclodepsipeptides; residues only rarely modified in Ahp-cyclodepsipeptides are depicted in green, and fully variable residues in blue. Note that th...

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