Photopharmacological agents exhibit light-dependent biological activity and may have potential in the development of new antimicrobial agents/modalities. Amidohydrolase enzymes homologous to the well-known human histone deacetylases (HDACs) are present in bacteria, including resistant organisms responsible for a significant number of hospital-acquired infections and deaths. We report photopharmacological inhibitors of these enzymes, using two classes of photoswitches embedded in the inhibitor pharmacophore: azobenzenes and arylazopyrazoles. Although both classes of inhibitor show excellent inhibitory activity (nM IC values) of the target enzymes and promising differential activity of the switchable E- and Z-isomeric forms, the arylazopyrazoles exhibit better intrinsic photoswitch performance (more complete switching, longer thermal lifetime of the Z-isomer). We also report protein-ligand crystal structures of the E-isomers of both an azobenzene and an arylazopyrazole inhibitor, bound to bacterial histone deacetylase-like amidohydrolases (HDAHs). These structures not only uncover interactions important for inhibitor binding but also reveal conformational differences between the two photoswitch inhibitor classes. As such, our data may pave the way for the design of improved photopharmacological agents targeting the HDAC superfamily.

Streptomyces mobaraensis is a key player for the industrial production of the protein cross‐linking enzyme microbial transglutaminase (MTG). Extra‐cellular activation of MTG by the transglutaminase‐activating metalloprotease (TAMP) is regulated by the TAMP inhibitory protein SSTI that belongs to the large Streptomyces subtilisin inhibitor (SSI) family. Despite decades of SSI research, the binding site for metalloproteases such as TAMP remained elusive in most of the SSI proteins. Moreover, SSTI is a MTG substrate, and the preferred glutamine residues for SSTI cross‐linking are not determined. To address both issues, that is, determination of the TAMP and the MTG glutamine binding sites, SSTI was modified by distinct point mutations as well as elongation or truncation of the N‐terminal peptide by six and three residues respectively. Structural integrity of the mutants was verified by the determination of protein melting points and supported by unimpaired subtilisin inhibitory activity. While exchange of single amino acids could not disrupt decisively the SSTI TAMP interaction, the N‐terminally shortened variants clearly indicated the highly conserved Leu40‐Tyr41 as binding motif for TAMP. Moreover, enzymatic biotinylation revealed that an adjacent glutamine pair, upstream from Leu40‐Tyr41 in the SSTI precursor protein, is the preferred binding site of MTG. This extension peptide disturbs the interaction with TAMP. The structure of SSTI was furthermore determined by X‐ray crystallography. While no structural data could be obtained for the N‐terminal peptide due to flexibility, the core structure starting from Tyr41 could be determined and analysed, which superposes well with SSI‐family proteins. Enzymes Chymotrypsin, http://www.chem.qmul.ac.uk/iubmb/enzyme/EC3/4/21/1.html; griselysin (SGMPII, SgmA), EC3.4.24.27; snapalysin (ScNP), http://www.chem.qmul.ac.uk/iubmb/enzyme/EC3/4/24/77.html; streptogrisin‐A (SGPA), http://www.chem.qmul.ac.uk/iubmb/enzyme/EC3/4/21/80.html; streptogrisin‐B (SGPB), http://www.chem.qmul.ac.uk/iubmb/enzyme/EC3/4/21/81.html; subtilisin BPN’, http://www.chem.qmul.ac.uk/iubmb/enzyme/EC3/4/21/62.html; transglutaminase, http://www.chem.qmul.ac.uk/iubmb/enzyme/EC2/3/2/13.html; transglutaminase‐activating metalloprotease (TAMP), EC3.4.‐.‐; tri‐/tetrapeptidyl aminopeptidase, EC3.4.11.‐; trypsin, http://www.chem.qmul.ac.uk/iubmb/enzyme/EC3/4/21/4.html. Databases The atomic coordinates and structure factors (PDB 6I0I) have been deposited in the Protein Data Bank (http://www.rcsb.org).

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Preliminary microprobe studies of the Aubrac lava flows (France) show basalts in the lower part of the series and porphyritic peridotites in the upper part. An increase in porphyries, ascending in the series, suggests an evolution towards a supersaturated limit. The basal basalts contain only one oxide--probably ulvoespinel a higher bed contains ulvoespinel and ilmenite, and the overlying peridotite contains ulvoespinel, ilmenite, and titanomagnetite. A comparison of the results shows a relationship between the mineralogy of the ferrotitanian oxides and the iddingsitization of the olivines.

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