Carbonic anhydrase IX (CAIX) is a transmembrane enzyme that regulates pH in hypoxic tumors and promotes tumor cell survival. Its expression is associated with the occurrence of metastases and poor prognosis. Here, we present nine derivatives of the cobalt bis­(dicarbollide)(1−) anion substituted at the boron or carbon sites by alkysulfamide group(s) as highly specific and selective inhibitors of CAIX. Interactions of these compounds with the active site of CAIX were explored on the atomic level using protein crystallography. Two selected derivatives display subnanomolar or picomolar inhibition constants and high selectivity for the tumor-specific CAIX over cytosolic isoform CAII. Both derivatives had a time-dependent effect on the growth of multicellular spheroids of HT-29 and HCT116 colorectal cancer cells, facilitated penetration and/or accumulation of doxorubicin into spheroids, and displayed low toxicity and showed promising pharmacokinetics and a significant inhibitory effect on tumor growth in syngenic breast 4T1 and colorectal HT-29 cancer xenotransplants.

SummaryRhomboid-family intramembrane proteases regulate important biological processes and have been associated with malaria, cancer, and Parkinson's disease. However, due to the lack of potent, selective, and pharmacologically compliant inhibitors, the wide therapeutic potential of rhomboids is currently untapped. Here, we bridge this gap by discovering that peptidyl α-ketoamides substituted at the ketoamide nitrogen by hydrophobic groups are potent rhomboid inhibitors active in the nanomolar range, surpassing the currently used rhomboid inhibitors by up to three orders of magnitude. Such peptidyl ketoamides show selectivity for rhomboids, leaving most human serine hydrolases unaffected. Crystal structures show that these compounds bind the active site of rhomboid covalently and in a substrate-like manner, and kinetic analysis reveals their reversible, slow-binding, non-competitive mechanism. Since ketoamides are clinically used pharmacophores, our findings uncover a straightforward modular way for the design of specific inhibitors of rhomboid proteases, which can be widely applicable in cell biology and drug discovery.

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Rutinosidases (α‐l‐rhamnosyl‐β‐d‐glucosidases) catalyze the cleavage of the glycosidic bond between the aglycone and the disaccharide rutinose (α‐l‐rhamnopyranosyl‐(1→6)‐β‐d‐glucopyranose) of specific flavonoid glycosides such as rutin (quercetin 3‐O‐rutinoside). Microbial rutinosidases are part of the rutin catabolic pathway, enabling the microorganism to utilize rutin and related plant phenolic glycosides. Here, we report the first three‐dimensional structure of a rutinosidase determined at 1.27‐Å resolution. The rutinosidase from Aspergillus niger K2 (AnRut), a member of glycoside hydrolase family GH‐5, subfamily 23, was heterologously produced in Pichia pastoris. The X‐ray structure of AnRut is represented by a distorted (β/α)8 barrel fold with its closest structural homologue being an exo‐β‐(1,3)‐glucanase from Candida albicans (CaExg). The catalytic site is located in a deep pocket with a striking structural similarity to CaExg. However, the entrance to the active site of AnRut has been found to be different from that of CaExg – a mostly unstructured section of ~ 40 residues present in CaExg is missing in AnRut, whereas an additional loop of 13 amino acids partially covers the active site of AnRut. NMR analysis of reaction products provided clear evidence for a retaining reaction mechanism of AnRut. Unexpectedly, quercetin 3‐O‐glucoside was found to be a better substrate than rutin, and thus, AnRut cannot be considered a typical diglycosidase. Mutational analysis of conserved active site residues in combination with in silico modeling allowed identification of essential interactions for enzyme activity and helped to reveal further details of substrate binding. The protein sequence of AnRut has been revised.DatabasesThe nucleotide sequence of the rutinosidase‐encoding gene is available in the GenBank database under the accession number MN393234. Structural data are available in the PDB database under the accession number 6I1A.Enzymeα‐l‐Rhamnosyl‐β‐d‐glucosidase (EC 3.2.1.168).

Glutamate carboxypeptidase III (GCPIII) is best known as a homologue of glutamate carboxypeptidase II [GCPII; also known as prostate-specific membrane antigen (PSMA)], a protease involved in neurological disorders and overexpressed in a number of solid cancers. However, mouse GCPIII was recently shown to cleave b-citrylglutamate (BCG), suggesting that these two closely related enzymes have distinct functions. To develop a tool to dissect, evaluate and quantify the activities of human GCPII and GCPIII, we analysed the catalytic efficiencies of these enzymes towards three physiological substrates. We observed a high efficiency of BCG cleavage by GCPIII but not GCPII. We also identified a strong modulation of GCPIII enzymatic activity by divalent cations, while we did not observe this effect for GCPII. Additionally, we used X-ray crystallography and computational modelling (quantum and molecular mechanical calculations) to describe the mechanism of BCG binding to the active sites of GCPII and GCPIII, respectively. Finally, we took advantage of the substantial differences in the enzymatic efficiencies of GCPII and GCPIII towards their substrates, using enzymatic assays for specific detection of these proteins in human tissues. Our findings suggest that GCPIII may not act merely as a complementary enzyme to GCPII, and it more likely possesses a specific physiological function related to BCG metabolism in the human body. DatabaseThe X-ray structure of GCPII Glu424Ala in complex with BCG has been deposited in the RCSB Protein Data Bank under accession code 5F09. Abbreviations 3D, three-dimensional; AAS, atomic absorption spectroscopy; ABS, arene-binding site; ACN, acetonitrile; BCG, b-citryl-L-glutamic acid (bcitrylglutamate, beta-citrylglutamate, beta-citryl-L-glutamic acid, beta-citryl-L-glutamate); BSA, bovine serum albumin; C12E8, octaethylene glycol monododecyl ether; DBU, 1,8-Diazabicycloundec-7-ene; DIEA, N,N-Diisopropylethylamine; EtOAc, ethylacetate; FolGlu n , folyl-n-c-Lglutamic acid; G3PDH, glyceraldehyde 3-phosphate dehydrogenase; GCPII, glutamate carboxypeptidase II; GCPIII, glutamate carboxypeptidase III; HPLC, high-performance liquid chromatography; HRMS, high-resolution mass spectrometry; MeOH, methanol; MD/ MM, molecular dynamical/molecular mechanical calculations; NAAG, N-acetyl-L-aspartyl-L-glutamic acid; NMR, nuclear magnetic resonance; OPA, orthophtalaldehyde; ORF, open reading frame; Pd(C), palladium on activated charcoal; PSMA, prostate-specific membrane antigen; QM/MM, quantum mechanical and molecular mechanical calculations; QM/MM/MD, quantum mechanical, molecular mechanical and molecular dynamical calculations; qPCR, quantitative polymerase chain reaction; rhGCPII, recombinant human glutamate carboxypeptidase II; SDS/PAGE, sodium dodecylsulfate polyacrylamide gel electrophoresis; TFA, trifluoroacetic acid; TLC, thin-layer chromatography.