This comparative analysis provides the first detailed quantitative characterization of the most commonly used mAbs against GCPII and can serve as a guideline for the scientific community to use them in a proper and efficient way.

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.

GCPII is present in human blood, and its concentration within a healthy population varies. Recombinant PGCP does not hydrolyze NAAG, suggesting that GCPII alone is responsible for the NAAG-hydrolyzing activity observed in human blood. The potential correlation between GCPII serum levels and the disease status of prostate cancer patients will be further investigated.

Activation and reaction energies for four model systems capturing the essential physicochemical features of the hydrolysis of the peptide bond have been calculated at various level of theory, including the presumably accurate CCSD(T) calculations. The models studied covered a part of the spectrum encountered in biological systems: the hydrolysis in the absence of metal ions (represented by formamide and Ala-Ala) and the hydrolysis in the presence of one and two zinc(II) ions, mimicking the active sites of mono- and dizinc metallopeptidases, respectively (by using thermolysin and glutamate carboxypeptidase II as the model catalytic systems and formamide as the model substrate). The results obtained using CCSD(T)/def2-TZVP and CCSD(T)/aug-cc-pVTZ calculations were used as the benchmark values to which the set of cheaper methods, such as (RI-)DFT, (RI-)MP2, and SCS-MP2, were referenced. It was shown that deviations of 3-5 kcal mol(-1) (translating to 2-3 orders in reaction constants) with respect to the reference CCSD(T) barriers are frequently encountered for many correlated methods and most of studied DFT functionals. It has been concluded that from the set of wave-function methods, both MP2 and SCS-MP2 methods can be recommended for smaller models (measured by the mean absolute deviation of the activation barriers over the four systems studied), whereas among the popular DFT functionals, B3LYP and especially M06-2X are likely to be reasonable choices for calculating the activation barriers of zinc metallopeptidases. Finally, with the model of glutamate carboxypeptidase II, issues related to the convergence of the calculated barriers with the size of the model system used as the representative of the enzyme active site were addressed. The intricacies related to system truncation are demonstrated, and suggest that the correlated wave-function methods may suffer from problems, such as intramolecular BSSE, which make their usage for the larger system questionable. Altogether, the presented data should contribute to efforts to understand enzymatic catalysis more deeply and to gain control of the accuracy and deficiencies of the available theoretical methods and computational approaches.

Glutamate carboxypeptidase II ( GCPII ), also known as prostate‐specific membrane antigen ( PSMA ) or folate hydrolase, is a metallopeptidase expressed predominantly in the human brain and prostate. GCPII expression is considerably increased in prostate carcinoma, and the enzyme also participates in glutamate excitotoxicity in the brain. Therefore, GCPII represents an important diagnostic marker of prostate cancer progression and a putative target for the treatment of both prostate cancer and neuronal disorders associated with glutamate excitotoxicity. For the development of novel therapeutics, mouse models are widely used. However, although mouse GCPII activity has been characterized, a detailed comparison of the enzymatic activity and tissue distribution of the mouse and human GCPII orthologs remains lacking. In this study, we prepared extracellular mouse GCPII and compared it with human GCPII . We found that mouse GCPII possesses lower catalytic efficiency but similar substrate specificity compared with the human protein. Using a panel of GCPII inhibitors, we discovered that inhibition constants are generally similar for mouse and human GCPII . Furthermore, we observed highest expression of GCPII protein in the mouse kidney, brain, and salivary glands. Importantly, we did not detect GCPII in the mouse prostate. Our data suggest that the differences in enzymatic activity and inhibition profile are rather small; therefore, mouse GCPII can approximate human GCPII in drug development and testing. On the other hand, significant differences in GCPII tissue expression must be taken into account when developing novel GCPII ‐based anticancer and therapeutic methods, including targeted anticancer drug delivery systems, and when using mice as a model organism.

Human diseases are often diagnosed by determining levels of relevant enzymes and treated by enzyme inhibitors. We describe an assay suitable for both ultrasensitive enzyme quantification and quantitative inhibitor screening with unpurified enzymes. In the DNA-linked Inhibitor ANtibody Assay (DIANA), the target enzyme is captured by an immobilized antibody, probed with a small-molecule inhibitor attached to a reporter DNA and detected by quantitative PCR. We validate the approach using the putative cancer markers prostate-specific membrane antigen and carbonic anhydrase IX. We show that DIANA has a linear range of up to six logs and it selectively detects zeptomoles of targets in complex biological samples. DIANA's wide dynamic range permits determination of target enzyme inhibition constants using a single inhibitor concentration. DIANA also enables quantitative screening of small-molecule enzyme inhibitors using microliters of human blood serum containing picograms of target enzyme. DIANA's performance characteristics make it a superior tool for disease detection and drug discovery.

We present here a structure-aided design of inhibitors targeting the active site as well as exosites of glutamate carboxypeptidase II (GCPII), a prostate cancer marker, preparing potent and selective inhibitors that are more than 1000-fold more active toward GCPII than its closest human homologue, glutamate carboxypeptidase III (GCPIII). Additionally, we demonstrate that the prepared inhibitor conjugate can be used for sensitive and selective imaging of GCPII in mammalian cells.

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