Rational design of specific inhibitors of PSA: A molecular docking study of beta-lactam covalent inhibitors

Abstract

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Prostate Specific Antigen (PSA) is classically used as a biomarker of prostate cancer diagnosis and therapeutic response but its precise role in prostate carcinogenesis and metastasis remains largely unknown. A number of studies arguing in favor of active role of PSA in prostate cancer development have implicated PSA in release and activation of growth factors such Insulin-like growth factor 1 (IGF1) and Latent Transforming Growth Factor β (TGF- β). In contrast, several studies indicate that PSA activity might hinder tumor development and progression. In light of these contradictory findings, efficient inhibitors of PSA are needed for exploring the precise biological role of PSA activity in the tumor development and spread. Towards the goal of developing specific and potent inhibitors of PSA, we have explored molecular mechanism of β-lactam binding to PSA using MALDI-TOF mass spectrometry and GOLD docking methodology. To enable the molecular docking in the absence of any crystallographic data on PSA, homology model of PSA was developed in its active form with its Kallikrein loop in semi-open configuration. This 3D structural model of PSA was used to perform the docking of beta-lactam inhibitors using GOLD docking program in both non-covalent and covalent binding modes. The mass spectrometry experiments confirmed the covalent binding mode of β-lactam compounds. The docking studies elucidated the molecular details of early non-covalent Michaelis complex, covalent acyl-enzyme product and the nature of conformational changes required for the long term stability of acyl-enzyme complex. Additionally, the molecular basis for the effect of stereochemistry of lactam ring on the inhibition potency was elucidated through docking of β-lactam enantiomers. As a validation of these docking studies, four novel enantiomers were synthesized and evaluated for their inhibition potency using fluoregenic substrate based activity assays. Additionally, a number of β-lactam compounds from a previous study were docked and their GOLD scores and binding modes were analyzed in order to assess the general applicability of our docking findings. The close agreement of experimental data with docking results validates the use of our docking insights for design and development of better PSA inhibitors.

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