Understanding a multifunctional biomolecule
Glutamate carboxypeptidase II (GCPII) is a zinc metalloenzyme attached to the cell membrane and extending into the extracellular space. Its catalytic activity yields the excitatory neurotransmitter glutamate and its inhibitors have been shown to prevent neurotoxicity in animal models. GCPII also goes by the name prostate-specific membrane antigen, or PSMA. It is expressed very highly in the prostate and its expression is upregulated about 10-fold in prostate cancer, making it a target for both imaging and therapy. GCPII is also associated with certain types of tumour vasculature. However, its normal role in areas outside the nervous system is poorly understood and further complicated by the existence of similar molecules. Scientists launched the EU-funded project (GCPII_SYSTEM) (Design and development of novel reagents, tools, and techniques targeting human glutamate carboxypeptidases II and III) to elucidate the roles of GCPII and its homologues/paralogues in normal and pathological states. The main thrust of the study was focused on identification of GCPII-specific ligands. The team produced 22 X-ray structures of complexes between GCPII and small-molecule inhibitors (i.e. ligands that bound it inhibiting its function). Work and findings are presented in five publications in peer-reviewed scientific journals. The knowledge gained should be instrumental in structure-assisted drug design. A comprehensive in vitro screening campaign of more than 50 000 compounds from small organic molecule libraries identified more 'hits' that were evaluated with structure-activity relationship studies. Finally, one million compounds from the virtual zinc library were screened and about 100 were tested in vitro. The best hits did in fact inhibit GCPII activity, demonstrating the ability of this methodology to screen many potential molecules and accurately identify the small percentage of relevance. In other work, the team identified a small engineered ligand-binding protein with very high affinity for GCPII in its native conformation that could be instrumental in targeted imaging of prostate cancer. Ongoing studies are evaluating GCPII and similar molecules in a variety of animal systems to determine their non-nervous system roles. GCPII_SYSTEM outcomes have shed important light on the role of GCPII and its analogues. Small and large molecules that bind the enzyme point the way to realisation of its full potential in targeted cancer therapy and imaging.
Keywords
Ligands, neurotoxicity, GCPII, prostate, membrane antigen, cancer
