Glutamate carboxypeptidase II (GCPII), also known as N-acetyl-L-aspartyl-L-glutamate peptidase I (NAALADase I), NAAG peptidase, or prostate-specific membrane antigen (PSMA) is an enzyme that in humans is encoded by the FOLH1 (folate hydrolase 1) gene. Human GCPII contains 750 amino acids and weighs approximately 84 kDa.
GCPII is a zinc metalloenzyme that resides in membranes. Most of the enzyme resides in the extracellular space. GCPII is a class II membrane glycoprotein. It catalyzes the hydrolysis of N-acetylaspartylglutamate (NAAG) to glutamate and N-acetylaspartate (NAA) according to the reaction scheme to the right.
Neuroscientists primarily use the term NAALADase in their studies, while those studying folate metabolism use folate hydrolase, and those studying prostate cancer or oncology, PSMA. All refer to the same protein glutamate carboxypeptidase II.
Discovery
GCPII is mainly expressed in four tissues of the body, including prostate epithelium, the proximal tubules of the kidney, the jejunal brush border of the small intestine and ganglia of the nervous system.
Indeed, the initial cloning of the cDNA encoding the gene expressing PSMA was accomplished with RNA from a prostate tumor cell line, LNCaP. PSMA shares homology with the transferrin receptor and undergoes endocytosis but the ligand for inducing internalization has not been identified. It was found that PSMA was the same as the membrane protein in the small intestine responsible for removal of gamma-linked glutamates from polygammaglutamate folate. This enables the freeing of folic acid, which then can be transported into the body for use as a vitamin. This resulted in the cloned genomic designation of PSMA as FOLH1 for folate hydrolase.
PSMA(FOLH1)+ folate polygammaglutamate(n 1-7)---> PSMA (FOLH1) + folate(poly)gammaglutamate(n-1) + glutamate continuing until releasing folate.
Structure
The three domains of the extracellular portion of GCPII—the protease, apical and C-terminal domains—collaborate in substrate recognition. The protease domain is a central seven-stranded mixed β-sheet. The β-sheet is flanked by 10 α-helices. The apical domain is located between the first and the second strands of the central β-sheet of the protease domain. The C-terminal domain is an Up-Down-Up-Down four-helix bundle. The apical, protease and C-terminal domains create a pocket that facilitates substrate binding.
The central pocket is approximately 2 nanometers in depth and opens from the extracellular space to the active site. This active site contains two zinc ions. During inhibition, each acts as a ligand to an oxygen in 2-PMPA or phosphate. There is also one calcium ion coordinated in GCPII, far from the active site. It has been proposed that calcium holds together the protease and apical domains. In addition, human GCPII has ten sites of potential glycosylation, and many of these sites (including some far from the catalytic domain) affect the ability of GCPII to hydrolyze NAAG.
The FOLH1 gene has multiple potential start sites and splice forms, giving rise to differences in membrane protein structure, localization, and carboxypeptidase activity based on the parent tissue.
Enzyme kinetics
The hydrolysis of NAAG by GCPII obeys Michaelis–Menten kinetics. Hlouchková et al. (2007) determined the Michaelis constant (Km) for NAAG to be 1.2*10−6 ± 0.5*10−6 M and the turnover number (kcat) to be 1.1 ± 0.2 s−1.
Role in cancer
Human PSMA is highly expressed in the prostate, roughly a hundred times greater than in most other tissues. In some prostate cancers, PSMA is the second-most upregulated gene product, with an 8- to 12-fold increase over levels in noncancerous prostate cells. Because of this high expression, PSMA is being developed as potential biomarker for therapy and imaging of some cancers. In human prostate cancer, the higher expressing tumors are associated with quicker time to progression and a greater percentage of patients suffering relapse. In vitro studies using prostate and breast cancer cell lines with decreased PSMA levels showed a significant decrease in the proliferation, migration, invasion, adhesion and survival of the cells.
Imaging
PSMA is the target of several nuclear medicine imaging agents for prostate cancer. PSMA expression can be imaged with gallium-68 PSMA or fluorine-18 PSMA for positron emission tomography. This uses a radiolabelled small molecule that binds with high affinity to the extra-cellular domain of the PSMA receptor. Previously, an antibody targeting the intracellular domain (indium-111 capromabpentide, marketed as Prostascint) was used, although detection rate was low.