Title : A structural model for the prostate disease marker, human
prostate-specific antigen
Abstract :
- Prostate-specific antigen ( PSA ) provides an excellent serum marker for prostate cancer, the most frequent form of cancer in American males
- PSA is a 237-residue protease based on sequence homology to kallikrein-like enzymes
- To predict the 3-dimensional structure of PSA , homology modeling studies were performed based on sequence and structural alignments with tonin, pancreatic kallikrein , chymotrypsin , and trypsin
- The structurally conserved regions of the 4 reference X-ray proteins provided the core structure of PSA , whereas the loop structures were modeled on the loops of tonin and kallikrein
- The unique " kallikrein loop" insert, between Ser 95b and Pro 95k of kallikrein , was constructed using molecular mechanics, dynamics, and electrostatics calculations
- In the resulting PSA structure, the catalytic triad, involving residues His 57, Asp 102, and Ser 195 , and hydrophobic and electrostatic interactions typical of serine proteases were extremely well conserved
- Similarly, the 5-disulfide bonds of kallikrein were also conserved in PSA
- These results, together with the fact that no major steric clashes arose during the modeling process, provide strong evidence for the validity of the PSA model
- Calculation of the electrostatic potential contours of kallikrein and PSA was carried out using the finite difference Poisson-Boltzmann method
- The calculations revealed matching areas of negative potential near the catalytic triad, but differences in the positive potential surrounding the active site
- The PSA glycosylation site, Asn 61 , is fully accessible to the solvent and is enclosed in a positive region of the isopotential map
- The bottom of the substrate specificity pocket, residue S1, is a serine ( Ser 189 ) as in chymotrypsin , rather than aspartate ( Asp 189 ) as in tonin, kallikrein , and trypsin
- This fact, plus other features of the S1 binding-pocket region , suggest that PSA would prefer substrates with hydrophobic residues at the P1 position
- The location of a potential zinc ion binding site involving the side chain of histidines 91, 101, and 233 is also suggested
- This PSA model should facilitate the understanding and prediction of structural and functional properties of this important cancer marker