Protein modelling using a chimera reference protein derived from exons

Bovine pancreatic beta-trypsin (PDB ID-code: 1TPO) which is registered in the Brookhaven Protein Data Bank (PDB) consists of four exons. The results of homology searches for each exon in the PDB showed that homologous proteins were tonin (PDB ID-code: 1TON), rat mast cell protease (PDB ID-code: 3RP2__A), kallikrein A (PDB ID-code: 2PKA__B) and kallikrein A (2PKA__B) respectively. Thus, for the three-dimensional structure prediction of 1TPO, a chimera protein was constructed from the three proteins mentioned above and the 3-D structure prediction was performed using this chimera reference protein. The modelled structure of 1TPO was energetically optimized by molecular mechanics and molecular dynamics simulation and was compared with its X-ray crystal structure registered in the PDB. The root mean square deviations (r.m.s.d.) of main chain atoms and the neighbouring active site (5 A sphere from His57, Asp102 and Ser195) between the modelled structure and the X-ray structure were 1.66 and 0.94 A respectively. Porcine pancreatic elastase (PDB ID-code: 3EST) which is registered in the PDB was used as the reference protein and the modelled structure from 3EST was also compared with the X-ray data. The r.m.s.d. of main chain atoms and that of the active site were 2.14 and 1.18 A respectively. These results clearly support the propriety of this method using the chimera reference protein.