Biochemical characterization of the arginine-specific proteases of Porphyromonas gingivalis W50 suggests a common precursor

Abstract

Extracellular proteases of Porphyromonas gingivalis specific for arginyl peptide bonds are considered to be important virulence factors in periodontal disease. In order to determine the number, inter-relationship and kinetic properties of these proteases, extracellular enzymes with this peptide-bond specificity were purified and characterized from P. gingivalis W50. Three forms, which we denote RI, RI-A and RI-B, accounted for all of the activity in the supernatant. All three enzymes contain an α chain of ∼54 kDa with the same N-terminal amino acid sequence. RI is a heterodimer of non-covalently linked α and β chains which migrate to the same position on SDS/PAGE but which can be resolved by 8 M urea/PAGE. RI-A and RI-B are both monomeric, but the molecular mass of RI-B (70–80 kDa) is significantly increased due to post-translational modification with lipopolysaccharide. All forms show absolute specificity for peptide bonds with Arg in the P1 position and are also capable of hydrolysing N-terminal Arg and C-terminal Arg–Arg peptide bonds. Thus they show limited amino- and carboxy-peptidase activity. For the hydrolysis of Nα-benzoyl-l-Arg-p-nitroanilide, the pH optimum is 8.0 at 30 °C. The Vmax for all three enzymes is controlled by ionization of two residues with apparent pKas at 30 °C of 6.5±0.05 and 9.7±0.05, and ΔH values of ∼29 kJ/mol and ∼ 24 kJ/mol in the enzyme–substrate complex. By analogy with papain, the pKa of 6.5 could be ascribed to a Cys and the pKa of 9.7 to a His residue. E-64 [l-trans-epoxysuccinyl-leucylamide-4-(4-guanidino)butane] is a competitive inhibitor of RI, RI-A and RI-B. Based on physical properties and kinetic behaviour, RI-A appears to be analogous to gingipain from P. gingivalis HG66. However the α/β structure of RI differs significantly from that of the high-molecular-mass multimeric complex of gingipain containing four haemagglutinins described by others. Since the genes for RI and high-molecular-mass gingipain are identical, the data indicate that an alternative processing pathway is involved in the formation of RI from the initial precursor. Furthermore, the identical N-termini and enzymic properties of the catalytic component of RI, RI-A and RI-B suggest that the maturation pathway of the RI precursor may also give rise to RI-A and RI-B. The physiological functions of these isoforms and their role in the disease process may become more apparent through examination of their interactions with host proteins.