The 1.7 Å crystal structure of the C5a peptidase from Streptococcus agalactiae (ScpB) reveals an active site competent for catalysis

A 1.7 Å structure is presented for an active form of the virulence factor ScpB, the C5a peptidase from Streptococcus agalactiae. The previously reported structure of the ScpB active site mutant exhibited a large separation (~20 Å) between the catalytic His and Ser residues. Significant differences are observed in the catalytic domain between the current and mutant ScpB structures resulting with a high RMSD_Cα (4.6 Å). The fold of the active form of ScpB is nearly identical to ScpA (RMSD_Cα 0.2 Å), the C5a-peptidase from Streptococcus pyogenes. Both ScpA and ScpB have comparable activity against human C5a, indicating neither enzyme require host proteins for C5a-ase activity. These studies are a first step in resolving reported differences in the specificities of these enzymes.