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

A 1.7 angstrom 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 (similar to 20 angstrom) 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 RMSDC alpha (4.6 angstrom). The fold of the active form of ScpB is nearly identical to ScpA (RMSDC alpha 0.2 angstrom), 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.