Catalytic mechanism of type C sialidase from Streptococcus pneumoniae: from covalent intermediate to final product.

Streptococcus pneumoniae is a Gram-positive human pathogenic bacterium, which is the main cause of pneumonia and meningitis in children and the elderly. Three sialidases (or neuraminidases) encoded from Streptococcus pneumoniae could catalyze the cleavage of sialic acid linkages. This mechanism is directly connected with infection, apoptosis, and signaling, and usually considered to be one of the critical virulence factors. Type C neuraminidase (NanC) is unique because its primary product of Neu5Ac2en is considered to be an inhibitor to the other two sialidases. Experimentally, there are two different pathways for the formation mechanism of Neu5Ac2en catalyzed by NanC. In this work, a combined quantum mechanical and molecular mechanical approach was employed in all calculations. Starting from the covalent sialylated intermediate, we first examined the reaction to Neu5Ac2en and found the reaction prefers a direct proton abstraction mechanism rather than the water mediated proton abstraction mechanism. Free energy profiles can confirm that Neu5Ac2en is the major product of NanC. Functional roles of some important residues were also investigated, e.g., D315 acts as the proton acceptor during the formation of Neu5Ac2en, while the general base for the hydrolytic reaction to Neu5Ac. This study can facilitate the understanding of the catalytic mechanism of NanC and has the potential to aid in future inhibitor design studies.