Fig 2. Scheme outlining the pathway of activation of Pgn by rSK-M1GAS and stimulation by Fgn.
The scheme follows a trigger and bullet mechanism where an initial activator complex of SK•Pgn (BG) is replaced by SK•Pm (BE) as Pm (E) is generated due to the higher affinity binding of E to SK. Fgn (F) associates weakly with Pgn (G), while formation of active Michaelis complexes, GFBG and GFBE have improved dissociation constants. An improved rate of Pgn activation is achieved by GFBE relative to GFBG due to lower KM, while the kcat for formation of Pm is unchanged. Derivation of the constants shown is detailed in Table 2.