Figure 6.

Possible mechanisms of mannose-binding lectin (MBL)-associated serine protease (MASP)-3 activation. The boxed part depicts activation and inactivation steps involving only lectin pathway proteases. Three forms of MASP-1 (zM1 for zymogen MASP-1, aM1 for active MASP-1, and iM1 for serpin-inhibited MASP-1), three forms of MASP-2 (zM2 for zymogen MASP-2, aM2 for active MASP-2, and iM2 for serpin-inhibited MASP-2), and two forms of MASP-3 (zM3 for zymogen MASP-3 and aM3 for active MASP-3) are shown as separate variants. Red arrows pointing from the enzyme to the substrate represent previously established activation reactions involving MASP-1 and MASP-2. Pink arrows (pointing from the enzyme to the substrate) represent possible activation reactions producing active MASP-3. Cleavage of zymogen MASP-3 by zymogen MASP-1 is highlighted by thick pink arrow (again pointing from the enzyme to the substrate). Inhibition by serpins is indicated by blue T-shaped symbols. Black arrows simply indicate conversion.