Figure 1.

(a) A schematic diagram of proposed binding of MBL to hexose sugars that have hydroxyl groups in the equatorial position and are thereby accommodated in the binding pocket that is stabilized by calcium ions. Also depicted is a model of MBL binding to complex ligands that require interactions of multiple binding pockets with ligand, which results in high affinity binding. (b) A model of the interactions of MBL with MASPs. Engagement of ligand by MBL activates MASP2, which then cleaves the C2C4 convertase and results in the cleavage of C3 and the generation of C3b. It has also been proposed that MASP1 can directly cleave C3; what is not known is whether this requires an MBL interaction with ligand. Our model proposes that this might be a prerequisite for this reaction to occur and that this in turn would activate the alternative complement pathway. Adapted from [1].