How Dextran Sulfate Affects C1-inhibitor Activity: A Model for Polysaccharide Potentiation.
C1-inhibitor is a key inhibitor of the complement and contact activation systems, and mutations in the protein can cause hereditary angioedema. Through an unknown mechanism, polysaccharides can increase C1-inhibitor activity against some of its target proteases. Here we present the crystal structures of the serine protease inhibitor (serpin) domain of active C1-inhibitor by itself and in complex with dextran sulfate. Unlike previously described interactions of serpins with polysaccharides, the structures and isothermal titration calorimetry experiments together reveal that dextran sulfate binds to C1-inhibitor's F1 helix with low affinity and does not invoke an allosteric change. Furthermore, one dextran sulfate molecule can bind multiple C1-inhibitor molecules. We propose that in a C1-inhibitor/protease/polysaccharide ternary complex, negatively charged polysaccharides link C1-inhibitor's positively charged F1 helix to positively charged autolysis loops of proteases. The proposed mechanism elegantly explains previous experiments showing that polysaccharide potentiation is increased against proteases with a greater positive charge in their autolysis loop.
Department of Biophysical Structural Chemistry, Gorlaeus Laboratories, Leiden University, Einsteinweg 55, 2333 CC Leiden, the Netherlands.
Madelon
M
Dijk
Pharming Group NV, Darwinweg 24, 2333 CR Leiden, the Netherlands.
Jolande
J
Holkers
Department of Biophysical Structural Chemistry, Gorlaeus Laboratories, Leiden University, Einsteinweg 55, 2333 CC Leiden, the Netherlands.
Patrick
P
Voskamp
Pharming Group NV, Darwinweg 24, 2333 CR Leiden, the Netherlands.
Bruno M
BM
Giannetti
Department of Biophysical Structural Chemistry, Gorlaeus Laboratories, Leiden University, Einsteinweg 55, 2333 CC Leiden, the Netherlands.
Willem-Jan
WJ
Waterreus
Pharming Group NV, Darwinweg 24, 2333 CR Leiden, the Netherlands. Electronic address: h.vanveen@pharming.com.
Harrie A
HA
van Veen
Department of Biophysical Structural Chemistry, Gorlaeus Laboratories, Leiden University, Einsteinweg 55, 2333 CC Leiden, the Netherlands. Electronic address: raj@chem.leidenuniv.nl.
Navraj S
NS
Pannu
D001665:Binding Sites; D002152:Calorimetry, Differential Scanning; D003174:Complement C1 Inactivator Proteins / Q000737:chemistry* / Q000378:metabolism*; D050718:Complement C1 Inhibitor Protein; D018360:Crystallography, X-Ray; D016264:Dextran Sulfate / Q000378:metabolism*; D006801:Humans; D008958:Models, Molecular; D011485:Protein Binding; D017433:Protein Structure, Secondary
D003174:Complement C1 Inactivator Proteins; D050718:Complement C1 Inhibitor Protein; C469952:SERPING1 protein, human; D016264:Dextran Sulfate
C1-inhibitor; complement system; contact system; polysaccharide potentiation; serine protease inhibitor