Clr and Cls subcomponents of human complement: two serine proteinases lacking the 'histidine-loop' disulphide bridge.

The N-terminal amino acid sequence of human C1s b chain has been extended to 52 residues. The histidine residue involved in the charge-relay system is located at position 38, whereas the histidine-loop disulphide bridge is missing. So far, human complement sub-components C1r and C1s are the only known mammalian serine proteinases lacking this disulphide bridge.