3.4.21.42	malfunction	hereditary angioedema is caused by C1 esterase inhibitor deficiency	708860	
3.4.21.42	metabolism	C1r and C1s pro-enzymes form a heterotetrameric structure that associates with the recognition molecule, C1q, in the C1 complex. Zymogen C1s is cleaved by the C1r protease resulting in C1s activation, kinetics of activation of C1s by C1r protease, overview	732102	
3.4.21.42	metabolism	complement is an important part of the immune system. It is initiated through three different pathways known as the classical, lectin, and alternative pathway. The multimolecular C1 complex of the classical pathway consists of a subcomponent, C1q, which binds to a tetramer comprising two C1r and two C1s proteases, EC 3.4.21.41 and EC 3.4.21.42, respectively	732889	
3.4.21.42	metabolism	hexameric complement C1q is a versatile recognition protein that senses a wide variety of immune and nonimmune ligands, including pathogens and altered self components, and triggers the classical complement pathway through activation of its associated proteases C1r, EC 3.4.21.41, and C1s. Residues LysB61 and LysC58 each play a key role in the interaction with C1s-C1r-C1r-C1s, with LysA59 being involved to a lesser degree	732824	
3.4.21.42	metabolism	In the classical complement pathway, immune complexes formed by IgM, IgG1, IgG2 and IgG3 antibodies result in the activation of the C1s protease that in turn cleaves C4 and then C4-bound-C2 yielding the proteolytic fragments C4b and C2a which associate to form a C3 convertase enzyme	731904	
3.4.21.42	additional information	synthesis of a series of fluorescence-quenched peptide substrates, comprising residues from the cleavage site in C1s and some peptides found among the phage-displayed peptides, very high concentrations of C1r are required to cleave such peptides	732102	
3.4.21.42	additional information	the C1s/C1r/C1r/C1s tetramer forms a complex with C1q by interacting with the stems. C1s is a homologous multidomain protease containing an N-terminal CUB module, an EGF-like module, a second CUB module, two complement control modules, and a serine protease domain. The CUB1-EGF-CUB2 fragments of C1s dimerize. Interaction analysis and structure-function relationship, formation of the C1 complex, molecular dynamics simulations and thermodynamics, detailed overview. Human C1s CUB1-EGF forms a head-to-tail homodimer in the absence of C1r	732889	
3.4.21.42	additional information	the large multicomponent assembly C1 complex is composed of a recognition subcomponent, C1q (460 kDa), and two serine protease subcomponents, C1r (90 kDa) and C1s (80 kDa) in a 1:2:2 ratio, with an overall molecular mass of about790 kDa. C1rs is a modular protease with two N-terminal complement C1r/C1s, Uegf and bone morphogenetic protein-1(CUB) domains, separated by an epidermal growth factor (EGF)-ike domain, followed by two complement control modules (CCP) and a C-terminal serine protease (SP) domain	732820	
3.4.21.42	physiological function	C1s is a zymogen, activation of C1 occurs when the C1q subcomponent binds to a pathogen via its globular heads resulting in autolytic activation of C1r followed, in turn, by C1r-mediated activation of C1s. Once activated C1s specifically cleaves its substrates C4 and C4b bound C2, to initiate the downstream complement cascade, a series of proteolytic reactions resulting in diverse biological activities aimed at providing a first line of defense against infection	732889	
3.4.21.42	physiological function	C1s is involved in the classical pathway of complement, whose activation is often associated with autoimmune disorders in which disease pathology is linked to the presence of an autoantibody. One such disorder is cold agglutinin disease, an autoimmune hemolytic anemia in which autoantibodies (cold agglutinins) bind to red blood cells at low temperatures. Anemia occurs as a result of autoantibody-mediated classical pathway activation on the surface of the erythrocyte, leading to the deposition of complement opsonins that drive extravascular hemolysis in the liver, overview. Classical pathway activity in cold agglutinin disease patients terminates prior to activation of the terminal cascade	731535	
3.4.21.42	physiological function	Cls is the second enzyme in the cascade of complement activation. This enzyme can be obtained as its inactive precursor, Cls, as the fully activated proteinase, Clg, or as a subunit of the first component of complement, Cl. In addition to Clg, CI contains two other proteins: Clq and a second proteinase, Clr. Activation of Cl by immune complexes, by freeze/thaw or by incubation with calcium plus lipopolysaccharide results first in conversion of inactive Cls to the proteinase Clr	749979	
3.4.21.42	physiological function	enzyme C1r is active with the substrate fragment consisting of complement control protein domains, CCP1 and CCP2, plus serine protease domain of wild-type and mutant Q462N, Q462G, I464A, and Q462N/I464A forms of C1s, C1s mutant Q462N/I464A gives very low activity as substrate for C1r	732102	
3.4.21.42	physiological function	multiprotein complex C1 triggers the destruction of invading pathogens via lysis or by stimulation of innate and adaptive immune processes	755267	
3.4.21.42	physiological function	the large multicomponent assembly C1 complex, binds to immune complexes, protein modulators (e.g., C-reactive protein), and polyanionic structures on pathogens to initiate complement activation. Binding to pathogens induces a conformational change that drives activation of the zymogen proteases in stepwise fashion: C1r first autoactivates, then activates C1s (2). C1s subsequently cleaves substrates C4 and C4b-bound C2, to form the C3 convertase (C4b2a), the downstream component of the reaction cascade	732820