3.4.21.41	evolution	binding residues are conserved in the CUB1 domains of C1r, MASP-1/-3, and MASP-2, indicating that the interaction mechanism is conserved in initiating complexes of the lectin and classical pathways	732820	
3.4.21.41	metabolism	C1r and C1s pro-enzymes form a heterotetrameric structure that associates with the recognition molecule, C1q, in the C1 complex	732102	
3.4.21.41	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	
3.4.21.41	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 and C1s, EC 3.4.21.42. 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	
3.4.21.41	additional information	subsite profiling of human C1r using a phage display library with a fixed P1 arginine, specificity determinants, overview. Gln and Ile residues at P2 and P1', respectively, are important for cleavage of phage displayed substrates, the enzyme displays considerable specificity at every position apart from P4, P3', and P4'	732102	
3.4.21.41	additional information	the C1s/C1r/C1r/C1s tetramer forms a complex with C1q by interacting with the stems. C1r is a homologous multidomain protease containing an N-terminal CUB module, an EGF-like module, a second CUB module, two complement control modules CCP, and a serine protease domain SP. The three domains that constitute the catalytic fragment of C1r (CCP1-CCP2-SP) readily form head-to-tail dimers. The CUB1-EGF-CUB2 fragments of C1r also dimerize. Interaction analysis and structure-function relationship, formation of the C1 complex, molecular dynamics simulations and thermodynamics, detailed overview	732889	
3.4.21.41	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. C1r 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	
3.4.21.41	physiological function	C1r 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	732889	
3.4.21.41	physiological function	multiprotein complex C1 triggers the destruction of invading pathogens via lysis or by stimulation of innate and adaptive immune processes	755267	
3.4.21.41	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. C1s subsequently cleaves substrates C4 and C4b-bound C2, to form the C3 convertase (C4b2a), the downstream component of the reaction cascade	732820