C1r enzyme is the activated form of C1r proenzyme. C1r is a subunit of the C1 complex which is the first complement component in the classical pathway of complement. C1r proenzyme is an inactive zymogen until C1 is activated. C1r is activated when C1 binds to and is activated by antibodies bound to antigens (immune complexes) yielding C1r enzyme, the first protease that initiates the cascade. C1 is a non-covalent calcium-dependent complex of one C1q, two C1r and two C1s molecules. Each C1q binds through two or more of its six arms to the Fc domains of IgG or IgM. The binding of multiple arms to immune complexes causes the two C1r proteins in the complex (protease zymogens) to activate producing two proteases that cleave and activate the two C1s protease zymogens in the complex. The activation of C1r results from cleavage of C1r into two fragments of 57,000 and 35,000 daltons. Activation of the bound C1s molecule is the only known function of C1r enzyme. Activated C1s cleaves complement component C4 releasing C4a and initiating covalent attachment of C4b to the activating surface. Activated C1s also cleaves C2 and the larger fragment of C2 binds to the surface-attached C4b forming C4b,C2a the C3/C5 convertase of the classical pathway.
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