Cleavage of the C3-alpha chain at peptide bond 77 results in production of C3a and C3b fragments. The C3b fragment is a glycoprotein composed of the modified C3-alpha chain and the intact C3-beta chain. Nascent C3b has the transient ability to form a covalent ester bond with a variety of target surfaces. Once bound to target surfaces C3b becomes an essential subunit of both the classical and alternative pathway C5 cleaving enzymes.