This group contains class II terpene cyclases, protein prenyltransferases beta subunit, two broadly specific proteinase inhibitors alpha2-macroglobulin (alpha (2)-M) and pregnancy zone protein (PZP) and, the C3 C4 and C5 components of vertebrate complement. Class II terpene cyclases include squalene cyclase (SQCY) and 2,3-oxidosqualene cyclase (OSQCY), these integral membrane proteins catalyze a cationic cyclization cascade converting linear triterpenes to fused ring compounds. The protein prenyltransferases include protein farnesyltransferase (FTase) and geranylgeranyltransferase types I and II (GGTase-I and GGTase-II) which catalyze the carboxyl-terminal lipidation of Ras, Rab, and several other cellular signal transduction proteins, facilitating membrane associations and specific protein-protein interactions. Alpha (2)-M is a major carrier protein in serum and involved in the immobilization and entrapment of proteases. PZP is a pregnancy associated protein. Alpha (2)-M and PZP are known to bind to and, may modulate, the activity of placental protein-14 in T-cell growth and cytokine production thereby protecting the allogeneic fetus from attack by the maternal immune system.
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