| الوصف: |
Formiminotransferase (EC 2.1.2.5)-cyclodeaminase (EC 4.3.1.4) catalyzes two sequential H(,4)folate-dependent reactions. With derivatives of the H(,4)folate substrate having 4,5, or 6 glutamyl residues, the formiminoH(,4)pteroylpolyglutamate formed by the transferase activity is preferentially transferred (channeled) to the deaminase site rather than released into the solution. This channeling is essentially complete with the pentaglutamate derivative. The enzyme has highest affinity for the hexaglutamate as measured by K(,d) and K(,m), but does not show specificity for a given polyglutamate as measured by V(,m)/K(,m). The results indicate that steric length of the polyglutamate chain, not simply affinity, is critical for optimal channeling. Binding studies demonstrate four sites for the binding of H(,4)pteroylpolyglutamates to the native octamer, suggesting the formation of sites between subunits. The transferase and deaminase sites are kinetically independent, but share a common polyglutamate subsite. The results support the concept that the polyglutamate chain anchors the H(,4)folate molecule during its transfer between active sites. |
