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Function
Mechanism
In CKX, the substrate displays a “plug-into-socket” binding mode that seals the catalytic site [3]. It interacts with the enzyme with polarising Hbond that stabilizes the tridimensional structure. The Asp169 is Hbonded to the N10 atom of the substrate (various studies have shown that it is involved in the recognition of the substrate) and to the Glu288. The substrate is recognized also by Glu381 which Hbonded to the N7 atom. The others Natoms are Hbonded with the solvent except for N1 atom. The FAD is Hbonded with the His105.
These interactions allow the positioning the C11 atom in close contact (3.0 A°) with the flavin N5 atom [3]. This carbon will be the site of oxidative attack [3]. This information suggests there is a direct transfer of the electrons and the proton to the flavin with a shortlived radical intermediate or the direct transfer of a hydride anion [3]. The carbocation is stabilized by the N10–Asp 169 H-bond interaction and the resonance effect in the oxidised imine product.
The reduced enzymeproduct complex is reoxidised by utilizing an organic electron acceptor, the hydroxamic acid 2,4 dihydroxy 7 methoxy 1,4benzoxazin3on (DIMBOA) or more precisely, the free radicals generated by LACCASE and PEROXIDASE with the DIMBOA. The reoxidation of FADH2 allows the product release.
Regulation
The reductive half reaction is fast, therefore, the reoxidation of FAD seems to be the rate limiting step in the catalytic cycle [8]. In absence of organic electron acceptor, oxygen can be used but it is a poor electron acceptor, so the regeneration of free radicals it is an important step. The coupling of CKX to LACCASE or PEROXIDASE involves the regulation of these enzyme implies the regulates of the [CKX] activity.
Different invivo competitive inhibitors of the [CKX] have been identified such as diphenylureas, which are known as cytokinin agonists [8] or Ureatype cytokinins [1].
Structural highlights
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