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'''Cytokinin deshydrogenase'''([http://en.wikipedia.org/wiki/Cytokinin_dehydrogenase Wikipedia]), also called CKX, is an enzyme which '''degrades cytokinin''' ([http://en.wikipedia.org/wiki/Cytokinin Wikipedia]) '''hormones''' in plants and which is encoded by the gene '''ZmCKX1'''. The one is located in '''Zea Maize''' on the chromosome 3 (NCBI Gene ID: 542585) and belongs to a multigene family called '''V'''anillyl-'''A'''lcohol '''O'''xidase (VAO) flavoprotein family. The gene is more particularly expressed in the kernel of maize, mainly in the embryo since it may protect the embryo from too much cytokinin and permit the correct development. <ref name="Frebortova">PMID: 19912568 </ref>

Cytokine deshydrogenase are '''extracellular''' and '''monomeric''' proteins with a molecular weight of 63kDa.<ref name="Kopečnýa">PMID: 20478354 </ref><ref name="Kopečnýa2">PMID: 18571199 </ref> For protein production purposes, ZmCKO1 precursor protein was truncated by deletion of 18 N-terminal amino acids to produce the expected mature enzyme. <ref name="Kopečnýa3">PMID: 15927342 </ref>

The Enzyme Classification number of CKX is EC 1.5.99.12 and this indicates that the enzyme is an '''oxydo reductase''' which acts on the CH-NH group of the donor. Consequently the reaction of CKX with its cytokinin substrate is a '''transfer of two electrons''' from the cytokinin to an '''electron acceptor''' which is in the case of CKX the '''F'''lavine '''A'''denine '''N'''ucleotide (FAD) cofactor. <ref name="Kopečnýa">PMID: 20478354 </ref>

In some papers the denomination '''CKO''' can be found for cytokinin deshydrogenase. Indeed in the vanillyl-alcohol oxidase flavoprotein family most of the enzyme use '''molecular oxygen''' as electron acceptor to '''reoxidize''' the FAD cofactor. That’s why the enzyme was first called '''Cytokinin Oxidase''' (CKO). CKX is an exception in the family since the enzyme uses '''other compounds''' ,such as quinone, for electron acceptor and poorly reacts with oxygen. Consequently the enzyme is now called '''CKX''' and enters the category of '''dehydrogenase'''. <ref name="Frebortova">PMID: 19912568 </ref> <ref name="Malitoa">PMID: 15321719 </ref>

== '''Structure & Function''' ==

=== '''Motifs & Domains''' ===

CKX has a '''two main domains''' structure, with an '''FAD-binding domain''' (residues 33–244 and 492–534) and a '''substrate-binding domain''' (residues 245–491). <ref name="Kopečnýa">PMID: 20478354 </ref>

The active site contains very '''highly conserved residues''' except for one residue which is located at the entrance of the active site (where a few amino acids such as Glu, Asp, Ser, Gly and other aliphatic amino acids can be found). <ref name="Kopečnýa">PMID: 20478354 </ref>

Three residues, Asp169, Glu288 and Glu381 may have an importance in '''cytokinin binding''' and '''enzyme action''' <ref name="Kopečnýa3">PMID: 15927342 </ref>

The presence of a conserved �GHS� at position 104-106 domain has been shown in the '''FAD binding domain''' which may have been highlighted in CKX structure like GlWeVPHPWLNL motif around position 390 and PGQxIF signature at the C-terminal ends. These sequences are specifically found in CKX family enzymes and their '''very high conservation inside the family''' shows that they have an important role for the enzyme functioning, such as in this case '''substrate recognition and electron transport'''. <ref name="Schmülling">PMID: 12721786 </ref>

== '''References''' ==

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