Structural Basis for CDK Activation

Binding of the corresponding cyclin and activating phosphorylation are required for full activation of the CDKs. Without the cyclin, the CDKs are inactive the CDK-cyclin complex possesses a basal protein kinase activity that is considerably increased by phosphorylation at Thr in position 160 (or equivalent position). The structural changes 

It is assumed that the activity increase is mainly due to better accessibility of the binding site for the protein substrate. In the unphosphorylated form, the T loop blocks access to the substrate binding site whereas in the phosphorylated form, this site is exposed. 

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