Editing Cyclin-dependent kinase complex (section)

=== Activation Loop ===
The [[activation loop]], also referred to as the T-loop, is the region of CDK (between the DFG and APE motifs in many CDK)<ref name=":1" /> that is enzymatically active when CDK is bound to its function-specific partner. In CDK-cyclin complexes, this activation region is composed of a conserved αL-12 Helix and contains a key phosphorylatable residue (usually [[Amino acid|Threonine]] for CDK-cyclin partners, but also includes Serine and Tyrosine) that mediates the enzymatic activity of the CDK. It is at this essential residue (T160 in CDK2 complexes, T177 in CDK6 complexes) that enzymatic ATP-phosphorylation of CDK-cyclin complexes by CAK (cyclin activating kinase, referring to the CDK7-Cyclin H complex in human cells) takes place. After the hydrolysis of ATP to phosphorylate at this site, these complexes are able to complete their intended function, the phosphorylation of cellular targets. It is important to note that in CDK 1, 2 and 6, the T-loop and a separate C-terminal region are the major sites of cyclin binding in the CDK, and which cyclins are bound to each of these CDK is mediated by the particular sequence of the activation site T-loop. These cyclin binding sites are the regions of highest variability in CDKs despite relatively high sequence homology surrounding the αL-12 Helix motif of this structural component.<ref name=":1" />