Cyclin-Dependent Kinase 2 (CDK2) is located in the cytoplasm of any cell. CDK2 is a regulator of the cell cycle, which governs when a cell is able proceed from one stage into the next (Fig 1). CDK2 is present throughout the entire cell cycle at moderate levels, but it is believed that its role is during cytokinesis when the cell physically divides into two seperate cells. It is also known that cyclin A regulates the activity of the substrate binding site for ATP. When cyclin A is not bound the ATP site is accessible to both ATP and the proteins to be phosphorylated but CDK2 is unable to "grab" onto the proteins for transfer of the phosphate group. When cyclin A is complexed with CDK2 it is now able to "grab" and "hold" the protein substrate in place while the phosphate group is transfered from ATP to the protein. The result of this reaction is ADP + phosphorylated protein. Once phosphorylated this protein usually becomes activated by a conformational change and is able to act on its substrate.
CDK2 also contains a magnesium atom located in the ATP binding site. It acts identically to the magnesium atom located in the Adenosine Kinase ATP binding site. Meaning that magnesium bonds with the beta and gamma phosphate groups on the ATP substrate and also to CDK2 which helps it to form the binding site. There is only one slight difference in the way that magnesium binds in CDK2. Instead of just binding the beta and gamma, Mg binds the alpha, beta, and gamma phosphate groups. Note the distances between phosphate oxygens and the Mg atom. Finally to further stabilize the ATP and Mg in the binding site gln131 bonds with the 5' hydroxyl group of the ATP and asn132 bonds with the Mg. By doing this both the substrate and co-factor are "locked" into place with in the protein and won't "run away".