An Investigation of the Roles of Magnesium Ions in the Catalytic Cycle of the Protein Kinase CDK2: A Molecular Link between Allosteric Activation and Catalytic Activity.

Abstract

Phosphorylation activity of the Cyclin Dependent Kinase (CDK) family of protein kinases regulates the progression of the cell cycle. Improper regulation of CDK activity is associated with the development of many cancers and other proliferative disorders. In this work, we have investigated the catalytic mechanism of fully activated CDK2 and have determined that under normal physiological conditions maximum activity is dependent on the binding of two Mg2+ ions in the kinase active site. We have shown that these two Mg2+ ions function to recruit ATP, stabilize the enzyme transition state to accelerate phosphoryl transfer, and then can limit and even slow the rate of ADP release. These are the first results that demonstrate that CDK2 requires two Mg2+ ions, resolving some ambiguities of the protein kinase mechanism. The complex, dynamic, and multiple activating and rate-limiting roles of the two Mg2+ ions observed for CDK2 may also exist in other kinases and explain why some kinases are observed to be activated by Mg2+ while others are inhibited. We have investigated the molecular details of the structure, dynamics, and activity of CDK2 at each stage of the catalytic cycle using X-ray crystallography, molecular dynamics calculations, and enzyme kinetics. We demonstrated the significance of the precise properties of Mg2+ binding by showing how allosteric effectors regulate the activity of CDK2 by tuning affinity of the second Mg2+ site. This same regulatory mechanism is conserved in other families of protein kinases.