title: Kinetic basis for activation of CDK2/cyclin A by phosphorylation.
abstract: The activation of most protein kinases requires phosphorylation at a conserved site within a structurally defined segment termed the activation loop. A classic example is the regulation of the cell cycle control enzyme, CDK2/cyclin A, in which catalytic activation depends on phosphorylation at Thr(160) in CDK2. The structural consequences of phosphorylation have been revealed by x-ray crystallographic studies on CDK2/cyclin A and include changes in conformation, mainly of the activation loop. Here, we describe the kinetic basis for activation by phosphorylation in CDK2/cyclin A. Phosphorylation results in a 100,000-fold increase in catalytic efficiency and an approximate 1,000-fold increase in the overall turnover rate. The effects of phosphorylation on the individual steps in the catalytic reaction pathway were determined using solvent viscosometric techniques. It was found that the increase in catalytic power arises mainly from a 3,000-fold increase in the rate of the phosphoryl group transfer step with a more moderate increase in substrate binding affinity. In contrast, the rate of phosphoryl group transfer in the ATPase pathway was unaffected by phosphorylation, demonstrating that phosphorylation at Thr(160) does not serve to stabilize ATP in the ATPase reaction. Thus, we hypothesize that the role of phosphorylation in the kinase reaction may be to specifically stabilize the peptide phosphoacceptor group.
authors.affiliation: Department of Molecular, Cellular and Developmental Biology, University of California, Santa Barbara, California 93106, USA.
authors.forename: J C
authors.initials: JC
authors.lastname: Hagopian
authors.forename: M P
authors.initials: MP
authors.lastname: Kirtley
authors.forename: L M
authors.initials: LM
authors.lastname: Stevenson
authors.forename: R M
authors.initials: RM
authors.lastname: Gergis
authors.forename: A A
authors.initials: AA
authors.lastname: Russo
authors.forename: N P
authors.initials: NP
authors.lastname: Pavletich
authors.forename: S M
authors.initials: SM
authors.lastname: Parsons
authors.forename: J
authors.initials: J
authors.lastname: Lew
mesh_terms: D000251:Adenosine Triphosphatases / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000378:metabolism; D042846:CDC2-CDC28 Kinases; D002852:Chromatography, Ion Exchange; D019925:Cyclin A / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism*; D051357:Cyclin-Dependent Kinase 2; D018844:Cyclin-Dependent Kinases / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism*; D004789:Enzyme Activation; D004795:Enzyme Stability; D006801:Humans; D007700:Kinetics; D010766:Phosphorylation; D017346:Protein Serine-Threonine Kinases / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism*; D011993:Recombinant Fusion Proteins; D012997:Solvents; D013816:Thermodynamics; D013912:Threonine / Q000378:metabolism; D014783:Viscosity
chemical_list: D019925:Cyclin A; D011993:Recombinant Fusion Proteins; D012997:Solvents; D013912:Threonine; D000255:Adenosine Triphosphate; D017346:Protein Serine-Threonine Kinases; D042846:CDC2-CDC28 Kinases; C495894:CDK2 protein, human; D051357:Cyclin-Dependent Kinase 2; D018844:Cyclin-Dependent Kinases; D000251:Adenosine Triphosphatases