TI - Autophosphorylation kinetics of protein kinases . AB - Protein kinases play a central role in cellular signal transduction , by transmitting biochemical information between activated membrane-bound receptors and physiological target proteins . In addition to phosphorylating other proteins , almost all protein kinases catalyse autophosphorylation reactions ( i.e. reactions in which the kinase serves as its own substrate ) . The autophosphorylation reactions can be intramolecular or intermolecular . In the present study , a detailed kinetic analysis of the intermolecular autophosphorylation reaction is presented . On the basis of the kinetic equations , a new procedure is developed to evaluate the kinetic parameters of the autophosphorylation reaction . This method was used to analyse the intermolecular autophosphorylation of an S6/H4 kinase from human placenta . At a fixed ATP concentration of 0.125 mM , the apparent catalytic-centre activity ( turnover number ; k ( cat ) ) and apparent Michaelis-Menten constant ( K (m) ) for the autophosphorylation reaction were determined to be 0.91 min ( -1 ) and 0.86 microM respectively .