Adenylate Kinase from Maize Leaves: True Substrates, Inhibition by P 1 , P 5-di(adenosine-5′) pentaphosphate and Kinetic Mechanism

Purified maize leaf adenylate kinase (AK) was shown to use one molecule each of free ADP and Mg-ADP as well as free AM P and Mg-ATP as substrates in the forward and reverse reaction, respectively. This was deduced from substrate kinetic studies which were carried out under conditions of strictly defined concentrations of free and Mg-complexed adenylate species and under controlled free magnesium levels. Apparent K m values of the substrates of AK were 3 and 6 μM for ADP and Mg-ADP, respectively (forward reaction), and 69 and 25 μM for free AMP and Mg-ATP, respectively (reverse reaction). The enzyme was competitively inhibited by P 1 ,P 5 -di(adenosine-5′)pentaphosphate (Ap 5 A), a bisubstrate analog of AK reaction, with apparent K i values in the range of 11 -80 nM , depending on variable substrate. Substrate kinetic studies and inhibition patterns with Ap 5 A suggested a sequential random kinetic mechanism in both directions of the reaction. These properties of a higher plant AK are similar or analogous to those previously established for the enzyme from yeast and non-plant tissues.