Base kinetic model. A, the base kinetic mechanism for p38 competition with two substrates in a random order bi-substrate reaction. After reversibly binding ATP and a second substrate (ATF2 or MK2) in random order, the ternary complex undergoes an irreversible phosphorylation step to create phosphorylated product, ADP and recover the active p38. Kinetic parameters are listed in Table 2. B, kinetic simulation of the base kinetic mechanism 0.5 nM p38, 50 uM ATP, 10 nM MK2 and 100 nM ATF2. Results are virtually indistinguishable from MK2 only condition for phospho-MK2 and from ATF2 only condition for phospho-ATF2.