Question

Members of the Mitogen-Activated Protein Kinase (MAPK) protein family are essential for many cellular responses to the extracellular environment. A MAPK protein becomes active after it becomes phosphorylated by a second kinase. These MAPKs contain a “ docking site” distant from the enzyme's active site that binds to conserved 10–15 amino acid “ docking motifs” found in a variety of proteins (see the Figure below). Docking motifs are found in proteins that bind MAPKs (scaffold proteins), proteins that are substrates of MAPKs, proteins that activate MAPKs by phosphorylating them (MAPK kinases), and proteins that inactivate MAPKs by dephosphorylating them (phosphatases).
The kinetic properties of a particular MAPK were measured with a small peptide substrate that contains only the preferred phosphorylation site. The Km is 300 nM and the kcat is 20/sec. (You may need to revise what these two constants refer to in terms of enzyme kinetics).
What do you expect the kcat for the natural protein substrate to be compared to that for the peptide substrate?