What exactly determines the ability of a protein kinase to actually bind and catalyze a specific location on some enzyme on some substrate molecule? Well, for a catalyzation reaction to actually take place, logically speaking, that substrate molecule has to be able to fit into the active side of that protein kinase. And not only that, the substrate molecule has to have a high enough affinity so that it remains long enough in the active side for that phosphorylation reaction to actually take place in the first place. And so what that means is to have a high affinity between the substrate molecule and the protein kinase. The sequence of amino acids around the side, around the amino acid, which is about to be phosphorylated, has to be correct because if the sequence of amino acids on the substrate molecule is not correct, that active side will not be able to bind onto that substrate molecule. So we conclude that the specificity of protein kinases depends on the amino acid sequence that directly surrounds that target residue that is about to be phosphorylated.