That is arginine and lysine. Chymotrypsin - cleaves on the carbonyl side of aromatic residues. That is tyrosine, phenylalanine and tryptophan. Thrombin DNA polymerase. This enzyme is capable of proofreading itself.
Enzymes are the sites of regulation for biochemical reactions pathways.
Varying the levels of reactants as a means of controlling a biochemical pathway is not always possible in addition to not being a very responsive means of control. An example of a control mechanism that is observed is feedback inhibition of the first enzyme in a biosynthetic pathway by a product of that pathway. When the concentration of the product increases it binds to an enzyme early in the pathway leading to the production of that product and inhibits its catalytic activity. This shuts down the production of that product and the intermediates leading up its production. If the binding is reversible, and the site on the enzyme to which it binds is different than the catalytic site, then the type of regulation is called allosteric regulation.