To accomplish the purification, rabbit brain tissue was homogenized and centrifuged to remove insoluble material. Next, the soluble preparation was loaded on top of an ATP-Sepharose column. This is an affinity column in which ATP is covalently linked to a polysaccharide bead. The sample is loaded on top of the column, washed with a low-salt buffer, followed by a wash with a high salt buffer. What is the rationale for this choice of column and binding-elution procedure?