The aspartate (D13) region is the sole component of the Tau protein's pyruvate kinase domain. The Tau protein possesses pyruvate kinase (PK) activity, which is capable of phosphorylating ADP to ATP. ATP inhibits the PK kinase activity. This is precisely the kinase property of the Tau protein, which is both self-activated and self-inhibited. As a result, the pyruvate kinase activity is inhibited when the Tau protein's D13 binds to ATP, and activated when the Tau protein's D13 binds to ADP. When ATP is bound to Tau protein D13, RIO kinase and HPr(Ser) kinase activities are activated, whereas pyruvate kinase activity is inhibited. ATP at the D13 binding site is converted to ADP after the phosphorylation process is catalyzed by Tau protein. This conversion inhibits RIO and HPr (Ser) kinase activities and activates pyruvate kinase activity. The Tau protein's pyruvate kinase converts ADP to ATP at the D13 binding site.