The architecture of PK is evolutionally highly conserved and is organized as a homotetramer with four distinct domains in each subunit. The activity of the enzyme is a combination of domain and subunits rotations coupled to active site geometry. Residues, located in the domain interfaces, play a crucial role in function and communication between the subunits of the PK. Much progress has been made in the understanding of the structure and property for RPK, which is expressed in erythrocytes and liver. Less is known about the two other iso-forms, PKM1 and PKM2, which are expressed in muscle, kidney and lung, and are the products of alternative splicing of the same mRNA.

Cations, such as H⁺, K⁺, Mg²⁺ and/or Mn²⁺, modulate the activity of PKs. Mono- or biphosphorylated sugars as well as PEP activate the enzyme and alter its properties. Phenylalanine is known as a physiological inhibitor of the enzyme. Indeed, we obtained well diffracting crystals when PKM2 was crystallized in presence of phenylalanine, Mg²⁺ and ADP. However, these ligands and Mg²⁺ were not located in the structure of PKM2, suggesting that they acted as additives for the crystallization of PMK2. The structures of PKM2 in the presence of various activators and inhibitors are needed to understand the nature of allosteric modulation of the subunits. These structures will provide the basis for understanding the mechanism of PKM2 activity and addressing the underlying principles of PK-related human diseases.

Materials and Methods