Structural and functional linkages between subunit interfaces in mammalian pyruvate kinase

Mammalian pyruvate kinase (PK) is a four-domain enzyme that is active as a homo-tetramer. Tissue-specific isozymes of PK exhibit distinct levels of allosteric regulation. PK expressed in muscle tissue (M₁-PK)shows hyperbolic steady-state kinetics, whereas PK expressed in kidney tissue (M₂-PK)displays sigmoidal kinetics. Rabbit M₁ and M₂-PK are isozymes whose sequences differ in only 22 out of 530 residues per subunit, and these changes are localized in an inter-subunit interface. Previous studies have shown that a single amino acid mutation to M₁-PK at either the Y (S402P) or Z (T340 M) subunit interface can confer a level of allosteric regulation that is intermediate to M₁-PK and M₂-PK. In an effort to elucidate the roles of the inter-subunit interaction in signal transmission and the functional/structural connectivity between these interfaces, the S402P mutant of M₁-PK was crystallized and its structure resolved to 2.8 Å. Although the overall S402P M₁-PK structure is nearly identical with the wild-type structure within experimental error, significant differences in the conformation of the backbone are found at the site of mutation along the Y interface. In addition, there is a significant change along the Z interface, namely, a loss of an inter-subunit salt-bridge between Asp177 of domain B and Arg341 of domain A of the opposing subunit. Concurrent with the loss of the salt-bridge is an increase in the degree of rotational flexibility of domain B that constitutes the active site. Comparison of previous PK structures shows a correlation between an increase in this domain movement with the loss of the Asp177: Arg341 salt-bridge. These results identify the structural linkages between the Y and Z interfaces in regulating the interconversion of conformational states of rabbit M₁-PK.