Pyruvate Kinase (PK) is expressed in a variety of tissues in mammals. Two isozymes, muscle and kidney PK, exhibit a distinct allosteric regulatory pattern. The affinity for subunit association differs by several orders of magnitude, muscle PK being a tetramer and kidney PK a dimer under normal assay conditions and in absence of effectors. However, there is only a five percent residue difference in the primary sequence. It is localized in a helix-loop-heju'x motif involved in intersubunit contacts. The muscle PK structure has been solved by x-ray crystallography. We therefore modeled the structure of the four helix bundle formed by association of the helix-loophelix motif of two submits. Using distance geometry calculations (program Diamod), the global folding predicted for the kidney isozyme was similar to the experimentally determined muscle PK with a root-mean square deviation of 0.35A for the helix backbone. Energy refinement with an ECEPP/2 force field (program Fantom) was then performed to assess the packing and electrostatic interactions between the two subunits. Lys 421, a strictly conserved residue in all known mammalian PK, accounts for 40% of the bundle solvent accessible surface buried upon dimerization. These critical intersubunit interactions are not conserved in the kidney PK as the loop structure might be altered residues Pro 402 and Arg 398 replacing Ser and Ala respectively. Exhaustive sampling of the loop conformations by Monte Carlo Simulation show at least two energetically favorable conformations. These sites are primed candidates for structural perturbations by site-directed mutagenesis.
|Original language||English (US)|
|State||Published - Dec 1 1996|
ASJC Scopus subject areas
- Molecular Biology