Refinement and comparisons of the crystal structures of pig cytosolic aspartate aminotransferase and its complex with 2-methylaspartate

J Biol Chem. 1997 Jul 11;272(28):17293-302.


Two high resolution crystal structures of cytosolic aspartate aminotransferase from pig heart provide additional insights into the stereochemical mechanism for ligand-induced conformational changes in this enzyme. Structures of the homodimeric native structure and its complex with the substrate analog 2-methylaspartate have been refined, respectively, with 1.74-A x-ray diffraction data to an R value of 0.170, and with 1.6-A data to an R value of 0.173. In the presence of 2-methylaspartate, one of the subunits (subunit 1) shows a ligand-induced conformational change that involves a large movement of the small domain (residues 12-49 and 327-412) to produce a "closed" conformation. No such transition is observed in the other subunit (subunit 2), because crystal lattice contacts lock it in an "open" conformation like that adopted by subunit 1 in the absence of substrate. By comparing the open and closed forms of cAspAT, we propose a stereochemical mechanism for the open-to-closed transition that involves the electrostatic neutralization of two active site arginine residues by the negative charges of the incoming substrate, a large change in the backbone (phi,psi) conformational angles of two key glycine residues, and the entropy-driven burial of a stretch of hydrophobic residues on the N-terminal helix. The calculated free energy for the burial of this "hydrophobic plug" appears to be sufficient to serve as the driving force for domain closure.

Publication types

  • Comparative Study
  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Acetic Acid / pharmacology
  • Animals
  • Aspartate Aminotransferases / chemistry*
  • Aspartate Aminotransferases / metabolism
  • Binding Sites
  • Crystallography, X-Ray
  • Cytosol / enzymology*
  • Formates / pharmacology
  • Macromolecular Substances
  • Models, Molecular
  • Molecular Sequence Data
  • N-Methylaspartate / analogs & derivatives*
  • N-Methylaspartate / metabolism
  • Protein Conformation / drug effects
  • Protein Structure, Secondary
  • Protein Structure, Tertiary
  • Swine


  • Formates
  • Macromolecular Substances
  • formic acid
  • N-Methylaspartate
  • 2-methylaspartic acid
  • Aspartate Aminotransferases
  • Acetic Acid

Associated data

  • PDB/1AJR
  • PDB/1AJS