Long-range effects and conformational flexibility of aldolase.

The conformational flexibility and long-range interactions in rabbit muscle aldolase induced by active-site ligand binding, cross-linking of the enzyme between Cys72 and Cys338, and removal of the C-terminal tyrosine residue were studied by following the changes in the microenvironments of Cys239 and Cys289 located outside the active site. It was found that substrates induced a conformational change in aldolase, which propagates from the active site to Cys239, which is located close to intersubunit contacts. The response of the enzyme is differential. Ligands having both C-1 and C-6 phosphates or C-1 phosphate only induce the enhancement of Cys239 reactivity, whereas those with C-6 phosphates only decrease Cys239 reactivity. This correlates well with a dramatic difference in kinetic parameters for a cleavage of fructose-1,6-P2 and fructose-1-P. Therefore, these changes can be interpreted as syncatalytic. Cross-linking of the aldolase subunit by an -S-S-bridge between Cys72 and Cys338 inactivates the enzyme, abolishes binding of active-site ligands, and induces a conformational change in the enzyme that can be detected far away (at Cys239 and Cys289) from the site of perturbation. Cys72 and Cys338 are not in the active site. This shows that the region of the active site and the environment of Cys72 and Cys338 are tightly coupled and that residues far away from the active site, through such coupling, can possess properties of active-site residues. Similar, although less dramatic changes are observed upon removal of the C-terminal tyrosine residue. In view of the results obtained in this paper, aldolase seems to be quite a flexible molecule, whose conformation is sensitive to the nature of a substrate bound to the enzyme and is able to transmit the information about a local perturbation over long distances within a molecule.