Guanidine-induced equilibrium unfolding of a homo-hexameric enzyme 4-oxalocrotonate tautomerase (4-OT).

Published

Journal Article

4-Oxalocrotonate tautomerase (4-OT) is a bacterial enzyme that is comprised of 6 identical 62 amino acid subunits. The 4-OT enzyme is an attractive model system in which to study the interrelationship between protein folding, subunit assembly, and catalytic function. Here we report on the GuHCl-induced equilibrium unfolding properties of wild-type 4-OT using catalytic activity measurements and using far-UV circular dichroism (CD) spectroscopy. We demonstrate that the unfolding of wild-type 4-OT in 50 mM phosphate buffers containing 6 M GuHCl is reversible at pHs 6.0, 7.4, and 8.5; and we find that there is both an enzyme concentration dependence and a pH dependence to the equilibrium unfolding properties of 4-OT. Our data suggests that the GuHCl-induced unfolding of 4-OT in 50 mM phosphate buffer at pH 8.5 can be modeled as a two-state process involving folded hexamer and unfolded monomer. On the basis of this model, we determined a free-energy value for the unfolding of 4-OT at pH 8.5 to be 68.7 +/- 3.2 kcal/mol under standard state conditions (1 M hexamer). In 50 mM phosphate buffers at pHs 6.0 and 7.4, only the catalytic activity denaturation curves are consistent with a two-state folding mechanism. At the lower pHs the far-UV-CD transitions are not well described by a two-state model. Our results at pHs 6.0 and 7.4 suggest that intermediate state(s) are populated in the equilibrium unfolding reaction at these lower pHs and that these intermediate state(s) have some helical content but no measurable catalytic activity.

Full Text

Duke Authors

Cited Authors

  • Silinski, P; Allingham, MJ; Fitzgerald, MC

Published Date

  • April 10, 2001

Published In

Volume / Issue

  • 40 / 14

Start / End Page

  • 4493 - 4502

PubMed ID

  • 11284706

Pubmed Central ID

  • 11284706

International Standard Serial Number (ISSN)

  • 0006-2960

Digital Object Identifier (DOI)

  • 10.1021/bi002752b

Language

  • eng

Conference Location

  • United States