Closed fumarase C active-site structures reveal SS Loop residue contribution in catalysis

FEBS Lett. 2020 Jan;594(2):337-357. doi: 10.1002/1873-3468.13603. Epub 2019 Sep 23.


Fumarase C (FumC) catalyzes the reversible conversion of fumarate to S-malate. Previous structural investigations within the superfamily have reported a dynamic structural segment, termed the SS Loop. To date, active-site asymmetry has raised the question of how SS Loop placement affects participation of key residues during the reaction. Herein, we report structural and kinetic analyses from Escherichia coli FumC variants to understand the contribution of SS Loop residues S318, K324, and N326. High-resolution X-ray crystallographic results reveal three distinct FumC active-site conformations; disordered-open, ordered-open, and the newly discovered ordered-closed. Surprisingly, each SS Loop variant has unaffected Michaelis constants coupled to reductions in turnover number. Based upon our structural and functional analyses, we propose structural and catalytic roles for each of the aforementioned residues.

Keywords: Krebs cycle; X-ray crystallography; circular dichroism; fumarase; fumarate hydratase; metabolism.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Amino Acid Sequence / genetics
  • Catalysis*
  • Catalytic Domain / genetics
  • Crystallography, X-Ray
  • Escherichia coli / enzymology
  • Fumarate Hydratase / chemistry
  • Fumarate Hydratase / genetics
  • Fumarate Hydratase / ultrastructure*
  • Kinetics
  • Models, Molecular
  • Protein Conformation*


  • fumarase C
  • Fumarate Hydratase