The role of extended Fe4S4 cluster ligands in mediating sulfite reductase hemoprotein activity

Biochim Biophys Acta Proteins Proteom. 2018 Sep;1866(9):933-940. doi: 10.1016/j.bbapap.2018.05.013. Epub 2018 May 28.


The siroheme-containing subunit from the multimeric hemoflavoprotein NADPH-dependent sulfite reductase (SiR/SiRHP) catalyzes the six electron-reduction of SO32- to S2-. Siroheme is an iron-containing isobacteriochlorin that is found in sulfite and homologous siroheme-containing nitrite reductases. Siroheme does not work alone but is covalently coupled to a Fe4S4 cluster through one of the cluster's ligands. One long-standing hypothesis predicted from this observation is that the environment of one iron-containing cofactor influences the properties of the other. We tested this hypothesis by identifying three amino acids (F437, M444, and T477) that interact with the Fe4S4 cluster and probing the effect of altering them to alanine on the function and structure of the resulting enzymes by use of activity assays, X-ray crystallographic analysis, and EPR spectroscopy. We showed that F437 and M444 gate access for electron transfer to the siroheme-cluster assembly and the direct hydrogen bond between T477 and one of the cluster sulfides is important for determining the geometry of the siroheme active site.

Keywords: Iron‑sulfur cluster; Metalloenzyme; Redox enzyme; Sulfite reductase hemoprotein.

Publication types

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

MeSH terms

  • Bacterial Proteins / chemistry
  • Bacterial Proteins / metabolism
  • Bacterial Proteins / physiology*
  • Catalytic Domain
  • Crystallography, X-Ray
  • Electron Spin Resonance Spectroscopy
  • Escherichia coli / enzymology*
  • Escherichia coli / genetics
  • Escherichia coli / metabolism
  • Ferrous Compounds / metabolism*
  • Genetic Complementation Test
  • Mutagenesis, Site-Directed
  • NADP / metabolism
  • Oxidation-Reduction
  • Point Mutation
  • Sulfite Reductase (Ferredoxin) / chemistry
  • Sulfite Reductase (Ferredoxin) / metabolism
  • Sulfite Reductase (Ferredoxin) / physiology*


  • Bacterial Proteins
  • Ferrous Compounds
  • NADP
  • Sulfite Reductase (Ferredoxin)