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Product release rather than chelation determines metal specificity for ferrochelatase
Amy E Medlock1, Michael Carter, Tamara A Dailey
1Biomedical and Health Sciences Institute, Department of Biochemistry and Molecular Biology, University of Georgia, Athens, 30602, USA.
Journal of Molecular Biology
|August 26, 2009
Summary
Ferrochelatase enzyme inhibition by certain metals like Mn, Pb, Cd, and Hg occurs after metal insertion, not due to inability to bind. Poor product release, not binding site issues, causes this ferrochelatase inhibition.
Area of Science:
- Biochemistry
- Structural Biology
- Enzymology
Background:
- Ferrochelatase (EC 4.99.1.1) is the final enzyme in heme biosynthesis, catalyzing ferrous iron insertion into protoporphyrin IX.
- Recent crystallographic data reveal significant structural changes in human ferrochelatase during catalysis, aiding metal insertion and product release.
Purpose of the Study:
- To elucidate the mechanism of ferrochelatase inhibition by various divalent metals (Fe, Co, Ni, Zn, Mn, Hg, Cd, Pb).
- To determine the structural basis for differential metal utilization versus inhibition by ferrochelatase.
Main Methods:
- High-resolution X-ray crystallography was employed to determine enzyme structures.
- Anomalous diffraction was used to characterize the identity and position of metal species within the enzyme-bound porphyrin macrocycle.
Main Results:
- Structures revealed that Pb, Hg, Cd, and Ni binding induced an "product release" conformation of the conserved pi helix.
- Mn binding resulted in a "substrate-bound" conformation, with the pi helix not extended or unwound.
- Inhibition by Mn, Pb, Cd, and Hg is linked to impaired product release, not failed metal insertion or secondary binding sites.
Conclusions:
- Ferrochelatase inhibition by Mn, Pb, Cd, and Hg is a consequence of poor product release after metal insertion.
- The study proposes explanations for the observed lack of product release in inhibited enzyme states.
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