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Ligand binding and slow structural changes in chlorocruorin from Spirographis spallanzanii
A Bellelli1, E Lendaro, R Ippoliti
1Department of Biochemical Sciences Alessandro Rossi Fanelli, University of Rome, La Sapienza, Italy.
Biochemistry
|August 3, 1993
Summary
Chlorocruorin, a respiratory pigment in worms, exhibits unique oxygen and carbon monoxide binding kinetics. Its complex structure influences slow conformational changes affecting ligand off-rates.
Area of Science:
- Biochemistry
- Comparative Physiology
- Structural Biology
Background:
- Chlorocruorin is a complex respiratory pigment in polychaete worms.
- It features a unique prosthetic group derived from iron protoporphyrin IX.
- Its quaternary structure is highly complex, assembling into large particles.
Purpose of the Study:
- To investigate the functional properties and kinetics of chlorocruorin from Spirographis spallanzanii.
- To compare its oxygen and carbon monoxide binding characteristics with human hemoglobin.
- To elucidate the conformational dynamics underlying its ligand-binding behavior.
Main Methods:
- Stopped-flow spectroscopy to measure reaction rates.
- Flash photolysis to study ligand rebinding kinetics.
- Analysis of oxygen and carbon monoxide binding affinities and rates.
Main Results:
- Chlorocruorin shows lower oxygen affinity but similar or higher carbon monoxide affinity than human hemoglobin.
- Rates of combination with oxygen and carbon monoxide are notably low.
- CO rebinding after photolysis is biphasic, suggesting a slow decay from the R to T state via an intermediate.
Conclusions:
- The complex structure of chlorocruorin leads to slow conformational transitions impacting ligand kinetics.
- These transitions are rate-limiting for oxygen but not carbon monoxide binding.
- Chlorocruorin's reactivity patterns provide insights into protein dynamics and cooperative binding.