关键词:
Electron Transport Chain Complex Proteins
Escherichia coli Proteins
Binding Sites
Carbon Monoxide
Circular Dichroism
Cyanates
Cytochromes
Escherichia coli
Heme
Ligands
Magnetics
Models
Chemical
Nitric Oxide
Oxidation-Reduction
Oxidoreductases
Research Support
Non-U.S. Gov't
Research Support
U.S. Gov't
P.H.S.
Temperature
摘要:
The interactions of the fully reduced and fully oxidized cytochrome bd from E. coli with ligands CO, NO, and CN- have been studied by a combination of absorption and magnetic circular dichroism (MCD) spectroscopy. In the reduced cytochrome bd, MCD resolves individual bands due to the high-spin heme b(595) and the low-spin heme b(558) components of the enzyme, allowing one to separately monitor their interactions along with ligand binding to the heme d component. The data show that at low concentrations, the ligands bind almost exclusively to heme d. At high concentrations, the ligands begin to interact with the low-spin heme b(558). At the same time, no evidence for significant binding of the ligands to the high-spin heme b(595) is revealed in either the reduced or the fully oxidized cytochrome bd complex, The data support the model [Borisov, V. B., Gennis, R. B., and Konstantinov, A. A. (1995) Biochemistry (Moscow) 60, 231-239] according to which the two high-spin hemes d and b(595) Share a high-affinity ligand binding site with a capacity for only a single molecule of the ligand;i.e., there is a strong negative cooperativity with respect to ligand binding to these two hemes with cytochrome d having an intrinsic ligand affinity much higher than that of heme b(595).