Spectroscopic Evidence for a Heme-Heme Binuclear Center in the Cytochrome bd Ubiquinol Oxidase from Escherichia coli

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 90; no. 12; pp. 5863 - 5867
• Main Authors: Hill, John J., Alben, James O., Gennis, Robert B.
• Format: Journal Article
• Language: English
• Published: Washington, DC National Academy of Sciences of the United States of America 15.06.1993; National Acad Sciences; National Academy of Sciences
• Subjects: Absorption spectra; Adducts; Analytical, structural and metabolic biochemistry; Bacteria; Binding Sites; Biochemistry; Biological and medical sciences; Carbon Dioxide - metabolism; Cellular biology; Cytochromes; Cytochromes - chemistry; Cytochromes - metabolism; Electron Transport Chain Complex Proteins; Enzymes; Enzymes and enzyme inhibitors; Escherichia coli; Escherichia coli - enzymology; Escherichia coli Proteins; Fundamental and applied biological sciences. Psychology; Heme - metabolism; Light; Low temperature; Models, Structural; Oxidases; Oxidoreductases; Oxidoreductases - chemistry; Oxidoreductases - metabolism; Oxygen; Photolysis; Physics; Protein Conformation; Scientific imaging; Spectrophotometry - methods; Spectrophotometry, Infrared - methods; Visible spectrum; Cytochrome; Fourier transformation; Molecular structure; Escherichia coli; Aerobe; Photodissociation; Proteins; Infrared spectrometry; Heme; Bacteria; Carbon monoxide; Enterobacteriaceae; Cell respiration
• DOI: 10.1073/pnas.90.12.5863
• ISSN: 0027-8424

The cytochrome bd complex is a ubiquinol oxidase, which is part of the aerobic respiratory chain of Escherichia coli. This enzyme is structurally unrelated to the heme-Cu oxidases such as cytochrome c oxidase. While the cytochrome bd complex contains no copper, it does have three heme prosthetic groups: heme b558, heme b595, and heme d (a chlorin). Heme b558appears to be involved in the oxidation of quinol, and heme d is known to be the site where oxygen binds and is reduced to water. The role of heme b595, which is high spin, is not known. In this paper, CO is used to probe the oxygen-binding site by use of Fourier transform infrared spectroscopy to monitor the stretching frequency of CO bound to the enzyme. Photodissociation at low temperature (e.g., 20 K) of the CO-heme d adduct results in CO associated with the protein within the heme binding pocket. This photodissociated CO can subsequently relax to form a kinetically trapped CO-heme b595adduct. The data clearly show that heme d and heme b595must reside within a common binding pocket in the enzyme. The catalytic active site where oxygen is reduced to water is, thus, properly considered to be a heme d-heme b595binuclear center. This is analogous to the heme a3-CuBbinuclear center in the heme-Cu oxidases. Heme b595may play roles analogous to those proposed for the CuBcomponent of cytochrome c oxidase.