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Iron: Heme Proteins & Electron Transport

  1. Bill Durham,
  2. Francis S. Millett

Published Online: 15 MAR 2006

DOI: 10.1002/0470862106.ia104

Encyclopedia of Inorganic Chemistry

Encyclopedia of Inorganic Chemistry

How to Cite

Durham, B. and Millett, F. S. 2006. Iron: Heme Proteins & Electron Transport. Encyclopedia of Inorganic Chemistry. .

Author Information

  1. University of Arkansas, Fayetteville, AR, USA

Publication History

  1. Published Online: 15 MAR 2006

Abstract

This chapter discusses the structure and function of cytochromes, which are a class of iron-containing heme proteins primarily involved in biological electron-transfer reactions. Subcellular organelles called mitochondria are responsible for carrying out oxidative phosphorylation, the major energy-transduction process in eukaryotic cells. Enormous progress has been made in our understanding of the molecular mechanism of the mitochondrial electron-transport chain over the past decade. X-ray crystal structures have been determined for three of the four electron-transfer complexes: succinate–ubiquinone reductase, cytochrome bc1, and cytochrome c oxidase (CcO). Cytochrome c (Cc) is a small heme protein with a molecular weight of 12 500 Da that transports electrons from cytochrome bc1 to CcO. It is a very positively charged protein, and is known to bind to both cytochrome bc1 and CcO by means of electrostatic interactions. Extensive chemical modification studies have demonstrated that the binding domain on Cc for both proteins involves lysines immediately surrounding the heme crevice, and Cc functions as a mobile shuttle during electron transport. The reactions of Cc with its redox partners are too fast to resolve by conventional techniques such as stopped-flow spectroscopy. A new method to study biological electron transfer has been introduced that utilizes a photoactive tris(bipyridine)ruthenium complex, Ru(II), which is covalently attached to a protein such as Cc. Photoexcitation of Ru(II) to the metal-to-ligand charge-transfer state, Ru(II*), a strong reductant, leads to rapid electron transfer to the ferric heme group in Cc. Subsequent electron transfer from photoreduced heme c to redox center(s) in another protein can be measured on a timescale as short as 50 ns. This technique has been used to measure intracomplex electron transfer between Cc and its physiological partners, CcO, cytochrome bc1, cytochrome c peroxidase (CcP), and cytochrome b5. The ruthenium technique was used to characterize sequential electron transfer in cytochrome bc1 from ubiquinol to the Rieske iron–sulfur protein (2Fe2S), cytochrome c1, and finally to Cc. Cytochrome oxidase contains four redox centers, CuA, heme a, heme a3, and CuB. A ruthenium Cc derivative was used to demonstrate that the initial site of electron entry into CcO is CuA, followed by electron transfer to heme a, and then heme a3.

Keywords:

  • cytochrome c;
  • cytochrome b5;
  • cytochrome c peroxidase;
  • cytochrome c oxidase;
  • cytochrome bc1;
  • ruthenium;
  • mitochondrial electron transfer