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The Structures of CuA and Cytochrome a in Cytochrome c Oxidase


Martin, Craig Timothy (1985) The Structures of CuA and Cytochrome a in Cytochrome c Oxidase. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/eft1-pp43.


Cytochrome c oxidase contains four metal centers, two heme irons and two copper centers. The two low-potential centers, CuA and cytochrome a, function in the transfer of electrons from cytochrome c to the oxygen binding site within cytochrome oxidase. Although almost nothing is currently known about the mechanism of proton pumping in cytochrome oxidase, it is very likely that at least one of these low-potential metal centers is directly involved in the translocation of protons across the inner mitochondrial membrane. Essential to a complete understanding of the mechanisms of electron transfer and proton pumping in cytochrome oxidase is a knowledge of the structures of the involved metal centers.

In this work, the spectroscopic tool of electron nuclear double resonance (ENDOR) is used in conjunction with the direct incorporation of isotopically substituted amino acids into the yeast protein to determine the identities of ligands to CuA and cytochrome a in cytochrome oxidase. For cytochrome a, the incorporation of [1,3-15N2]histidine into the yeast protein results in the appearance of an ENDOR signal associated with hyperfine coupling to the histidine ring 15N, thereby unambiguously identifying histidine as an axial ligand to cytochrome a. Comparison of this result with similar results in the native and 15N-substituted bis-imidazole model compounds metmyoglobin-imidazole and bis-imidazole tetraphenyl porphyrin, provides strong evidence that cytochrome a is coordinated by two axial histidine ligands.

Similar studies on the effect of the incorporation of [1,3-15N2]histidine into yeast cytochrome oxidase demonstrate that histidine is a ligand to CuA. The incorporation of [β,β-2H2]cysteine into the protein reveals that the two strongly coupled protons previously observed in the ENDOR spectrum of CuA (H. L. Van Camp, et al. (1978) Biochim. Biophys. Acta 537, 238) can be assigned to the methylene protons on cysteine, proving conclusively that cysteine is a ligand to [β-13C]cysteine-substituted protein. These results, combined with the resolution in the native enzyme of a new hyperfine coupling to a third strongly coupled proton for CuA, provide strong evidence for the coordination of two cysteine ligands to CuA.

Armed with the identification of one histidine and two cysteine ligands to CuA, we present a model for the physical and electronic structure of the oxidized CuA center. This model is characterized by the delocalization of a substantial amount of unpaired spin density away from the copper atom and into an orbital between the two cysteine sulfur ligands. This model satisfactorily explains many of the unusual spectroscopic features of CuA and should prove to be of use in future studies of the nature of the CuA site.

With the positive identification of ligands to cytochrome a and CuA, we have analyzed the available protein sequence data for the subunits of cytochrome oxidase from a wide variety of organisms, and are able to reach some conclusions regarding the physical location of these metal centers in the subunit structure of the protein. The cysteine ligand(s) to CuA most certainly derive from the two conserved cysteine residues in subunit II. Furthermore, it is predicted that all of the metal centers in cytochrome oxidase cannot be located in subunit II, and are quite likely shared with subunit I. Finally, we present a model for the protein coordination of CuA in which the two cysteine sulfur ligands to CuA originate in two cysteine residues which are physically adjacent on the face of an α-helix in subunit II.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Chemistry
Degree Grantor:California Institute of Technology
Division:Chemistry and Chemical Engineering
Major Option:Chemistry
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Gray, Harry B.
Thesis Committee:
  • Chan, Sunney I. (chair)
  • Campbell, Judith L.
  • Janda, Kenneth C.
  • Gray, Harry B.
Defense Date:17 August 1984
Record Number:CaltechTHESIS:04052019-144431487
Persistent URL:
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:11443
Deposited By: Mel Ray
Deposited On:10 Apr 2019 15:44
Last Modified:16 Apr 2021 23:21

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