What Role Does COA6 Play in Cytochrome C Oxidase Biogenesis: A Metallochaperone or Thiol Oxidoreductase, or Both?

Abstract

Complex IV (cytochrome c oxidase; COX) is the terminal complex of the mitochondrial electron transport chain. Copper is essential for COX assembly, activity, and stability, and is incorporated into the dinuclear Cu_A and mononuclear Cu_B sites. Multiple assembly factors play roles in the biogenesis of these sites within COX and the failure of this intricate process, such as through mutations to these factors, disrupts COX assembly and activity. Various studies over the last ten years have revealed that the assembly factor COA6, a small intermembrane space-located protein with a twin CX₉C motif, plays a role in the biogenesis of the Cu_A site. However, how COA6 and its copper binding properties contribute to the assembly of this site has been a controversial area of research. In this review, we summarize our current understanding of the molecular mechanisms by which COA6 participates in COX biogenesis.

Keywords: COA6; assembly factor; copper; cytochrome c oxidase; mitochondria; structure.

Figure 1

(a) The overall structure of the monomeric bovine COX (PDB 1OCC) [5]. All secondary structures are shown as cartoons. COX1, COX2, and COX3 subunits are colored in cyan, red and pink, respectively. Copper ions are shown as orange spheres and heme a and heme a₃ are shown as yellow sticks. Other remaining subunits are colored in gray for clarity. (b) The mononuclear Cu_B site, which is located in close proximity to the heme a₃ (4.7 Å), binds one copper ion via coordination with residues His240, His290, and His291. Residues located (labeled) at the Cu_B site are shown as cyan sticks. The copper (Cu) and iron (Fe) atoms are shown as orange and pink spheres, respectively. Heme a and heme a₃ are shown as yellow sticks, where the distance between their Fe atoms is 13.4 Å which is shown as a dashed line. Carbon, oxygen, nitrogen and sulfur atoms are colored cyan, red, blue and yellow, respectively. (c) Residues located at the Cu_A site (labeled) are shown as sticks. Copper ions are shown as orange spheres. A cluster of two copper atoms, bridged by Cys196 and Cys200 residues constitutes the Cu_A site. One copper ion coordinated by the imidazole group of His161 and the thioether group of Met207 while the other copper ion is coordinated by His204 and the carbonyl group of Glu198. Carbon, oxygen, nitrogen and sulfur atoms are colored cyan, red, blue and yellow, respectively.

Figure 2

(a) Cartoon representation of the crystal structure of COA6 (PDB 6PCE) [56]. (i) The structures are colored from blue at the N-terminus to red at the C-terminus. Cysteine residues are shown as yellow sticks; (ii) Salt bridges between residues Arg83 (α2) and Asp99 (α3) and residues Asp70 (α1) and Arg102 (α3), which stabilize the relative orientations of these secondary structural elements, are indicated as dashed lines; (b) Cartoon representation of the solution structure of COA6 (PDB 6NL3) [52] (i) The structures are colored from blue at the N-terminus to red at the C-terminus. Cysteine residues are shown as yellow sticks; (ii) Residues Arg83 (α2) and Asp99 (α3) and residues Asp70 (α1) and Arg102 (α3) (labeled), which form salt bridges in the crystal structure, are located on ‘opposite’ sides of the molecule and separated by ~25 Å.

Figure 3

(a) The crystal structure of COA6, with residues that are mutated in patient samples shown as pink sticks (PDB 6PCE) [56]. Secondary structures are represented as cartoons with monomers colored in cyan and gray. Cysteine residues are shown as yellow spheres; (b) Cartoon representation of the crystal structure ^W59CCOA6 (PDB 6PCF) [56]. Individual protomers are colored in cyan, gray, purple, and green. Each monomer (cyan and gray) is linked to another monomer (green and purple, respectively) by an intermolecular disulfide bond (shown as yellow sticks) through the introduced Cys59 residue.