Cytochrome oxidase subunit protein

The half lives and the pool sizes of the precursor polypeptides were derived from the time courses of labeling shown in Fig. 9. They are summarized in Table II. As can be seen, the precursor pool of the 20,000 Mr subunit of cytochrome oxidase is relatively small compared to the precursor pools of the other cytochrome oxidase subunits. This 20,000 Mr subunit was shown to be produced by the chloramphenicol-sensitive mitochondrial protein synthesis (Fig. 7B). Consequently, soon after the addition of chloramphenicol, the precursor pool of this subunit is exhausted, making a further assembly of the holoenzyme impossible. This explains why, after... 

It is known that protein kinase C can phosphorylate a number of key oxidase components, such as the two cytochrome b subunits and the 47-kDa cytoplasmic factor. This process is prevented by protein kinase C inhibitors such as staurosporine (although it is now recognised that this inhibitor is not specific for protein kinase C), which also inhibits the respiratory burst activated by agonists such as PMA. However, when cells are stimulated by fMet-Leu-Phe, translocation of pAl-phox to the plasma membrane can occur even if protein kinase C activity is blocked - that is, phosphorylation is not essential for the translocation of this component in response to stimulation by this agonist. Similarly, the kinetics of phosphorylation of the cytochrome subunits do not follow the kinetics of oxidase activation, and protein kinase C inhibitors have no effect on oxidase activity elicited by some agonists -for example, on the initiation of the respiratory burst elicited by agonists such as fMet-Leu-Phe (Fig. 6.14). Furthermore, the kinetics of DAG accumulation do not always follow those of oxidase activity. Hence, whilst protein kinase C is undoubtedly involved in oxidase activation by some agonists, oxidase function is not totally dependent upon the activity of this kinase. 

SH3 domains occur in signal proteins that are involved in Tyr kinase signaling pathways (see Cohen et al., 1995 Pawson, 1995). They are also foimd in proteins of the cytoskeleton and in a subunit of the neutrophilic cytochrome oxidase. Ligand binding at SH3 domains takes place via Pro-rich sequences of ca. 10 amino acids, and Pro-rich peptides are very good binding substrates. 

Boerner JL, Demory ML, Silva C et al. Phosphorylation of Y845 on the epidermal growth factor receptor mediates binding to the mitochondrial protein cytochrome c oxidase subunit II. Mol Cell Biol 2004 24 7059-7071. 

FIGURE 19-13 Critical subunits of cytochrome oxidase (Complex IV). The bovine complex is shown here (PDB ID 10CC). (a) The core of Complex IV, with three subunits. Subunit I (yellow) has two heme groups, a and a3 (red), and a copper ion, CuB (green sphere). Heme a3 and CuB form a binuclear Fe-Cu center. Subunit II (blue) contains two Cu ions (green spheres) complexed with the —SH groups of two Cys residues in a binuclear center, CuA, that resembles the 2Fe-2S centers of iron-sulfur proteins. This binuclear center and the cytochrome... 

It has long been established that the functional unit of cytochrome oxidase contains four redox-active metal centres. Two of these, cytochromes a and a3, contain heme A (Fig. 5-23) coordinated in different ways in subunit I. The heme group is not covalently linked to the protein. The main structural features are the carbonyl group at position 8 and the isoprenoid chain at position 2 of the porphyrin ring. 

S. acidocaldarius (strain 7) contains a cyanide-sensitive cytochrome oxidase [24], The purified cytochrome (M, 150000) is composed of three subunits (M, 37000, 23 000, and 14000). Difference spectra following reduction with dithionite show a Soret band at 441 nm and a maximum at 603 nm characteristic of aa3-type cytochromes. In addition, there is a band at 558 nm whose connection to the oxidase is not clear. This oxidase is stimulated by cholate, but unlike the oxidase from the DSM 639 strain it is inhibited by low concentrations of cyanide (pM as opposed to mM) and oxidizes horse-heart cytochrome c, TMPD-ascorbate, and caldariella quinol. The rates of oxidation (pmol/min/mg protein) for cytochrome c, TMPD-ascorbate, and quinol are 63, 6.1, and 0.2, respectively. Another cytochrome oxidase that has an absorption maximum at 602 nm, oxidizes caldariella quinol, but does not oxidize cytochrome c, is also present in strain 7 so that the terminal portion of the electron transport system in S. acidocaldarius consists of at least three oxidases. It is suggested [8] that the presence of three oxidases in 5. acidocaldarius is unlikely and that the cyanide-sensitive oxidase was isolated from a different species, namely S. solfataricus. There is little taxonomic information in this assertion to judge whether strain 7 and DSM 639 are indeed different species. However, based on growth conditions reported by the investigators [12,28], which are unique for S. acidocaldarius and S. solfataricus [ 22, there is no reason to suspect that these organisms are different species. 

All three respiratory complexes are typical integral membrane proteins that span the inner mitochondrial membrane. Each consists of several different subunits, the exact number of which is still under debate. The genes of some subunits of cytochrome oxidase and the />c, complex are in mitochondrial DNA (mtDNA). These proteins are synthesised inside the mitochondrion. However, most proteins of these complexes, as well as cytochrome c, are synthesised on cytoplasmic ribosomes and coded by the nuclear genome. This raises intriguing questions of how the latter are imported into the mitochondrion and inserted into the mitochondrial membrane, as well as of how mitochondrial and cytoplasmic transcription and translation are synchronised [3-5]. 

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