There are numerous reports of heavy metals depleting cellular glutathione pools, leading to diminished antioxidant protection in the cell and resulting in ROS accumulation ( Schutzendubel and Polle, 2002). The redox-inactive heavy metal cadmium exhibits strong affinity for oxygen, nitrogen, and sulfur atoms ( Nieboer and Richardson, 1980) and can inhibit enzyme activity by direct blocking of protein function or displacement of natural metal centers. Metal ions, however, can also be highly toxic to cells and cell organelle functions. Cobalt (Co) is known to substitute for other metal ions in the activation of NAD-malic enzyme and succinyl-CoA ligase from plant mitochondrial extracts ( Palmer and Wedding, 1966 Macrae, 1971), but it is not known whether there is an in vivo requirement for trace amounts of Co for plant respiratory metabolism. For example, copper (Cu) and iron (Fe) ions facilitate the transfer of electrons in the electron transport chain (ETC Bligny and Douce, 1977 Pascal and Douce, 1993), proteins of the tricarboxylic acid (TCA) cycle utilize metal ion cofactors to catalyze primary metabolic reactions ( Miernyk and Randall, 1987 Jordanov et al., 1992), manganese (Mn) and Fe are required for antioxidant defense enzymes ( Alscher et al., 2002), and zinc (Zn) is required for the protein import apparatus in both carrier protein transport to the inner membrane ( Lister et al., 2002) and presequence degradation ( Moberg et al., 2003).
In plant mitochondria, key functions of metal cofactors include metabolism, electron transport, ATP synthesis, and the detoxification of reactive oxygen species (ROS). Transition metal ions are essential in myriad biochemical functions by being incorporated into or associating with proteins to elicit functions in living cells.
A detailed study of oxidized residues and predicted metal interaction sites in the tricarboxylic acid cycle enzyme aconitase identified selective oxidation of residues in the active site and showed an approach for broader screening of functionally significant oxidation events in the mitochondrial proteome. Mitochondrial respiratory chain pathways and matrix enzymes varied widely in their susceptibility to metal-induced loss of function, showing the selectivity of the process. There were strong correlations between the sets of immobilized metal affinity chromatography-interacting proteins, proteins predicted to contain metal-binding motifs, and protein sets known to be oxidized or degraded during abiotic stress. Immobilized metal affinity chromatography charged with Cu 2+, Zn 2+, and Co 2+ was used to identify over 100 mitochondrial proteins with metal-binding properties. We show that selective changes occur in mitochondrial copper and iron content following in vivo and in vitro oxidative stresses. We have analyzed the metal content of isolated Arabidopsis ( Arabidopsis thaliana) mitochondria, revealing a 26:8:6:1 molar ratio for iron:zinc:copper:manganese and trace amounts of cobalt and molybdenum. Understanding the metal ion content of plant mitochondria and metal ion interactions with the proteome are vital for insights into both normal respiratory function and the process of protein damage during oxidative stress.
0 kommentar(er)
