Description
The bioavailability of copper in earth dramatically increased after the advent of oxygen in the atmosphere. These physicochemical changes also resulted in the oxidation of sulphide group of proteins that became accessible to complex soft metals such as copper (Cu). As a consequence, many proteins evolved to use Cu as a cofactor, integrating it into different electron transfer and metabolic pathways [1]. At the same time, the increased availability of reactive Cu species demanded the evolution of a number of strategies to handle its intracellular concentration and counteract unwanted damage [2]. This involved the development of specific sensor/response systems to tightly control Cu uptake and removal of its excess according to the metabolic requirements. The ability to maintain the intracellular copper quota allows microorganisms to adapt to a variety of environments, and recent evidence indicates that pathogens may have evolved copper handling mechanisms to survive in the host [3, 4]. Accordingly, both the essentiality and toxicity of Cu, and the ability of the host to control Cu availability would influence host-pathogen inter-actions. The outcome of this balance could determine if it results either in a pro-ductive infection or elimination of the pathogen. Recent evidence also indicates that mammalian macrophages can actively accumulate Cu ions in subcellular com-partments, restricting bacterial growth [3]. As a consequence, the genes involved in Cu resistance acquire particular relevance in pathogens that undergo intracellular survival and replication during their infective cycle.
In recent years, the number of newly identified or proposed Cu-containing polypeptides or proteins involved in copper handling has increased as the result of the direct detection or of newly posted genomic sequences. In this chapter, we summarize the current knowledge of the mechanisms that bacteria employ to fulfil their Cu demands and, at the same time, to defend themselves from the harmful effects of this metal, focusing on pathogens. We discuss how these pathways may serve to develop new strategies against infection diseases.
In recent years, the number of newly identified or proposed Cu-containing polypeptides or proteins involved in copper handling has increased as the result of the direct detection or of newly posted genomic sequences. In this chapter, we summarize the current knowledge of the mechanisms that bacteria employ to fulfil their Cu demands and, at the same time, to defend themselves from the harmful effects of this metal, focusing on pathogens. We discuss how these pathways may serve to develop new strategies against infection diseases.
Copper as an Essential yet Highly Toxic Element
Copper is an ideal cofactor for redox enzymatic reactions because it can cycle between two oxidation states, Cu(I) and Cu(II). This distinctive attribute has made this transition metal suitable for driving many biological processes that involve single electron shuttling, such as energy transduction, iron handling, and free radical neutralization. Examples of enzymes that build their catalytic mechanism on copper are oxidases [5], in which copper catalyzes the reduction of a dioxygen molecule to H2O2 or to two molecules of H2O, and oxygenases, which use copper to activate O2 and catalyze the incorporation of one or two atoms of oxygen into organic molecules [6]. Copper is also a catalytic metal in azurins and plastocyanins, small families of proteins involved in transfer of electrons for diverse processes [7]. Despite these beneficial roles, the imbalance in copper levels can be harmful. Failureincopperhomeostasiscanleadtoseveral humandiseasessuchasMenkes syndrome, Wilson’s disease, as well as Parkinson and Alzheimer’s diseases [8–10]. The toxicity of Cu has been linked to different mechanisms. Firstly, Cu(I)/(II) is at the top of the Irving–Williams series that highlights the ability of a metal ion to react with available ligands. Inside cells, Cu ions interact with sulfur, oxygen and imidazole ligands, displacing other cations from their active site in enzymes [11]. Secondly, the redox potential of the Cu(I)/Cu(II) pair is close to the redox value of the bacterial cytoplasm, which makes copper an extremely dangerous cation. Redox cycling of Cu ions can generate deleterious free radicals derived from oxygen through Fenton-like reactions, resulting in lipid peroxidation as well as protein and DNA damage [12]. Iron–sulfur clustersiron of proteins that perform key cellular metabolic functions have been also shown to be direct targets for Cu toxicity. The first observation, made in Escherichia coli, showed that Cu can block branched-chain amino acid biosynthesis by inactivating isopropylmalate dehydratase, an enzyme with a solvent-exposed Fe-S cluster in its active site [13]. Further studies demonstrated that Cu excess not only displaces iron from their coordinating sulfur ligands in these clusters, but also affects the formation of new iron-sulfur clusters [14].
Copper is an ideal cofactor for redox enzymatic reactions because it can cycle between two oxidation states, Cu(I) and Cu(II). This distinctive attribute has made this transition metal suitable for driving many biological processes that involve single electron shuttling, such as energy transduction, iron handling, and free radical neutralization. Examples of enzymes that build their catalytic mechanism on copper are oxidases [5], in which copper catalyzes the reduction of a dioxygen molecule to H2O2 or to two molecules of H2O, and oxygenases, which use copper to activate O2 and catalyze the incorporation of one or two atoms of oxygen into organic molecules [6]. Copper is also a catalytic metal in azurins and plastocyanins, small families of proteins involved in transfer of electrons for diverse processes [7]. Despite these beneficial roles, the imbalance in copper levels can be harmful. Failureincopperhomeostasiscanleadtoseveral humandiseasessuchasMenkes syndrome, Wilson’s disease, as well as Parkinson and Alzheimer’s diseases [8–10]. The toxicity of Cu has been linked to different mechanisms. Firstly, Cu(I)/(II) is at the top of the Irving–Williams series that highlights the ability of a metal ion to react with available ligands. Inside cells, Cu ions interact with sulfur, oxygen and imidazole ligands, displacing other cations from their active site in enzymes [11]. Secondly, the redox potential of the Cu(I)/Cu(II) pair is close to the redox value of the bacterial cytoplasm, which makes copper an extremely dangerous cation. Redox cycling of Cu ions can generate deleterious free radicals derived from oxygen through Fenton-like reactions, resulting in lipid peroxidation as well as protein and DNA damage [12]. Iron–sulfur clustersiron of proteins that perform key cellular metabolic functions have been also shown to be direct targets for Cu toxicity. The first observation, made in Escherichia coli, showed that Cu can block branched-chain amino acid biosynthesis by inactivating isopropylmalate dehydratase, an enzyme with a solvent-exposed Fe-S cluster in its active site [13]. Further studies demonstrated that Cu excess not only displaces iron from their coordinating sulfur ligands in these clusters, but also affects the formation of new iron-sulfur clusters [14].