2004). Following the reduction of Fe
3+ ion to Fe
2+ or the Mn
4+ ion to Mn
2+ ,
solubility is enhanced. Microbial species have the capacity to immobilize heavy
metals perfectly by reducing heavy metal ions to a lower oxidation state, to produce
less bioactive metallic elements (load zero) (Valls and de Lorenzo 2002; Gadd
2004).
12.5.4.9 Biomethylation
Metal ions can be changed from highly toxic to less harmful form by the
microorganisms through biomethylation process. In aerobic as well as anaerobic
environment, different varieties of bacteria, filamentous fungi and yeasts can methylate Hg, As, Cd, Se, Sn, Te and Pb ions making them highly immobile and prepare
them for participating in those processes which lessen their toxicities. In this
enzymatic process, a methyl group (CH 3 ) is transferred to metals and metalloids.
The resultant methylated compounds are highly diverse in solubility and volatility as
well as damaging (Roane and Pepper 2001; Gadd 2004), such as intermediates of
processing for Hg
0 , methyl and dimethyl mercury, which are more harmful in
comparison to inorganic Hg ions. Inorganic forms of As are more damaging in
comparison to methylated species (acids and methyl-As dimethyl-As), while the
methylated and inorganic forms of Se and Cd are more harmful (Roane and Pepper
2001; Tabak et al. 2005).
12.5.4.10 Metal-Binding Cysteine-Rich Peptides
Cells produce cysteine-rich peptides, metallothioneins (MTs), glutathione (GSH) or
phytochelatin (PCs) when exposed to toxic levels of heavy metals. All of them are
low-molecular-weight and non-enzymatic compounds which can withstand thermocoagulation as well as acid precipitation. They make complexes with divalent metals
and metal-thiols which are significant metabolites for fighting ROS (Bae et al. 2000,
2001).
Metallothioneins (MTs)
A group of well-preserved structures of proteins which behave like antioxidants is
known as metallothioneins (MTs) and are found in all living beings. They are low
molecular weight as well as cysteine rich. Thiol groups (SH) of cysteine chemical
structure have the capability to arrest metal ions like Cd
2+ , Fe
2+ , Hg
2+ , Cu
2+ and Zn
2
+
. MTs possess two different domains: one in the N-terminal region is known as β
domain, while the other present in the C-terminal region is called as α domain. The β
domain has nine cysteine residues which bind three divalent ions, while the α
domain already consists of eleven cysteine residues which bind four ions; therefore,
a single molecule binds total seven ions (Cobbett and Goldsbrough 2002;
Thirumoorthy et al. 2007). The MTs play various roles like heavy metal detoxification and defence to the presence of ROS. Therefore, MTs are accountable for
decreasing the influence of oxidative stress due to these ions, but they also retain
homeostatic cellular redox balance. According to these features, only metals can
induce protein synthesis (Cobbett and Goldsbrough 2002; Smith et al. 2007).
12 Remediation of Heavy Metals Through Genetically Engineered Microorganism
329
3+ ion to Fe
2+ or the Mn
4+ ion to Mn
2+ ,
solubility is enhanced. Microbial species have the capacity to immobilize heavy
metals perfectly by reducing heavy metal ions to a lower oxidation state, to produce
less bioactive metallic elements (load zero) (Valls and de Lorenzo 2002; Gadd
2004).
12.5.4.9 Biomethylation
Metal ions can be changed from highly toxic to less harmful form by the
microorganisms through biomethylation process. In aerobic as well as anaerobic
environment, different varieties of bacteria, filamentous fungi and yeasts can methylate Hg, As, Cd, Se, Sn, Te and Pb ions making them highly immobile and prepare
them for participating in those processes which lessen their toxicities. In this
enzymatic process, a methyl group (CH 3 ) is transferred to metals and metalloids.
The resultant methylated compounds are highly diverse in solubility and volatility as
well as damaging (Roane and Pepper 2001; Gadd 2004), such as intermediates of
processing for Hg
0 , methyl and dimethyl mercury, which are more harmful in
comparison to inorganic Hg ions. Inorganic forms of As are more damaging in
comparison to methylated species (acids and methyl-As dimethyl-As), while the
methylated and inorganic forms of Se and Cd are more harmful (Roane and Pepper
2001; Tabak et al. 2005).
12.5.4.10 Metal-Binding Cysteine-Rich Peptides
Cells produce cysteine-rich peptides, metallothioneins (MTs), glutathione (GSH) or
phytochelatin (PCs) when exposed to toxic levels of heavy metals. All of them are
low-molecular-weight and non-enzymatic compounds which can withstand thermocoagulation as well as acid precipitation. They make complexes with divalent metals
and metal-thiols which are significant metabolites for fighting ROS (Bae et al. 2000,
2001).
Metallothioneins (MTs)
A group of well-preserved structures of proteins which behave like antioxidants is
known as metallothioneins (MTs) and are found in all living beings. They are low
molecular weight as well as cysteine rich. Thiol groups (SH) of cysteine chemical
structure have the capability to arrest metal ions like Cd
2+ , Fe
2+ , Hg
2+ , Cu
2+ and Zn
2
+
. MTs possess two different domains: one in the N-terminal region is known as β
domain, while the other present in the C-terminal region is called as α domain. The β
domain has nine cysteine residues which bind three divalent ions, while the α
domain already consists of eleven cysteine residues which bind four ions; therefore,
a single molecule binds total seven ions (Cobbett and Goldsbrough 2002;
Thirumoorthy et al. 2007). The MTs play various roles like heavy metal detoxification and defence to the presence of ROS. Therefore, MTs are accountable for
decreasing the influence of oxidative stress due to these ions, but they also retain
homeostatic cellular redox balance. According to these features, only metals can
induce protein synthesis (Cobbett and Goldsbrough 2002; Smith et al. 2007).
12 Remediation of Heavy Metals Through Genetically Engineered Microorganism
329