Microbial Enzymes in Nanotechnology …
195
2.4 Biosynthetic Mechanism of Microbial Enzymes
in the Nanomaterials Fabrication
By their universal characteristics, enzymes catalyze the synthesis but not used up in
the reactions, but sometimes, may function as agents causing reduction and stabilization. The process may be catalyzed by the whole enzyme or amino acids liberated
after denaturation of the enzyme as a result of the reaction conditions (Adelere
and Lateef 2016). Microbial enzymes can facilitate the biosynthesis of nanomaterials either by intracellularly or extracellularly produced enzymes. Microbes such
as fungi, bacteria, viruses, actinomycetes and yeasts serve as biofactories for the
bioreduction of silver, gold–silver alloy, gold, cadmium, silica, platinum, selenium,
titania, magnetite, palladium and other metals to their nanoparticles (Narayanan and
Sakthivel 2010) by employing their intracellularly produced enzymes. In the intracellular method, enzymes with ability to bioreduce the metal ions to their particular nanoparticles are secreted in the cell wall of the microbe. The inter-relations
of intracellular enzymes and positively charged groups are exploited in enthralling
of metallic ions from the solution leading to consequent reduction within the cell
(Thakkar et al. 2010; Dauthal and Mukhopadhyay 2016). Microscopic examinations
have revealed that nanoparticles are stored in the cytoplasmic membrane, periplasmic
space and cell wall and this was as a consequence of the diffusion of metal ions across
these membranes and enzymatic reduction (Ovais et al. 2018b).
Alkalo-tolerant and alkalo-thermophilic actinomycetes species, Rhodococcus sp.
and Thermomonospora sp. respectively were employed for the intracellular synthesis
of AuNPs (Ahmad et al. 2003a, b), and the Au
3+ reduction was observed to be
mediated by enzymes located at the cytoplasmic membrane and mycelia surface.
According to Mukherjee et al. (2001), Verticillium biomass was reacted with AgNO 3
solution which brought about the intracellular reduction and subsequent development of AgNPs. Electron microscopy illustrated that the AgNPs formed were located
under the cell wall surface owing to enzymatic bioreduction, which is nontoxic to the
fungi. Brevibacterium casei reportedly biosynthesize AuNPs and AgNPs via intracellular enzymes which were spherically shaped (Kalishwaralal et al. 2010). Likewise,
Konishi et al. (2007) reported Shewanella algae as an efficient bioreducer of AuCl 4
ions to elemental gold. AuNPs biosynthesized were located in the periplasmic space
which was facilitated via intracellular enzymes.
In extracellular scheme, reductase enzymes are secreted by microbial cell and are
applied in the reduction of metal ions (Hulkoti and Taranath 2014). Studies have
suggested that cofactors like both nicotinamide adenine dinucleotide (NADH) and
reduced form of nicotinamide adenine dinucleotide phosphate (NADPH)-dependent
enzymes play important responsibilities as reducing agents through the electron
transfer from NADH by NADH-contingent enzymes that functions as electron
carriers (Bose and Chatterjee 2016). He et al. (2007) emphasized that the bioreduction of gold was instigated via electron transfer from NADH by NADH-contingent
reductase enzymes that are at hand in bacterium Rhodopseudomonas capsulata in
195
2.4 Biosynthetic Mechanism of Microbial Enzymes
in the Nanomaterials Fabrication
By their universal characteristics, enzymes catalyze the synthesis but not used up in
the reactions, but sometimes, may function as agents causing reduction and stabilization. The process may be catalyzed by the whole enzyme or amino acids liberated
after denaturation of the enzyme as a result of the reaction conditions (Adelere
and Lateef 2016). Microbial enzymes can facilitate the biosynthesis of nanomaterials either by intracellularly or extracellularly produced enzymes. Microbes such
as fungi, bacteria, viruses, actinomycetes and yeasts serve as biofactories for the
bioreduction of silver, gold–silver alloy, gold, cadmium, silica, platinum, selenium,
titania, magnetite, palladium and other metals to their nanoparticles (Narayanan and
Sakthivel 2010) by employing their intracellularly produced enzymes. In the intracellular method, enzymes with ability to bioreduce the metal ions to their particular nanoparticles are secreted in the cell wall of the microbe. The inter-relations
of intracellular enzymes and positively charged groups are exploited in enthralling
of metallic ions from the solution leading to consequent reduction within the cell
(Thakkar et al. 2010; Dauthal and Mukhopadhyay 2016). Microscopic examinations
have revealed that nanoparticles are stored in the cytoplasmic membrane, periplasmic
space and cell wall and this was as a consequence of the diffusion of metal ions across
these membranes and enzymatic reduction (Ovais et al. 2018b).
Alkalo-tolerant and alkalo-thermophilic actinomycetes species, Rhodococcus sp.
and Thermomonospora sp. respectively were employed for the intracellular synthesis
of AuNPs (Ahmad et al. 2003a, b), and the Au
3+ reduction was observed to be
mediated by enzymes located at the cytoplasmic membrane and mycelia surface.
According to Mukherjee et al. (2001), Verticillium biomass was reacted with AgNO 3
solution which brought about the intracellular reduction and subsequent development of AgNPs. Electron microscopy illustrated that the AgNPs formed were located
under the cell wall surface owing to enzymatic bioreduction, which is nontoxic to the
fungi. Brevibacterium casei reportedly biosynthesize AuNPs and AgNPs via intracellular enzymes which were spherically shaped (Kalishwaralal et al. 2010). Likewise,
Konishi et al. (2007) reported Shewanella algae as an efficient bioreducer of AuCl 4
ions to elemental gold. AuNPs biosynthesized were located in the periplasmic space
which was facilitated via intracellular enzymes.
In extracellular scheme, reductase enzymes are secreted by microbial cell and are
applied in the reduction of metal ions (Hulkoti and Taranath 2014). Studies have
suggested that cofactors like both nicotinamide adenine dinucleotide (NADH) and
reduced form of nicotinamide adenine dinucleotide phosphate (NADPH)-dependent
enzymes play important responsibilities as reducing agents through the electron
transfer from NADH by NADH-contingent enzymes that functions as electron
carriers (Bose and Chatterjee 2016). He et al. (2007) emphasized that the bioreduction of gold was instigated via electron transfer from NADH by NADH-contingent
reductase enzymes that are at hand in bacterium Rhodopseudomonas capsulata in