90
G. Grasso et al.
The filamentous bacteria actinomycetes can mediate both intracellular and extracellular synthesis of metal nanoparticles. It has been suggested that the first step of
intracellular synthesis could involve the electrostatic binding of metal ions to the
negatively charged carboxylate groups in enzymes present both on the cell wall and
on the cytoplasmic membrane. Concerning extracellular synthesis, nitrogen cycle
enzymes, cell wall reductive enzymes, and/or soluble secreted enzymes could exert
a biocatalytic function; (Manimaran and Kannabiran 2017).
Compared to intracellular biosynthesis of nanoparticles, extracellular biosynthesis
poses several undoubted advantages in terms of reduced nanoparticle downstream
purification processes required as well as a possible reuse of cell cultures for new
biosynthetic processes because lysis of cell to extract the nanoparticles is not required.
The reducing hydroxyl groups of pectins located in the outer cell wall of the green
alga Spirogyra insignis have been suggested to act as reducing agents to mediate
AuNPs and AgNPs synthesis (Castro et al. 2013). The interesting role of the surfacelayer in D. radiodurans as biological scaffold and biotemplate for the synthesis of
nanomaterials, including noble metal and bimetallic nanoparticles, quantum dots,
and nanopillars have been reviewed by Li et al. (2019).
The set of proteins and enzymes secreted by fungi, for either anchored to the cell
surface or freed in the extracellular environment; i.e., fungal secretome components
have been suggested to act as reducing and capping agents in nanoparticles biosynthesis (Kitching et al. 2015). In particular, molds belonging to Penicillium, Fusarium,
Trichoderma, and Aspergillus genera have been extensively studied for nanosynthesis (Guilger-Casagrande and Lima 2019; Barabadi et al. 2019; Elegbede et al.
2018, 2019, 2020). However, the biochemical mechanisms involved in nanoparticles
synthesis by fungal secretome have not yet been completely elucidated and characterized. Furthermore, fungal secretome possess a greater variety of extracellular
enzymes in comparison with bacterial secretome and the identity and functions of
many components of fungal secretome have not been completely elucidated (Bouws
et al. 2008). To date, the promising potential of fungal secretome in nanobiosynthesis has yet to be explored. Ballottin et al. (2016) have studied the interactions
between the secretome of Aspergillus tubingensis AY876924 and the biosynthesized
AgNPs, and they have also characterized the components of AgNPs capping layer.
In particular, mass spectrometry analyses have enabled the identification of eight
proteins present in the capping layer of AgNPs, including glycoamilase, acid phosphatase, serine carboxypeptidase, and glucanosyltransferase, all involved in carbon,
phosphorous and nitrogen uptake, and for the fungal growth.
Despite the current good level of knowledge about the biofabrication mechanisms of nanomaterials at cellular and molecular level, for the outlining of solutions
concerning the control of monodispersity and nanoparticles size and in general to
gain a greater control and tuning over all the biosynthetic processes steps, further
biomolecular studies will be required.
G. Grasso et al.
The filamentous bacteria actinomycetes can mediate both intracellular and extracellular synthesis of metal nanoparticles. It has been suggested that the first step of
intracellular synthesis could involve the electrostatic binding of metal ions to the
negatively charged carboxylate groups in enzymes present both on the cell wall and
on the cytoplasmic membrane. Concerning extracellular synthesis, nitrogen cycle
enzymes, cell wall reductive enzymes, and/or soluble secreted enzymes could exert
a biocatalytic function; (Manimaran and Kannabiran 2017).
Compared to intracellular biosynthesis of nanoparticles, extracellular biosynthesis
poses several undoubted advantages in terms of reduced nanoparticle downstream
purification processes required as well as a possible reuse of cell cultures for new
biosynthetic processes because lysis of cell to extract the nanoparticles is not required.
The reducing hydroxyl groups of pectins located in the outer cell wall of the green
alga Spirogyra insignis have been suggested to act as reducing agents to mediate
AuNPs and AgNPs synthesis (Castro et al. 2013). The interesting role of the surfacelayer in D. radiodurans as biological scaffold and biotemplate for the synthesis of
nanomaterials, including noble metal and bimetallic nanoparticles, quantum dots,
and nanopillars have been reviewed by Li et al. (2019).
The set of proteins and enzymes secreted by fungi, for either anchored to the cell
surface or freed in the extracellular environment; i.e., fungal secretome components
have been suggested to act as reducing and capping agents in nanoparticles biosynthesis (Kitching et al. 2015). In particular, molds belonging to Penicillium, Fusarium,
Trichoderma, and Aspergillus genera have been extensively studied for nanosynthesis (Guilger-Casagrande and Lima 2019; Barabadi et al. 2019; Elegbede et al.
2018, 2019, 2020). However, the biochemical mechanisms involved in nanoparticles
synthesis by fungal secretome have not yet been completely elucidated and characterized. Furthermore, fungal secretome possess a greater variety of extracellular
enzymes in comparison with bacterial secretome and the identity and functions of
many components of fungal secretome have not been completely elucidated (Bouws
et al. 2008). To date, the promising potential of fungal secretome in nanobiosynthesis has yet to be explored. Ballottin et al. (2016) have studied the interactions
between the secretome of Aspergillus tubingensis AY876924 and the biosynthesized
AgNPs, and they have also characterized the components of AgNPs capping layer.
In particular, mass spectrometry analyses have enabled the identification of eight
proteins present in the capping layer of AgNPs, including glycoamilase, acid phosphatase, serine carboxypeptidase, and glucanosyltransferase, all involved in carbon,
phosphorous and nitrogen uptake, and for the fungal growth.
Despite the current good level of knowledge about the biofabrication mechanisms of nanomaterials at cellular and molecular level, for the outlining of solutions
concerning the control of monodispersity and nanoparticles size and in general to
gain a greater control and tuning over all the biosynthetic processes steps, further
biomolecular studies will be required.