specific, and environmentally sound. In some cases, it serves as simple replacement
for existing toxic, hazardous, complex, or uneconomical chemical- or chemophysical-based processes [124, 125]. While in other cases such as when the substrate/product is complex and hardly possible to synthesize it using the conventional
chemical methods, biotransformation or biocatalysis processes may be the only way
forward. Here, a good example could be PCR, a reaction that is the heartbeat of the
modern-day biotechnology. The polymerases used in this reaction duplicate the
template (DNA strands) of largely unknown sequence (often except that of primers)
which is impossible to achieve with the conventional chemistry approach.
Synthesis reactions may require harsh conditions such as extreme pH, low water
activity, high temperature, vigorous mixing, etc. Hence, the biocatalysts used for
synthesis reactions should be robust enough, and extremophiles and extremozymes
have been known more suitable to catalyze such kind of synthesis reactions.
Synthetic reactions that require high pH conditions can deploy alkaliphiles and
their enzymes. Thus, alkaliphiles, through simple whole-cell biotransformation or
enzymatic biocatalysis, can potentially be used to produce an array of valuable
organic compounds.
Alkaliphiles and their enzymes have been tried in various synthesis reactions, and
peptide synthesis is one of the interesting mediations achieved by these biocatalysts.
Unlike the conventional method of chemical synthesis, enzymatic peptide synthesis
is stereoselective, clean, and mild and avoids the time-consuming protectiondeprotection procedures [126]. Moreover, it may offer other technical advantages
which are difficult to achieve using the chemical method. For instance, production of
peptides which are longer than dipeptides has been a challenge for the chemicalbased synthetic route. However, it is possible to synthesize such peptides through
enzymatic synthesis route [127, 128]. Efficiency of peptide synthesis is better in
alkaline condition, which is considered as a natural process that happens around
hydrothermal vents and believed to be part of the prebiotic chemical evolution of
biological molecules. Recently, this reaction is mimicked and successfully used
in vitro to synthesize peptides on rock surfaces at pH 10–11 [129]. Thus, by
combining the effect of alkaline condition and catalytic efficiency of enzymes, one
expects better synthesis of longer peptides. Indeed, this has been demonstrated by
[130] who reported higher degree of peptide polymerization (DP) by enzymatic
reactions at alkaline medium than at neutral condition. This shows the potential of
alkaline active proteases in the synthesis of peptides for a variety of applications.
However, not all alkaline active proteases are equally applicable. Since the peptide
synthesis reactions are more efficient in nonaqueous medium, proteases which are
alkaline active and operationally stable in low water activity are more suitable.
There has been an effort in obtaining alkaline active proteases which are active at
low water activity. Alkaline active proteases from Aspergillus flavus, Bacillus
pseudofirmus SVB1, Pseudomonas aeruginosa PseA, Bacillus pumilus strain
CBS, Streptomyces sp. strain AB1, and Alcalase (the most well-known alkaline
active proteases) are known to be highly active and stable in the presence of organic
solvents, and their promising potential in peptide synthesis has been verified [131–
135]. Although it is possible that alkaline active proteases can be obtained from
Alkaliphiles: The Versatile Tools in Biotechnology
19
for existing toxic, hazardous, complex, or uneconomical chemical- or chemophysical-based processes [124, 125]. While in other cases such as when the substrate/product is complex and hardly possible to synthesize it using the conventional
chemical methods, biotransformation or biocatalysis processes may be the only way
forward. Here, a good example could be PCR, a reaction that is the heartbeat of the
modern-day biotechnology. The polymerases used in this reaction duplicate the
template (DNA strands) of largely unknown sequence (often except that of primers)
which is impossible to achieve with the conventional chemistry approach.
Synthesis reactions may require harsh conditions such as extreme pH, low water
activity, high temperature, vigorous mixing, etc. Hence, the biocatalysts used for
synthesis reactions should be robust enough, and extremophiles and extremozymes
have been known more suitable to catalyze such kind of synthesis reactions.
Synthetic reactions that require high pH conditions can deploy alkaliphiles and
their enzymes. Thus, alkaliphiles, through simple whole-cell biotransformation or
enzymatic biocatalysis, can potentially be used to produce an array of valuable
organic compounds.
Alkaliphiles and their enzymes have been tried in various synthesis reactions, and
peptide synthesis is one of the interesting mediations achieved by these biocatalysts.
Unlike the conventional method of chemical synthesis, enzymatic peptide synthesis
is stereoselective, clean, and mild and avoids the time-consuming protectiondeprotection procedures [126]. Moreover, it may offer other technical advantages
which are difficult to achieve using the chemical method. For instance, production of
peptides which are longer than dipeptides has been a challenge for the chemicalbased synthetic route. However, it is possible to synthesize such peptides through
enzymatic synthesis route [127, 128]. Efficiency of peptide synthesis is better in
alkaline condition, which is considered as a natural process that happens around
hydrothermal vents and believed to be part of the prebiotic chemical evolution of
biological molecules. Recently, this reaction is mimicked and successfully used
in vitro to synthesize peptides on rock surfaces at pH 10–11 [129]. Thus, by
combining the effect of alkaline condition and catalytic efficiency of enzymes, one
expects better synthesis of longer peptides. Indeed, this has been demonstrated by
[130] who reported higher degree of peptide polymerization (DP) by enzymatic
reactions at alkaline medium than at neutral condition. This shows the potential of
alkaline active proteases in the synthesis of peptides for a variety of applications.
However, not all alkaline active proteases are equally applicable. Since the peptide
synthesis reactions are more efficient in nonaqueous medium, proteases which are
alkaline active and operationally stable in low water activity are more suitable.
There has been an effort in obtaining alkaline active proteases which are active at
low water activity. Alkaline active proteases from Aspergillus flavus, Bacillus
pseudofirmus SVB1, Pseudomonas aeruginosa PseA, Bacillus pumilus strain
CBS, Streptomyces sp. strain AB1, and Alcalase (the most well-known alkaline
active proteases) are known to be highly active and stable in the presence of organic
solvents, and their promising potential in peptide synthesis has been verified [131–
135]. Although it is possible that alkaline active proteases can be obtained from
Alkaliphiles: The Versatile Tools in Biotechnology
19