worth noting that maltooligosaccharide-forming amylases are thought to be the
evolutionary link between α-amylases and CGTases and are referred to
as “intermediary” enzymes, thereby exhibiting features from both groups of enzymes
[57]. CGTases and α-amylases are the most closely related enzymes within the
α-amylase family, differing in their product specificity and reaction, whereby
α-amylases carry out hydrolysis while CGTases perform transglycosylation reactions
[3, 57]. An α-amylase from Anaerobranca gottschalkii, a thermoalkaliphile, was
reported to exhibit transglycosylation on maltooligosaccharides and also displayed
CGTase activity [58], while some amylases have been reported to hydrolyse CDs
with varying rates of hydrolysis [59–62].
Initial industrial production of cyclodextrins from starch employed the use of
a CGTase from a neutralophilic Bacillus macerans strain. However, the enzyme
did not yield much product due to the low conversion rate, and the use of toxic
solvents for precipitating the cyclodextrins posed a major production problem [63].
These problems were overcome by the use of a CGTase from the alkaliphilic
Bacillus sp. strain 38-2, which resulted in yields of cyclodextrins at 85–90% from
amylose and 70–80% from potato starch on a laboratory scale, allowing the direct
crystallization of cyclodextrins from the hydrolysate without the use of organic
solvents [64]. As a result, the cost of β-CD reduced from $1,000/kg to $5/kg and
that of α-CD to within $15/kg [63].
Over the years, several other reports describing CGTase production from
alkaliphiles have been published, with a majority of the CGTases reported to
produce mixtures of α-, β- and γ-cyclodextrin at varying ratios [65–72]. However,
in a number of reports, the ratio of the final products was found to change depending
on the reaction time, type and concentration of substrate [68, 73]. Production of
mixtures of cyclodextrins makes the separation of the individual cyclodextrins
costly, and hence CGTases that produce predominantly a single type of cyclodextrin
are of more interest [74]. All known wild-type CGTases essentially produce a
mixture of α-, β- and γ-cyclodextrins, and as such, they have been further classified
into α-, β- and γ-CGTases according to their major cyclodextrin products [75].
Examples of alkaliphilic microorganisms that predominantly produce one
type of cyclodextrin include the CGTase from the thermoalkaliphilic anaerobic
Anaerobranca gottschalkii which preferentially produces α-cyclodextrin [68],
β-cyclodextrin producing CGTases from Bacillus agaradhaerens LS-3C [74],
Bacillus firmus [76], Amphibacillus sp. NPST-10 [77], Bacillus clausii E16a [78],
Bacillus pseudalcaliphilus 20RF [67] and γ-cyclodextrin producing CGTases from
Bacillus sp. G-825-6 [73], Brevibacterium sp. No. 9605 [79] and Bacillus clarkii
[80, 81]. In many of these reports, the reaction conditions and choice of substrate
were found to influence the type of cyclodextrin produced. Furthermore, genetic
manipulation of CGTases to enhance production of a specific cyclodextrin has
also been conducted on several enzymes [82–86].
There are also reports of production of large ring cyclodextrins – cyclodextrins
with greater than nine glucose units (CD 9 ) from various microorganisms, including
alkaliphiles [87]. However, only a few have been characterized, owing to their low
yields and difficulty in isolation from commercial cyclodextrin mixtures [88]. The
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S. O. Hashim
evolutionary link between α-amylases and CGTases and are referred to
as “intermediary” enzymes, thereby exhibiting features from both groups of enzymes
[57]. CGTases and α-amylases are the most closely related enzymes within the
α-amylase family, differing in their product specificity and reaction, whereby
α-amylases carry out hydrolysis while CGTases perform transglycosylation reactions
[3, 57]. An α-amylase from Anaerobranca gottschalkii, a thermoalkaliphile, was
reported to exhibit transglycosylation on maltooligosaccharides and also displayed
CGTase activity [58], while some amylases have been reported to hydrolyse CDs
with varying rates of hydrolysis [59–62].
Initial industrial production of cyclodextrins from starch employed the use of
a CGTase from a neutralophilic Bacillus macerans strain. However, the enzyme
did not yield much product due to the low conversion rate, and the use of toxic
solvents for precipitating the cyclodextrins posed a major production problem [63].
These problems were overcome by the use of a CGTase from the alkaliphilic
Bacillus sp. strain 38-2, which resulted in yields of cyclodextrins at 85–90% from
amylose and 70–80% from potato starch on a laboratory scale, allowing the direct
crystallization of cyclodextrins from the hydrolysate without the use of organic
solvents [64]. As a result, the cost of β-CD reduced from $1,000/kg to $5/kg and
that of α-CD to within $15/kg [63].
Over the years, several other reports describing CGTase production from
alkaliphiles have been published, with a majority of the CGTases reported to
produce mixtures of α-, β- and γ-cyclodextrin at varying ratios [65–72]. However,
in a number of reports, the ratio of the final products was found to change depending
on the reaction time, type and concentration of substrate [68, 73]. Production of
mixtures of cyclodextrins makes the separation of the individual cyclodextrins
costly, and hence CGTases that produce predominantly a single type of cyclodextrin
are of more interest [74]. All known wild-type CGTases essentially produce a
mixture of α-, β- and γ-cyclodextrins, and as such, they have been further classified
into α-, β- and γ-CGTases according to their major cyclodextrin products [75].
Examples of alkaliphilic microorganisms that predominantly produce one
type of cyclodextrin include the CGTase from the thermoalkaliphilic anaerobic
Anaerobranca gottschalkii which preferentially produces α-cyclodextrin [68],
β-cyclodextrin producing CGTases from Bacillus agaradhaerens LS-3C [74],
Bacillus firmus [76], Amphibacillus sp. NPST-10 [77], Bacillus clausii E16a [78],
Bacillus pseudalcaliphilus 20RF [67] and γ-cyclodextrin producing CGTases from
Bacillus sp. G-825-6 [73], Brevibacterium sp. No. 9605 [79] and Bacillus clarkii
[80, 81]. In many of these reports, the reaction conditions and choice of substrate
were found to influence the type of cyclodextrin produced. Furthermore, genetic
manipulation of CGTases to enhance production of a specific cyclodextrin has
also been conducted on several enzymes [82–86].
There are also reports of production of large ring cyclodextrins – cyclodextrins
with greater than nine glucose units (CD 9 ) from various microorganisms, including
alkaliphiles [87]. However, only a few have been characterized, owing to their low
yields and difficulty in isolation from commercial cyclodextrin mixtures [88]. The
232
S. O. Hashim