Endo-xylanases are the major enzymes that hydrolyze the main chain of xylan.
Since xylan cannot be directly transported to cell, the initial hydrolysis happens
extracellularly. Therefore, the endo-β-1,4-xylanases should be secreted extracellularly, a phenomenon that often requires leader peptide as shown in Table 3. In the
early phase of the hydrolysis, these enzymes form xylooligosaccharides from xylan,
and toward the end xylobiose, xylotriose, and xylotetraose accumulate as major end
products, while xylose appears upon prolonged incubation. The smaller oligos with
side chains can be transported to the cell and probably the completion of the
hydrolysis accomplished within the cell. Thus, most of the accessory enzymes
might be cell associated, and this may one of the reasons why there is no detectable
leader peptide for the B. halodurans accessory enzymes (Table 3). However, the
absence of the leader peptide cannot confirm that the enzymes are intracellular.
5.1.2 Mannanases
Mannan and heteromannan (glucomannan and galactomannan) are among the
important hemicelluloses, and there has been biotechnological interest in enzymes
that modify these carbohydrates. Among the enzymes, β-mannanases which break
the β-1,4-mannosidic linkages in the main chain of mannan and heteromannan have
attracted a great deal of interest and got several applications in food, detergent, and
pulp and paper industries [113–115]. The requirement of alkaline active mannanases
such as in detergent formulation and Kraft pulp production has contributed to
initiation of the search for alkaline active mannanases. Today, several alkaline active
Table 3 Hemicellulose-degrading enzymes identified in the genome sequence of B. halodurans
C-125
Enzyme
Number Gene name GenBank ID
Family Leader peptide
Endo-β-1,4-xylanase
2
BH0899
BAB04618.1 GH11
Yes
BH2120
BAA00055.1 GH10
Yes
Exo-β-1,4-xylosidase
3
BH1068
BAB04787.1 GH39
Not detected
BH3683
BAB07402.1 GH43
Not detected
BH2114
BAB05833.1 GH52
Not detected
Rex exo-oligoxylanase
a
1
BH2105
BAB05824.1 GH8
Not detected
α-L-Arabinofuranosidase 2
BH1861
BAB05580.1 GH51
Not detected
BH1874
BAB05593.1 GH51
Not detected
Acetylxylan esterase
2
BH3326
BAB07045.1 CE7
Not detected
BH1115
BAB04834.1 CE12
Not detected
α-Glucuronidase
1
BH1061
BAB04780.1 GH67
Not detected
α-Galactosidase
3
BH2223
BAB05942.1 GH36
Not detected
BH2228
BAB05947.1 GH4
Not detected
BH1870
b
BAB05589.1 GH27
Not detected
β-Glucosidase
1
BH1923
BAB05642.1 GH1
Not detected
a Reducing end xylose-releasing exo-oligoxylanase
b
Only the first part clearly belongs to galactosidase
Alkaline Active Hemicellulases
261
Since xylan cannot be directly transported to cell, the initial hydrolysis happens
extracellularly. Therefore, the endo-β-1,4-xylanases should be secreted extracellularly, a phenomenon that often requires leader peptide as shown in Table 3. In the
early phase of the hydrolysis, these enzymes form xylooligosaccharides from xylan,
and toward the end xylobiose, xylotriose, and xylotetraose accumulate as major end
products, while xylose appears upon prolonged incubation. The smaller oligos with
side chains can be transported to the cell and probably the completion of the
hydrolysis accomplished within the cell. Thus, most of the accessory enzymes
might be cell associated, and this may one of the reasons why there is no detectable
leader peptide for the B. halodurans accessory enzymes (Table 3). However, the
absence of the leader peptide cannot confirm that the enzymes are intracellular.
5.1.2 Mannanases
Mannan and heteromannan (glucomannan and galactomannan) are among the
important hemicelluloses, and there has been biotechnological interest in enzymes
that modify these carbohydrates. Among the enzymes, β-mannanases which break
the β-1,4-mannosidic linkages in the main chain of mannan and heteromannan have
attracted a great deal of interest and got several applications in food, detergent, and
pulp and paper industries [113–115]. The requirement of alkaline active mannanases
such as in detergent formulation and Kraft pulp production has contributed to
initiation of the search for alkaline active mannanases. Today, several alkaline active
Table 3 Hemicellulose-degrading enzymes identified in the genome sequence of B. halodurans
C-125
Enzyme
Number Gene name GenBank ID
Family Leader peptide
Endo-β-1,4-xylanase
2
BH0899
BAB04618.1 GH11
Yes
BH2120
BAA00055.1 GH10
Yes
Exo-β-1,4-xylosidase
3
BH1068
BAB04787.1 GH39
Not detected
BH3683
BAB07402.1 GH43
Not detected
BH2114
BAB05833.1 GH52
Not detected
Rex exo-oligoxylanase
a
1
BH2105
BAB05824.1 GH8
Not detected
α-L-Arabinofuranosidase 2
BH1861
BAB05580.1 GH51
Not detected
BH1874
BAB05593.1 GH51
Not detected
Acetylxylan esterase
2
BH3326
BAB07045.1 CE7
Not detected
BH1115
BAB04834.1 CE12
Not detected
α-Glucuronidase
1
BH1061
BAB04780.1 GH67
Not detected
α-Galactosidase
3
BH2223
BAB05942.1 GH36
Not detected
BH2228
BAB05947.1 GH4
Not detected
BH1870
b
BAB05589.1 GH27
Not detected
β-Glucosidase
1
BH1923
BAB05642.1 GH1
Not detected
a Reducing end xylose-releasing exo-oligoxylanase
b
Only the first part clearly belongs to galactosidase
Alkaline Active Hemicellulases
261