Purification and characterization of an exo-1,4-beta-galactanase from a strain of Bacillus subtilis

Peculiarities and applications of galactanolytic enzymes that act on type I and II arabinogalactans

Arabinogalactans (AGs) are branched galactans to which arabinose residues are bound as side chains and are widely distributed in plant cell walls. They can be grouped into two types based on the structures of their backbones. Type I AGs have β-1,4-galactan backbones and are often covalently linked to the rhamnogalacturonan-I region of pectins. Type II AGs have β-1,3-galactan backbones and are often covalently linked to proteins. The main enzymes involved in the degradation of AGs are endo-β-galactanases, exo-β-galactanases, and β-galactosidases, although other enzymes such as α-L-arabinofuranosidases, β-L-arabinopyranosidases, and β-D-glucuronidases are required to remove the side chains for efficient degradation of the polysaccharides. Galactanolytic enzymes have a wide variety of potential uses, including the bioconversion of AGs to fermentable sugars for production of commodity chemicals like ethanol, biobleaching of cellulose pulp, modulation of pectin properties, improving animal feed, and determining the chemical structure of AGs. This review summarizes our current knowledge about the biochemical properties and potential applications of AG-degrading enzymes.

Metabolism of a plant derived galactose-containing polysaccharide by Bifidobacterium breve UCC2003

In this study, we describe the functional characterization of the Bifidobacterium breve UCC2003 gal locus, which is dedicated to the utilization of galactan, a plant‐derived polysaccharide. Using a combination of molecular approaches we conclude that the galA gene of B. breve UCC2003 encodes a β‐1,4‐endogalactanase producing galacto‐oligosaccharides, which are specifically internalized by an ABC transport system, encoded by galBCDE, and which are then hydrolysed to galactose moieties by a dedicated intracellular β‐galactosidase, specified by galG. The generated galactose molecules are presumed to be fed into the fructose‐6‐phosphate phosphoketolase pathway via the Leloir pathway, thereby allowing B. breve UCC2003 to use galactan as its sole carbon and energy source. In addition to these findings we demonstrate that GalR is a LacI‐type DNA‐binding protein, which not only appears to control transcription of the galCDEGR operon, but also that of the galA gene.

Bifidobacterium longum endogalactanase liberates galactotriose from type I galactans

A putative endogalactanase gene classified into glycoside hydrolase family 53 was revealed from the genome sequence of Bifidobacterium longum strain NCC2705 (Schell et al., Proc. Natl. Acad. Sci. USA 99:14422-14427, 2002). Since only a few endo-acting enzymes from bifidobacteria have been described, we have cloned this gene and characterized the enzyme in detail. The deduced amino acid sequence suggested that this enzyme was located extracellularly and anchored to the cell membrane. galA was cloned without the transmembrane domain into the pBluescript SK(-) vector and expressed in Escherichia coli. The enzyme was purified from the cell extract by anion-exchange and size exclusion chromatography. The purified enzyme had a native molecular mass of 329 kDa, and the subunits had a molecular mass of 94 kDa, which indicated that the enzyme occurred as a tetramer. The optimal pH of endogalactanase activity was 5.0, and the optimal temperature was 37 degrees C, using azurine-cross-linked galactan (AZCL-galactan) as a substrate. The K(m) and V(max) for AZCL-galactan were 1.62 mM and 99 U/mg, respectively. The enzyme was able to liberate galactotrisaccharides from (beta1-->4)galactans and (beta1-->4)galactooligosaccharides, probably by a processive mechanism, moving toward the reducing end of the galactan chain after an initial midchain cleavage. GalA's mode of action was found to be different from that of an endogalactanase from Aspergillus aculeatus. The enzyme seemed to be able to cleave (beta1-->3) linkages. Arabinosyl side chains in, for example, potato galactan hindered GalA.

Transglycosylation catalyzed by a Penicillium chrysogenum exo-1,5-α-l-arabinanase

Penicillium chrysogenum exo-arabinanase (Abnx), which releases arabinobiose from the nonreducing terminus of alpha-1,5-L-arabinan, was found to possess trans-arabinobiosylation activity on various acceptors, such as aliphatic alcohols, sugars, and sugar alcohols. Abnx was found to prefer primary hydroxyl groups in polyhydric alcohols as acceptors over primary hydroxyl groups in monohydric alcohols. Among the 21 different compounds tested, glycerol was the best acceptor for the enzyme. The transfer product of glycerol was identified as O-alpha-L-arabinosyl-(1-->5)-O-alpha-L-arabinosyl-(1-->1)-glycerol on the basis of the spectral data, fast atom bombardment-mass and 1H- and 13C-NMR. Unlike endo-arabinanases, Abnx catalyzed the hydrolysis of linear arabinan without inverting the anomeric configuration.

The beta-1,4-endogalactanase A gene from Aspergillus niger is specifically induced on arabinose and galacturonic acid and plays an important role in the degradation of pectic hairy regions

The Aspergillus nigerbeta-1,4-endogalactanase encoding gene (galA) was cloned and characterized. The expression of galA in A. niger was only detected in the presence of sugar beet pectin, d-galacturonic acid and l-arabinose, suggesting that galA is coregulated with both the pectinolytic genes as well as the arabinanolytic genes. The corresponding enzyme, endogalactanase A (GALA), contains both active site residues identified previously for the Pseudomonas fluorescensbeta-1,4-endogalactanase. The galA gene was overexpressed to facilitate purification of GALA. The enzyme has a molecular mass of 48.5 kDa and a pH optimum between 4 and 4.5. Incubations of arabinogalactans of potato, onion and soy with GALA resulted initially in the release of d-galactotriose and d-galactotetraose, whereas prolonged incubation resulted in d-galactose and d-galactobiose, predominantly. MALDI-TOF analysis revealed the release of l-arabinose substituted d-galacto-oligosaccharides from soy arabinogalactan. This is the first report of the ability of a beta-1,4-endogalactanase to release substituted d-galacto-oligosaccharides. GALA was not active towards d-galacto-oligosaccharides that were substituted with d-glucose at the reducing end.

Exo-arabinanase of Penicillium chrysogenum able to release arabinobiose from alpha-1,5-L-arabinan

An exo-arabinanase, designated Abnx, was purified from a culture filtrate of Penicillium chrysogenum 31B by ammonium sulfate precipitation, anion-exchange chromatography, and hydrophobic chromatography. Abnx had an apparent molecular mass of 47 kDa. The enzyme released only arabinobiose from the nonreducing terminus of alpha-1,5-L-arabinan and showed no activity towards p-nitrophenyl-alpha-L-arabinofuranoside and alpha-1,5-L-arabinofuranobiose. Abnx is the first enzyme with this mode of action.

Preliminary characterization of a new exo-beta-(1,4)-galactanase with transferase activity

As a prerequisite to the study of the fine chemical structure of the branched region of pectin, an exo-beta-(1,4)-galactanase was purified from a commercial preparation (Pectinex AR). Purification was carried out by precipitation with 70% saturated ammonium sulfate, preparative electrofocusing, anion-exchange chromatography and affinity chromatography on cross-linked alginate. Exogalactanase specific activity was 992 nkat mg-1 and the enzyme was devoid of beta-(1,3)- or beta-(1,6)-galactanase, arabinanase, beta-D-galactosidase and alpha-L-arabinofuranosidade activities. Residual exopolygalacturonase activity represented 2.9% of the galactanase activity. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and isoelectric focusing showed two close bands with molecular weights of 120,000 and 90,000 and pHi of 3.8 and 4.1, respectively. The enzyme acted in an exo manner and its activity was optimum at pH 3.5 and 60 degrees C. When incubated with galacto-oligosaccharides, new oligosaccharides with a higher degree of polymerization appeared, indicating the ability of the enzyme to transfer galactose residues.