Characterization of Bacillus halodurans alpha-galactosidase Mel4A encoded by the mel4A gene (BH2228)

The melREDCA Operon Encodes a Utilization System for the Raffinose Family of Oligosaccharides in Bacillus subtilis

Bacillus subtilis is a heterotrophic soil bacterium that hydrolyzes different polysaccharides mainly found in the decomposed plants. These carbohydrates are mainly cellulose, hemicellulose, and the raffinose family of oligosaccharides (RFOs). RFOs are soluble α-galactosides, such as raffinose, stachyose, and verbascose, that rank second only after sucrose in abundance. Genome sequencing and transcriptome analysis of B. subtilis indicated the presence of a putative α-galactosidase-encoding gene (melA) located in the msmRE-amyDC-melA operon. Characterization of the MelA protein showed that it is a strictly Mn²⁺- and NAD⁺-dependent α-galactosidase able to hydrolyze melibiose, raffinose, and stachyose. Transcription of the msmER-amyDC-melA operon is under control of a σ^A-type promoter located upstream of msmR (P _msmR ), which is negatively regulated by MsmR. The activity of P _msmR was induced in the presence of melibiose and raffinose. MsmR is a transcriptional repressor that binds to two binding sites at P _msmR located upstream of the -35 box and downstream of the transcriptional start site. MsmEX-AmyCD forms an ATP-binding cassette (ABC) transporter that probably transports melibiose into the cell. Since msmRE-amyDC-melA is a melibiose utilization system, we renamed the operon melREDCA IMPORTANCE Bacillus subtilis utilizes different polysaccharides produced by plants. These carbohydrates are primarily degraded by extracellular hydrolases, and the resulting oligo-, di-, and monosaccharides are transported into the cytosol via phosphoenolpyruvate-dependent phosphotransferase systems (PTS), major facilitator superfamily, and ATP-binding cassette (ABC) transporters. In this study, a new carbohydrate utilization system of B. subtilis responsible for the utilization of α-galactosides of the raffinose family of oligosaccharides (RFOs) was investigated. RFOs are synthesized from sucrose in plants and are mainly found in the storage organs of plant leaves. Our results revealed the modus operandi of a new carbohydrate utilization system in B. subtilis.

Both Chemical and Non-Chemical Steps Limit the Catalytic Efficiency of Family 4 Glycoside Hydrolases

The glycoside hydrolase family 4 (GH4) α-galactosidase from Citrobacter freundii (MelA) catalyzes the hydrolysis of fluoro-substituted phenyl α-d-galactopyranosides by utilizing two cofactors, NAD⁺ and a metal cation, under reducing conditions. In order to refine the mechanistic understanding of this GH4 enzyme, leaving group effects were measured with various metal cations. The derived β_lg value on V/ K for strontium activation is indistinguishable from zero (0.05 ± 0.12). Deuterium kinetic isotope effects (KIEs) were measured for the activated substrates 2-fluorophenyl and 4-fluorophenyl α-d-galactopyranosides in the presence of Sr²⁺, Y³⁺, and Mn²⁺, where the isotopic substitution was on the carbohydrate at C-2 and/or C-3. To determine the contributing factors to the virtual transition state (TS) on which the KIEs report, kinetic isotope effects on isotope effects were measured on these KIEs using doubly deuterated substrates. The measured ^D V/ K KIEs for MelA-catalyzed hydrolysis of 2-fluorophenyl α-d-galactopyranoside are closer to unity than the measured effects on 4-fluorophenyl α-d-galactopyranoside, irrespective of the site of isotopic substitution and of the metal cation activator. These observations are consistent with hydride transfer at C-3 to the on-board NAD⁺, deprotonation at C-2, and a non-chemical step contributing to the virtual TS for V/ K.

Hooked on α-d-galactosidases: from biomedicine to enzymatic synthesis

α-d-Galactosidases (EC 3.2.1.22) are enzymes employed in a number of useful bio-based applications. We have depicted a comprehensive general survey of α-d-galactosidases from different origin with special emphasis on marine example(s). The structures of natural α-galactosyl containing compounds are described. In addition to 3D structures and mechanisms of action of α-d-galactosidases, different sources, natural function and genetic regulation are also covered. Finally, hydrolytic and synthetic exploitations as free or immobilized biocatalysts are reviewed. Interest in the synthetic aspects during the next years is anticipated for access to important small molecules by green technology with an emphasis on alternative selectivity of this class of enzymes from different sources.

Characterization and site-directed mutagenesis of an α-galactosidase from the deep-sea bacterium Bacillus megaterium

A novel gene (BmelA) (1323bp) encoding an α-galactosidase of 440 amino acids was cloned from the deep-sea bacterium Bacillus megaterium and the protein was expressed in Escherichia coli BL21 (DE3) with an estimated molecular mass of about 45 kDa by SDS-PAGE. The enzyme belongs to glycoside hydrolase family 4, with the highest identity (74%) to α-galactosidase Mel4A from Bacillus halodurans among the characterized α-galactosidases. The recombinant BmelA displayed its maximum activity at 35 °C and pH 8.5-9.0 in 50 mM Tris-HCl buffer, and could hydrolyze different substrates with the Km values against p-nitrophenyl-α-D-galactopyranoside (pNP-α-Gal), raffinose and stachyose being 1.02±0.02, 2.24±0.11 and 3.42±0.17 mM, respectively. Besides, 4 mutants (I38 V, I38A, I38F and Q84A) were obtained by site-directed mutagenesis based on molecular modeling and sequence alignment. The kinetic analysis indicated that mutants I38 V and I38A exhibited a 1.7- and 1.4-fold increase over the wild type enzyme in catalytic efficiency (k(cat)/K(m)) against pNP-α-Gal, respectively, while mutant I38F showed a 3.5-fold decrease against pNP-α-Gal and mutant Q84A almost completely lost its activity. All the results suggest that I38 and Q84 sites play a vital role in enzyme activity probably due to their steric and polar effects on the predicted "tunnel" structure and NAD+ binding to the enzyme.

α-Galacturonidase(s): a new class of Family 4 glycoside hydrolases with strict specificity and a unique CHEV active site motif

The catalytic activity of the Family 4 glycosidase LplD protein, whose active site motif is CHEV, is unknown despite its crystal structure having been determined in 2008. Here we identify that activity as being an α-galacturonidase whose natural substrate is probably α-1,4-di-galacturonate (GalUA2). Phylogenetic analysis shows that LplD belongs to a monophyletic clade of CHEV Family 4 enzymes, of which four other members are also shown to be galacturonidases. Family GH 4 enzymes catalyze the cleavage of the glycosidic bond, via a non-canonical redox-assisted mechanism that contrasts with Koshland's double-displacement mechanism.