Bifunctional family 3 glycoside hydrolases from barley with alpha -L-arabinofuranosidase and beta -D-xylosidase activity. Characterization, primary structures, and COOH-terminal processing

• Published in: The Journal of biological chemistry 2003-02, Vol.278 (7), p.5377-5387
• Main Authors: Lee, Robert C, Hrmova, Maria, Burton, Rachel A, Lahnstein, Jelle, Fincher, Geoffrey B
• Format: Article
• Language: English
• Subjects: Amino Acid Sequence; Glycoside Hydrolases - analysis; Glycoside Hydrolases - metabolism; Hordeum - enzymology; Hydrolysis; Molecular Sequence Data; Plant Proteins - analysis; Plant Proteins - metabolism; Protein Processing, Post-Translational; Sequence Alignment; Substrate Specificity; Xylosidases - analysis; Xylosidases - metabolism
• ISSN: 0021-9258
• DOI: 10.1074/jbc.M210627200

An alpha-l-arabinofuranosidase and a beta-d-xylosidase, designated ARA-I and XYL, respectively, have been purified about 1,000-fold from extracts of 5-day-old barley (Hordeum vulgare L.) seedlings using ammonium sulfate fractional precipitation, ion exchange chromatography, chromatofocusing, and size-exclusion chromatography. The ARA-I has an apparent molecular mass of 67 kDa and an isoelectric point of 5.5, and its catalytic efficiency during hydrolysis of 4'-nitrophenyl alpha-l-arabinofuranoside is only slightly higher than during hydrolysis of 4'-nitrophenyl beta-d-xyloside. Thus, the enzyme is actually a bifunctional alpha-l-arabinofuranosidase/beta-d-xylosidase. In contrast, the XYL enzyme, which also has an apparent molecular mass of 67 kDa and an isoelectric point of 6.7, preferentially hydrolyzes 4'-nitrophenyl beta-d-xyloside, with a catalytic efficiency approximately 30-fold higher than with 4'-nitrophenyl alpha-l-arabinofuranoside. The enzymes hydrolyze wheat flour arabinoxylan slowly but rapidly hydrolyze oligosaccharide products released from this polysaccharide by (1 --> 4)-beta-d-xylan endohydrolase. Both enzymes hydrolyze (1 --> 4)-beta-d-xylopentaose, and ARA-I can also degrade (1 --> 5)-alpha-l-arabinofuranohexaose. ARA-I and XYL cDNAs encode mature proteins of 748 amino acid residues which have calculated molecular masses of 79.2 and 80.5 kDa, respectively. Both are family 3 glycoside hydrolases. The discrepancies between the apparent molecular masses obtained for the purified enzymes and those predicted from the cDNAs are attributable to COOH-terminal processing, through which about 130 amino acid residues are removed from the primary translation product. The genes encoding the ARA-I and XYL have been mapped to chromosomes 2H and 6H, respectively. ARA-I transcripts are most abundant in young roots, young leaves, and developing grain, whereas XYL mRNA is detected in most barley tissues.