Aspergillus oryzae is a safe microorganism that is commonly used in food production. We constructed a self-cloning vector capable of high expression in A. oryzae. Using the vector, three putative pectin methylesterase (PME) genes belonging to Carbohydrate Esterase family 8 derived from A. oryzae were expressed, and several characteristics of the gene products were examined. The effects of temperature and pH on the three enzymes (AoPME1, 2, and 3) were similar, with optimal reaction temperatures of 50 - 60 °C and optimal reaction pH range of 5 - 6. The specific activities of AoPME1, 2, and 3 for apple pectin were significantly different (34, 7,601, and 2 U/mg, respectively). When the substrate specificity was examined, AoPME1 showed high activity towards pectin derived from soybean and pea. Although AoPME2 showed little activity towards these pectins, it showed very high activity towards apple- and citrus-derived pectins. AoPME3 showed low specific activity towards all substrates tested. Sugar composition analysis revealed that apple- and citrus-derived pectins were rich in homogalacturonan, while soybean- and pea-derived pectins were rich in xylogalacturonan. When pea pectin was treated with endo-polygalacturonase or endo-xylogalacturonase in the presence of each PME, specific synergistic actions were observed (endo-polygalacturonase with AoPME1 or AoPME2 and endo-xylogalacturonase with AoPME1 or AoPME3). Thus, AoPME1 and AoPME3 hydrolyzed the methoxy group in xylogalacturonan. This is the first report of this activity in microbial enzymes. Our findings on the substrate specificity of PMEs should lead to the determination of the distribution of methoxy groups in pectin and the development of new applications in the field of food manufacturing.
Keywords: Aspergillus oryzae; Homogalacturonan; Pectin methylesterase; Self-cloning vector; Substrate specificity; Xylogalacturonan.
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