pgaE encodes a fourth member of the endopolygalacturonase gene family from Aspergillus niger

Abstract

In the present study, the molecular and basic biochemical characterization of endopolygalacturonase E, the fourth Aspergillus niger N400 endopolygalacturonase, is reported. The entire endopolygalacturonase E gene consists of 1293 bp interrupted by three short introns (50, 50, and 59 bp, respectively) as concluded from the cDNA sequence. The deduced amino acid sequence comprises 378 residues that include 39 N-terminal amino acids of the prepropeptide. The calculated M(r) and pI of the mature protein are 35,584 and 3.6, respectively. Compared with other endopolygalacturonases from A. niger N400, the mature protein endopolygalacturonase E has the highest sequence identity with endopolygalacturonase C (77.6%) followed by endopolygalacturonase I (57.6%) and endopolygalacturonase II (54.3%). For overproduction of endopolygalacturonase E, an A. niger multicopy strain was used that was transformed with a promoter gene fusion construct that directs expression from the glycolytic A. niger pyruvate kinase promoter. The enzyme was purified and characterized as an endopolygalacturonase based on product analysis after polygalacturonate hydrolysis and on bond cleavage frequencies of oligogalacturonates of different degree of polymerisation (n = 2-7). The pH optimum was 3.8. The K(m) and V(max) for polygalacturonate hydrolysis were 2.5 ± 0.4 mg · ml-1 and 1.3 ± 0.2 μkat · mg-1, respectively. A subsite map was calculated by the combination of the methods of Suganuma et al. [Suganuma, T., Matsuno, R., Ohnishi, M. and Hiromi, K. (1978) J. Biochem. (Tokyo) 84, 293-316] and Nitta et al. [Nitta, Y., Mizushima, M., Hiromi, K. and One, S. (1971) J. Biochem. (Tokyo) 69, 567-576]. This indicated that the enzyme was composed of at least five subsites.