Relationship of Exo-β-D-Galactofuranosidase Kinetic Parameters to the Number of Phosphodiesters in Penicillium fellutanum Peptidophosphogalactomannan: Enzyme Purification and Kinetics of Glycopeptide and Galactofuran Chain Hydrolysis

Extracellular Penicillium fellutanum exo-β-D-galactofuranosidase, with a mass of 70 kDa, was purified to apparent homogeneity. The enzyme was used to investigate the influence of phosphodiesters of the peptidophosphogalactomannans pP₂GMⁱⁱ and pP₂₅GMⁱⁱ (containing 2 and 25 phosphodiester residues, respectively, per mol of polymer) on the kinetic parameters of galactofuranosyl hydrolysis of these two polymers, of 1-O-methyl-β-D-galactofuranoside, and of two galactofuranooligosaccharides. The enzyme did not hydrolyze phosphorylated galactose residues of pP₂GMⁱⁱ or pP₂₅GMⁱⁱ The k_cat/K_m value for pP₂₅GMⁱⁱ is 1.7 × 10³ M^-1 s^-1, that for l-O-methyl-β-D-galactofuranoside is 1.1 × 10⁴ M^-1 s^-1 that for pP₂GMⁱⁱ is 1.7 × 10 ⁴ M^-1 s^-1, and those for 5-O-β-D-galactofuranooligosaccharides with degrees of polymerization of 3.4 and 5.5 are 1.7 × 10⁵ and 4.1 × 10⁵ M^-1 s^-1, respectively. Variability in the k_cat/K_m values is due primarily to differences in K_m values; the k_-1/k₁ ratio likely provides the most influence on K_m, k_cat increases as the degree of polymerization of galactofuranosyl residues increases. Most of the galactofuranosyl and phosphocholine residues were removed by day 8 in vivo from pP_xGMⁱⁱ added to day 3 cultures initiated in medium containing 2 mM phosphate but not from those initially containing 20 mM phosphate. The filtrates from day 9 cultures initiated in 2 mM inorganic phosphate in modified Raulin-Thom medium contained 0.2 mM inorganic phosphate and 2.2 U of galactofuranosidase ml^-1h^-1. No galactofuranosidase activity but 15 mM inorganic phosphate was found in filtrates from day 9 cultures initiated in 20 mM phosphate. In vivo the rate of galactofuranosyl hydrolysis of pP_xGMⁱⁱ and of related polymers is proportional to the k_cat/K_m value of each polymer. The kinetic data show that the k_cat/K_m value increases as the number of phosphodiesters of pP_xGMⁱⁱ decreases, also resulting in an increase in the activity of exo-β-D-galactofuranosidase.