Figure 12.3 Production of extracellular H2O2 by wood rotting fungi. (From Hammel (1987). With permission of CAB International.)

secreted by the fungus (Figure 12.3). Wood decay by white rot fungi requires the cooperative action of lignin peroxidases and glyoxal oxidase. White-rot fungi synthesize and secrete veratryl alcohol, a substrate for its own peroxidase enzymes.

In liquid-grown P. chrysosporium cultures, ligninolytic activity appeared after primary growth of the fungus ceased due to nutrient limitation (Jeffries et al., 1981), raising the question whether lignin degradation is a growth-associated process. A different picture emerged using a technique that allowed for simultaneous monitoring of growth as well as secretion of enzymes by the hyphae. Moukha et al. (1993) grew the fungus on an agar medium sandwiched between two perforated membranes (Figure 1.9). This arrangement allowed the free uptake of nutrients, autoradiographic detection of growth by the application of radioactively labeled acetylglucosamine (a chitin precursor) and the detection of secreted enzyme using lignin peroxidase antibodies. Though the radial growth of the fungal colony stopped, new short branches were initiated at the colony center that secreted Mn2+-dependent lignin peroxidase. The results suggested that although primary growth of the fungus was over, nonetheless, secondary branches were produced, apparently by the recycling of intracellular metabolites. In surface-grown cultures, this period coincided with accumulation of RNA transcripts and secretion of ligninase. Electron microscopy, cytochemistry and immunogold labeling of P. chrysosporium growing in wood show that lignin degrading activity (LiP and MnP) is associated with the mucilage (glucan) sheath at the apical regions of the hyphal cell-wall surface (Daniel et al., 1989; Daniel et al., 1990; Ruel and Joseleau, 1991).

12.2.2 Cellulose Degradation

Cellulose is the most abundant organic compound in nature. It is a homopolymer of 10,000 or more D-glucose units in (3(1-4) glycosidic linkage (Figure 12.4). This linkage results in the aggregation of the flat glucan chains, side-by side and above each other. Hydrogen bonds between the ring oxygen atom and the hydroxyl groups of glucose molecules result in the tight aggregation of the glucan chains in microfibrils. Their packing is so ordered

Figure 12.4 Cellulose, a polymer of 4 linked glucose.

0 0

Post a comment