ABSTRACT
The first rhamnogalacturonan-degrading enzyme was discovered during fractionation of a commercial enzyme preparation derived from Aspergillus aculeatus (13). The enzyme acted exclusively on RG, and showed no activity on homogalacturonan. A few years later, this enzyme, as well as another rhamnogalacturonase from A. aculeatus, was cloned. The enzymes were termed RGaseA and RGaseB, respectively (14, 15). It appeared that the two enzymes differed in their catalytic mechanism. RGaseA is a hydrolase, meaning that it cleaves the RG backbone with the concomitant uptake of a water molecule. Degradation of RG by RGaseB follows a -eliminative reaction; a double bond is formed between C-4 and C-5 of the nonreducing GalA residues of the reaction products (16, 17). This reaction can be monitored spectrophotometrically at 235 nm. Therefore, RGaseA and RGaseB were renamed to endorhamnogalacturonan hydrolase (endoRGH) and endorhamnogalacturonan lyase (endoRGL), respectively. Thus, it seems that fungi have acquired a similar set of enzymes for RG degradation as for that of homogalacturonan (endoPG, endoPAL, endoPL).
