Ill CARBON METABOLISM II 



(166, 107). As might be expected, the wood-destroying basidiomycetes 

 are especially active in the hydrolysis of hemicelluloses (31, 189); the 

 gross utilization of hemicelluloses has been mentioned earlier (Chapter 

 3). The only other fungi which have been studied are some members 

 of the Saprolegniaceae (21, 201), and of the genus Phytophthora (147), 

 all of which form reducing sugars from the crude hemicellulose of 

 date endosperm. 



The most abundant of the pentosans are xylans; the rarer arabans 

 are found in association with pectic substances. The limited literature 

 on fungus metabolism of xylans is reviewed by Voss and Butter (240). 

 Enzymes (xylanases) attacking xylan have been found in Aspergillus 

 oryzac (90) and Glomerclla cingulata (96). The xylanase of Chae- 

 tomium globosum is formed in appreciable amounts only if the 

 organism is grown on xylan, and consists of an exocellular component 

 converting xylan to xylobiose and an intracellular enzyme hydrolyzing 

 thexylobiose (217, 218). 



Arabans — pentosans made up largely of L-arabinose — are hydrolyzed 

 by a crude enzyme preparation from Aspergillus oryzae (67). 



Simpson (212) surveyed the action of 112 isolates of 24 genera on a 

 purified pentosan of unspecified composition. An active hydrolysis 

 was brought about by the culture filtrate of about half of the isolates 

 tested; the most active filtrates were from cultures of strains of 

 Aspergillus, Fusarium, Tricothecium, and Trichoderma. Of the 54 

 active strains, only 6 formed the enzyme during growth on glucose; 

 in the rest, growth on the pentosan — replaceable in some instances by 

 free pentoses — appeared to be necessary for enzyme synthesis. In the 

 genus Streptomyces, 5 of the 19 isolates tested formed an extracellular 

 enzyme active on the pentosan. 



A purified arabogalactan from Larix is split by an enzyme from 

 Aspergillus niger (189). Mannans are hydrolyzed by an enzyme of 

 Chaetomium globosum which is apparently distinct from the enzyme 

 acting on xylans (217). 



The chemistry of the substrates is poorly known and the nature of 

 the enzymes involved in their hydrolysis is uncertain. In particular, 

 it is not possible from the data available to decide on the possible 

 identity or non-identity of enzymes which have been studied by 

 different workers. It is generally accepted that the enzymes attacking 

 the pentosans and the crude hemicelluloses are distinct from cellulose- 

 hydrolyzing enzymes and from lichenase (180), but beyond this point 

 specificity is not known. 



The Breakdown of Pectic Substances. Perhaps nowhere in the 

 field of carbon metabolism is the confusion so great as it is in the 



