50 PHYSIOLOGY OF THE FUNGI 



easily hydrolyzed. Glucose is likewise the end product of hydrolysis. 

 The enzyme (or enzymes) which catalyzes the hydrolysis of starch is 

 called amylase. In general, the end product of enzymatic hydrolysis of 

 starch is maltose and glucose. The various intermediate degradation 

 products are called dextrins. 



Starch appears to be composed of two main types of compounds: 

 amylose (20 to 25 per cent) and amylopectin. Amylose appears to con- 

 sist of long, unbranched molecules containing some 300 glucose residues, 

 whereas amylopectin has a branched structure. There are two types of 

 amylase: jS-amylase, which hydrolyzes off two glucose residues at a time 

 to form maltose, and oi-amylase, which attacks the 1,4-glucosidic linkages 

 in such a way as to produce starch fragments (dextrins) as the primary 

 products. The dextrins are further hydrolyzed to form maltose and some 

 glucose. The primary function of a-amylase is thus liquefaction; that of 

 the /3-amylase is saccharification. The Aspergillus amylases are of the 

 alpha type. The student is referred to the excellent reviews of Hopkins 

 (1946) and Myrback (1948) for critical summaries of amylase activity. 

 Amylase is widely distributed among the fungi but is not universal. 



Pectinase. The pectins are colloidal carbohydrate-like compounds 

 found in fruits and in the middle lamellae of plants. Many fungi produce 

 pectinase, which catalyzes the hydrolysis of pectin. When the pectin is 

 hydrolyzed, the cells fall apart. Harter and Weimer (1921) tested the 

 ability of nine species of Rhizoyus to produce pectinase in culture but were 

 unable to correlate the pathenogenicity of these species with the amount 

 of pectinase secreted. In fact, some of the pathogenic species {R. 

 nigricans and R. autocarpi) secreted less pectinase than did two non- 

 pathogenic species {R. chinensis and R. microsporus) . 



Pectins were formerly believed to yield a considerable variety of hydro- 

 lytic products, including acetic acid, galactose, and arabinose in addition 

 to methyl alcohol and D-galacturonic acid. More recent work indicates 

 that pectins are methylated polymers of D-galacturonic acid (Schneider 

 and Bock, 1937). The chemistry and physiology of the pectins have been 

 reviewed by Bonner (1936). 



Proteinases and peptidases. These enzymes, also called proteolytic 

 enzymes, catalyze the hydrolysis (and synthesis) of proteins and peptides. 

 These enzymes have been separated into two groups upon the basis of 

 ability to attack native protein. Those enzymes which act upon intact 

 proteins are called proteinases, while those which attack peptides are 

 called peptidases. It seems that the fundamental difference between 

 these two classes of enzymes lies in the point of attack. The proteinases 

 attack the protein molecule in such a way as to produce various peptides 

 as well as amino acids, while the peptidases act only on the ends of the 

 peptide chains. This is analogous to the action of the two amylases. 



