CARBON SOURCES 117 



carbon in the chain. Sugars having an aldehyde group are called aldoses, 

 those having ketone group, ketoses; the ending -ose denotes a sugar. In 

 addition, the sugars are further classified according to the number of 

 carbon atoms in the chain, e.g., pentoses, hexoses, or more specifically 

 as aldopentoses, ketohexoses, etc. While it will be necessary in the 

 discussion to follow to include some information about the chemistry 

 and structure of the sugars, the reader is advised to consult suitable 

 texts for further information. Those of Oilman (1943) and Pigman and 

 Goepp (1948) are recommended. 



Compounds which have the same composition and the same molecular 

 weight are called isomers. There are 16 aldohexoses (32, if the alpha 

 and beta forms are considered), which have the same percentage com- 

 position and the same functional groups as glucose (dextrose). There 

 are eight possible ketohexoses isomeric with fructose. Two kinds of 

 isomers exist among the sugars: First, there are those which have the 

 same physical properties but differ in the direction in which they rotate 

 plane-polarized light (enantiomorphs) . Isomers of this kind occur in 

 pairs, and the configuration of the functional groups of one isomer is the 

 mirror image of the configuration of the other. Enantiomorphs usually 

 differ physiologically. One such isomer may be utilized and the other 

 not, or one may be utilized much more rapidly than the other. Pasteur 

 (1860) was the first to demonstrate that fungi are able to distinguish 

 between such isomers. Penicillium glaucum utilized c?-tartrate more 

 rapidly than Z-tartrate {d and / refer to optical rotation). Second, there 

 are those isomers which, although they have the same functional groups, 

 have these groups arranged in a different order, so that one isomer is not 

 the mirror image of the other (diastereoisomers). It is usually safe to 

 assume that one member of a pair of enantiomorphs will be better 

 utilized than the other, but such an assumption about utilization of 

 diastereoisomers is not possible. 



Since not all sugars of a group such as the aldohexoses are utilized by 

 fungi, it is of interest to compare chemical structure or configuration with 

 utilization. Not all fungi are able to utilize exactly the same sugars 

 (Fig. 20). Whether a sugar is utilized or not depends upon both the 

 configuration of the sugar and the particular abihties of the specific 

 fungus. By configuration is meant the spatial arrangement of the 

 hydrogen and hydroxyl groups. The long history of chemical investi- 

 gation which established the configuration of the simple sugars must be 

 passed by. Inasmuch as glucose is the key compound in sugar chemistry, 

 as well as in physiology, particular emphasis will be devoted to this 

 aldose. 



The structures of the glucose enantiomorphs are given at the top of 

 page 119. 



