PLANT STRUCTURE AND COMPOSITION 



H2SO4 , , . H2SO4 

 Xylans or arabans > xylose or arabinose > HC— 



HC 



185 



-CH 



II 



C— CHO 



O 



furfural 



Pectic Substances 



Pectic substances form an important group of compounds occurring 

 in the primary cell wall and the intercellular layers of land plants. 

 Structurally, the pectins contain an admixture of three polymers: (1) 

 D-galacturonic methyl ester, (2) D-galactans, and (3) L-arabans. The 

 close association of these three polymers has led some workers to sug- 

 gest that the uronic acid polymer arises from the D-galactan through 

 the oxidation of the primary alcohol (number 6 carbon). Decarboxy- 

 lation of the uronic polymer could give rise to the L-araban polymer. 



OH 



OH 



OH 



HC- 



HCOH 



1 

 HOCH 



1 

 HOCH 



1 

 HC 



HC- 



O 



HCOH 

 -^"^2 HOCH 



HC- 



O 



■411 



HOCH 



1 

 HC— 



HCOH 

 ^^ HOCH 



HOCH 



1 

 H2C— 



O 



CH2OH 



D-galactose 



COOH 



D-galacturonic 

 acid 



L-arabiiiose 



The theory, while attractive, is untenable because: (1) the araban in 

 pectin contains arabinose with the furano ring, while the galactose is 

 pyranose in character, and (2) the galactan polymer is linear as com- 

 pared with the highly branched araban structure. The synthetic 

 routes involved in the building of the pectin components remain 

 obscure. 



Young growing fruits first lay down protopectin, a water-insoluble 

 pectic substance of high molecular weight. As the fruit ripens, the 

 protopectin is hydrolyzed to the smaller water-soluble pectins by the 

 enzyme protopectinase. Upon further ripening, the resulting pectin is 

 subjected to attack by two more enzymes, pectases and pectinases. The 

 pectases hydrolyze the ester groups to produce pectic acid, and pec- 

 tinases hydrolyze the glycosidic linkages to produce uronic acid and 

 intermediate polymers. 



