Besides glucose, the digestion of foodstuffs and the utilization of endogenous metabolites can supply a variety of carbohydrates for glycolysis. These include the other monosaccharides (sugars), disaccharides, and polysaccharides. The metabolic routes for utiilizing substrates other than glucose in glycolysis are summarized in Figure 13.12.

Monosaccharide Sugars

Galactose, mannose, and fructose are the main sugars of interest. All can be phosphorylated at position six by hexokinase and ATP. In addition, fructose can enter the glycolytic pathway as fructose-1-phosphate, which is then broken down to D-glyceraldehyde and DHAP (see here). Galactose can also be converted to glucose via the UDP-galactose-4-epimerase catalyzed pathway (Figure 13.14)

Disaccharides

The enzyme lactase breaks down the disaccharide lactose to yield the monosaccharides galactose and glucose. Maltase breaks down maltose to yield two molecules of glucose. Sucrase breaks down sucrose to yield fructose and glucose. All of these monosaccharides subsequently enter glycolysis as shown in Figure 13.12.

 

Polysaccharides

Glycogen (from animals), starch (e.g., amylose and amylopectin from plants), and cellulose (from plants) are the main polysaccharides of interest. Amylose and cellulose are linear polymers, whereas glycogen and amylopectin are branched polymers.

The breakdown of glycogen requires two enzymes, glycogen phosphorylase and (1,4 ->1,4) glucantransferase (a so-called "debranching enzyme). Glycogen phosphorylase cleaves the (1->4) linkages between glucose units in the linear chains in glycogen, yielding glucose-1-phosphate in the process. Glycogen phosphorylase cannot cleave the (1->6) linkages at the branch points, however, but the debranching enzyme can, yielding glucose and more (1-> 4) linked chains (see Figure 13.17)

The breakdown of starch (amylose and amylopectin) also requires two enzymes, -amylase, and (1 ->6) glucosidase (another "debranching enzyme"). -Amylase cleaves the (1->4) linkages in amylose and amylopectin. Because amylose is a linear polymer, -amylase breaks it down completely to maltose and a small amount of glucose. However, because amylopectin is a branched polymer, -amylase breaks it down until a branch point is encountered. The debranching enzyme then cleaves the (1->6) bond of the branch point, thus exposing a new set of (1->4) linkages for -amylase to attack (see Figure 13.16).

Cellulose is the single most abundant polymer in the biosphere. Like amylose, it is a polymer. Unlike amylose, however, the linkages between glucose units in cellulose are (1->4) bonds instead of (1->4) bonds. Thus, enzymes called cellulases are needed to cleave the (1->4) bonds in cellulose. It turns out, however, that only ruminants, such as cows, termites, and certain fungi, such as mushrooms, possess the necessary cellulases to digest cellulose. Ruminants and termites have the required enzymes only because their respective digestive tracts contain symbiotic bacteria that produce cellulases.