2.4.1  Starch and sucrose degradation

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Figure 2.19 Pathways of starch metabolism. Numbers refer to the following enzymes: 1, ß-amylase; 2, α-amylase; 3, starch phosphorylase; 4, glucosidase; 5, hexose kinase; 6, phosphoglucomutase; 7, glucose 6-phosphate isomerase. (Original drawing courtesy David Day)

Starch is the major carbon reserve in most plants. It is a mixture of amylose and amylopectin and is deposited as granules inside plastids (chloroplasts in leaves, amyloplasts in non-photosynthetic tissues). The initial attack on starch granules in leaves and non-photosynthetic tissues is by a-amylase and a debranching enzyme. Oligosaccharides released during starch degradation, such as maltose, maltotriose and maltotetraose, are hydrolysed to glucose by a-glucosidase. These processes are summarised in Figure 2.19.

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Figure 2.20 A scheme for breakdown of sucrose in plants. Broken lines indicate possible interconnecting reactions. Numbers refer to the following enzymes: 1, invertase; 2, sucrose synthase 3, UDP-glucose pyrophosphorylase; 4, hexokinase; 5, fructokinase; 6, phosphogluconiutase; 7 phosphohexose isomerase; 8, phosphofructophosphotransferase; 9, phosphofructokinase. (Original drawing courtesy David Day)

Reactions in plant tissues leading to degradation of sucrose to hexose monophosphates are outlined in Figure 2.20. The first step is cleavage of the glycosidic bond by either invertase (Equation 2.1) or sucrose synthase (Equation 2.2).

Sucrose + H2O → D-glucose + D-fructose (2.1)

Sucrose + UDP → UDP-glucose + D-fructose (2.2)

Plant tissues contain two types of invertases which hydrolyse sucrose to glucose and fructose in an essentially irreversible reaction: acid invertase, which has optimum activity near pH 5, and is present

In vacuoles, the free space outside cells, and may be associated with the cell wall; and alkaline or neutral invertase, which is maximally active at about pH 7 to 7.5 and is located in the cytosol. Sucrose synthase is a cytosolic enzyme that catalyses a readily reversible reaction, but probably acts only in the breakdown of sucrose in vivo. Sucrose appears to be partitioned between alkaline invertase and sucrose synthase in the cytosol on the basis of differences in affinity of the two enzymes for the substrate. (Km values for sucrose of alkaline invertase and sucrose synthase generally fall within the ranges 10–15 mM and 20–30 mM, respectively.) Glucose and fructose are metabolised further following phosphorylation to the corresponding hexose-6-P, probably by separate enzymes for the two hexoses. Plant tissues contain several hexose kinases that have specificity towards either glucose or fructose. A substantial portion of the glucose kinase in plant tissues is associated with the outer surface of the outer mitochondrial membrane, while fructokinases appear to be soluble in the cytosol.

 Two molecules of ATP are required to metabolise the hexoses formed upon cleavage of sucrose by invertase. How-ever, when sucrose is cleaved by sucrose synthase, part of the energy in the glycosidic bond is conserved in the UDP-glucose formed and only one molecule of ATP is required for the further metabolism of fructose. UDP-glucose may be converted to glucose-1-P by UDP-glucose pyrophosphorylase. Glucose-1-P is converted to glucose-6-P, and glucose-6-P to fructose-6-P, by phosphoglucomutase and phosphohexose isomerase, respectively.

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