Page 315 - Biology_F5

Biology for Advanced Level Secondary Schools

pathway enables cells to convert glucose- group from ATP is transferred to glucose
6-phosphate, a derivative of glucose, producing glucose-6-phosphate, a more
into ribose-5-phosphate and other types reactive form of glucose. The enzyme
of monosaccharides. The NADH is an hexokinase, with broad specificity
important cellular reducing agent, which is catalyses the phosphorylation of six-
also produced by this pathway. Generally, carbon sugars by addition of a phosphate
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glycolysis involves three main stages, group to glucose in the cell’s cytoplasm.
namely phosphorylation of glucose, lysis The enzyme hexokinase splits ATP into
and oxidation by dehydrogenation. ADP and the Pi is added on to the glucose.
Phosphorylation prevents transport of
(a) Phosphorylation of glucose glucose out of the cell and increases the
Phosphorylation refers to the addition of reactivity of the oxygen in the resulting
the phosphate group (Pi) in an organic phosphate ester. The negative charge of the
molecule. In this stage of phosphorylation, phosphate prevents passage of the sugar
two phosphate groups attach to the phosphate through the plasma membrane,
glucose in order to make it more reactive. trapping glucose inside the cell.
In the first step of glycolysis, a phosphate

Hexokinase
→
Glucose (C H O ) + ATP ⎯⎯⎯⎯ Glucose 6-phosphate (C H O P) + ADP + H
6 12 6 Mg 6 11 6
H H
HC OH HC
HC O P
H O H O H
H Hexokinase H H
OH H + ATP Mg + ADP + H
OH OH OH OH H OH
H OH H OH
Glucose Glucose -6- phosphate

The first product of phosphorylation of glucose is glucose-6-phosphate, which is
isomerized to fructose-6-phosphate by phosphoglucose isomerase enzyme.
H H H
HC O P P O CH O HC OH
6
H 5 O H 5 6 1 2
H
4 OH H 1 phosphoglucose isomerase H OH
OH 3 2 OH H 3 OH
4
H OH OH H
Glucose -6-phosphate Fructose -6- phosphate

Fructose-6-phosphate is further phosphorylated to fructose-1, 6-biphosphate, under
the enzyme phosphofructokinase (PFK) and another ATP molecule is used to transfer

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