Pentose phosphate pathway
Pentose phosphate pathway (PPP) is a secondary pathway
of glucose metabolism, primary being glycolysis. Glucose
enters PPP as glucose 6-phosphate. It occurs in the
cytosol of tissues active in fatty acid biosynthesis.
Those tissues are liver, mammary and adrenal glands and
adipose tissue. There are two phases in the pathway:
oxidative and non-oxidative. There are four steps
involved in oxidative phase, during which glucose-6
phosphate gets converted to ribose 5-phosphate. In the
non-oxidative phase, transaldolase and transketolase
enzymes catalyze the reversible reactions for the
regeneration of glucose 6-phosphate.
Glyoxylate cycle
Glyoxylate cycle is a variant of TCA cycle and occurs in bacteria
and glyoxysomes of plants. It bypasses decarboxylations of citric
acid cycle and allows net synthesis of glucose from acetyl CoA.
Gluconeogenesis
Gluconeogenesis is a process of biosynthesis of glucose from simpler
non carbohydrate precursors such as oxaloacetate or pyruvate. It is
a universal pathway in animals, plants, microbes and fungi. It
converts pyruvate to glucose (reverse of glycolysis). The
irreversible steps of glycolysis are circumvented by four key
enzymes of gluconeogenesis. They are pyruvate carboxylase,
phosphoenol pyruvate carboxy kinase, fructose-1, 6-bisphosphatase
and glucose-6-phosphatase.
Glycogen synthesis
Glycogen synthesis takes place in the liver and muscle. All the
enzymes of glycogen synthesis are located in the cytosol. Activation
of glucose takes place followed by the sequential addition of
activated glucose units by glycogen synthase. A brancher enzyme
creates alpha 1-6 linkage.
Starch synthesis
Starch is synthesized in the chloroplasts of plants with the help of
enzymes ADP-glucose phosphorylase and starch synthase.
Disaccharide synthesis
Lactose is synthesized in lactating mammary glands of animals with
the help of enzyme lactose synthase. Lactose is made up of galactose
and glucose. Sucrose is synthesized in plants.
