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 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 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 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 is synthesized in the chloroplasts of plants with the help of enzymes ADP-glucose phosphorylase and starch synthase.
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.