Feeder’S Pathways� for �Glycolysis

Submitted by

Dr. Sakshi Verma

Assistant Professor

Zoology Department

HMV, Jalandhar

Feeder Pathways for Glycolysis

• Many carbohydrates besides glucose meet their catabolic fate in glycolysis, after being transformed into one of the glycolytic intermediates.
• The most significant are the storage polysaccharides glycogen and starch; the disaccharides maltose, lactose, trehalose, and sucrose; and the monosaccharides fructose, mannose, and galactose

Feeder Pathways for Glycolysis

• 1. Glycogen and Starch Are Degraded by Phosphorolysis
• Glycogen in animal tissues and in microorganisms (and starch in plants) can be mobilized for use within the same cell by a phosphorolytic reaction catalyzed by glycogen phosphorylase (starch phosphorylase in plants). These enzymes catalyze an attack by Pi on the (α1-4) glycosidic linkage that joins the last two glucose residues at a nonreducing end, generating glucose 1-phosphate and a polymer one glucose unit shorter.
• Glycogen phosphorylase (or starch phosphorylase) acts repetitively until it approaches an (α1-6) branch point where its action stops. A debranching enzyme removes the branches.

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Feeder Pathways for Glycolysis

• 1. Glycogen and Starch Are Degraded by Phosphorolysis
• Glucose 1-phosphate produced by glycogen phosphorylase is converted to glucose 6-phosphate by phosphoglucomutase, which catalyzes the reversible reaction.
• The glucose 6-phosphate thus formed can enter glycolysis.

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The general name mutase is given to enzymes that catalyze the transfer of a functional group from one position to another in the same molecule. Mutases are a subclass of isomerases, enzymes that interconvert stereoisomers or structural or positional isomers

Feeder Pathways for Glycolysis

• 2. Dietary Polysaccharides and Disaccharides Undergo Hydrolysis to Monosaccharides:
• For most humans, starch is the major source of carbohydrates in the diet. Digestion begins in the mouth, where salivary α-amylase hydrolyzes the internal glycosidic linkages of starch, producing short polysaccharide fragments or oligosaccharides. (Note that in this hydrolysis reaction, water, not Pi, is the attacking species.)
• In the stomach, salivary α-amylase is inactivated by the low pH, but a second form of α-amylase, secreted by the pancreas into the small intestine, continues the breakdown process. Pancreatic α-amylase yields mainly maltose and maltotriose (the di- and trisaccharides of (α 1-4) glucose) and oligosaccharides such as dextrins, fragments of amylopectin containing (α 1-6) branch points.

Feeder Pathways for Glycolysis

• 2. Dietary Polysaccharides and Disaccharides Undergo Hydrolysis to Monosaccharides:
• Intestinal disaccharides and dextrins are hydrolyzed by enzymes attached to the outer surface of the intestinal epithelial cells as follow:

Feeder Pathways for Glycolysis

• 2. Dietary Polysaccharides and Disaccharides Undergo Hydrolysis to Monosaccharides:
• Maltose and dextrins are degraded by enzymes of the intestinal microvilli.
• Dietary glycogen has essentially the same structure as starch, and its digestion proceeds by the same pathway.
• Thus the monosaccharides so formed are actively transported into the epithelial cells, then passed into the blood to be carried to various tissues, where they are phosphorylated and enter into the glycolytic sequence.

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Feeder Pathways for Glycolysis

• 3. Other Monosaccharides Enter the Glycolytic Pathway at Several Points:
• In most organisms, hexoses other than glucose can undergo glycolysis after conversion to a phosphorylated derivative.
• D-Fructose is phosphorylated by hexokinase. This is a major pathway of fructose entry into glycolysis in the muscles and kidney. In the liver, however, fructose enters by a different pathway.

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• The liver enzyme fructokinase catalyzes the phosphorylation of fructose at C-1 rather than C-6.

Feeder Pathways for Glycolysis

• 3. Other Monosaccharides Enter the Glycolytic Pathway at Several Points:
• The fructose 1-phosphate is then cleaved to glyceraldehyde and dihydroxyacetone phosphate by fructose 1-phosphate aldolase.
• Dihydroxyacetone phosphate is converted to glyceraldehyde 3-phosphate by the glycolytic enzyme triose phosphate isomerase. Glyceraldehyde is phosphorylated by ATP and triose kinase to glyceraldeh.yde 3-phosphate.

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• Thus both products of fructose 1-phosphate hydrolysis enter the glycolytic pathway as glyceraldehyde 3- phosphate.

Feeder Pathways for Glycolysis

• 3. Other Monosaccharides Enter the Glycolytic Pathway at Several Points:D-
• Mannose, released in the digestion of various polysaccharides and glycoproteins of foods, can be phosphorylated at C-6 by hexokinase.

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• Mannose 6-phosphate is isomerized by phosphomannose isomerase to yield fructose 6-phosphate, an intermediate of glycolysis.

Feeder Pathways for Glycolysis

• 3. Other Monosaccharides Enter the Glycolytic Pathway at Several Points:
• D-Galactose, a product of hydrolysis of the disaccharide lactose (milk sugar), passes in the blood from the intestine to the liver, where it is first phosphorylated at C-1, at the expense of ATP, by the enzyme galactokinase.

The galactose 1-phosphate is then converted to its epimer at C-4, glucose 1-phosphate, by a set of reactions in which uridine diphosphate (UDP) functions as a coenzyme-like carrier of hexose groups.

The epimerization involves first the oxidation of the C-4 OOH group to a ketone, then reduction of the ketone to an OOH, with inversion of the configuration at C-4.

NAD is the cofactor for both the oxidation and the reduction

Feeder Pathways for Glycolysis

Feeder Pathways for Glycolysis

Conversion of galactose to glucose 1-phosphate

• Defects in any of the three enzymes in this pathway cause galactosemia in humans. In galactokinase deficiency galactosemia, high galactose concentrations are found in blood and urine. Infants develop cataracts, caused by deposition of the galactose metabolite galactitol in the lens.