Harper's Biochemistry Chapter 20 - The Pentose Phosphate Pathway & Other Pathways of Hexose Metabolism

Pentose Phosphate Pathway

• The pentose phosphate pathway is active in liver, adipose tissue, adrenal cortex, thyroid, erythrocytes, testis and lactating mammary glands.
• Pathway activity is low in non-lactating mammary glands and skeletal muscle.
• The pentose phosphate pathway is involved in producing NADPH for reductive biosynthesis, which is used in the biosynthesis of various molecules:
  • Fatty acids
  • Steroids
  • Amino acids through glutamate dehydrogenase
  • Reduced glutathione
• NADPH is also used by NADPH oxidase in phagocytes and neutrophils to destroy engulfed cells and bacteria using superoxide.
• Ribose synthesis occurs in virtually all tissues.
• Ribulose-5-phosphate serves as a substrate in the pentose phosphate pathway and can be converted into xylulose-5-phosphate and ribose-5-phosphate.
• Genetic deficiency of glucose-6-phosphate dehydrogenase, the first enzyme of the pentose phosphate pathway, causes acute hemolysis of red blood cells.

The oxidative phase of pentose phosphate pathway

• The pentose phosphate pathway can be divided into two phases: the oxidative phase and the non-oxidative phase.
• The oxidative phase involves the production of NADPH and CO2 using glucose-6-phosphate as a substrate.
• The first step is catalyzed by glucose-6-phosphate dehydrogenase, which is NADP+ dependent and converts glucose-6-phosphate to 6-phosphogluconate.
• 6-phosphogluconate is then decarboxylated by 6-phosphogluconate dehydrogenase , producing ribulose-5-phosphate.

The non-oxidative phase of pentose phosphate pathway

• The non-oxidative phase involves the reversible conversion of ribulose-5-phosphate back to glucose-6-phosphate.
• This phase involves two key enzymes, transketolase and transaldolase.
• Transketolase catalyzes the transfer of a 2-carbon unit, from a ketose sugar to an aldose sugar.
• Transaldolase catalyzes the transfer of a 3-carbon unit, from a ketose sugar to an aldose sugar.
• The end products of the non-oxidative phase are two molecules of glucose-6-phosphate and one molecule of glyceraldehyde-3-phosphate
• The pathway can account for the complete oxidation of glucose as two molecules of glyceraldehyde-3-phosphate can regenerate glucose-6-phosphate.

Coupling the pentose phosphate pathway with glycolysis

• Xylulose-5-phosphate, a product of the pentose phosphate pathway, has regulatory roles in glycolysis.
• Xylulose-5-phosphate activates protein phosphatase that dephosphorylates the 6-phosphofructo-2-kinase/fructose 2,6-bisphosphatase bifunctional enzyme.
• This activation leads to an increase in fructose 2,6-bisphosphate production, which boosts the activity of phosphofructokinase-1, accelerating glycolytic flux.
• Xylulose-5-phosphate also activates protein phosphatase, initiating the nuclear translocation and DNA binding of carbohydrate response element-binding protein, stimulating fatty acid synthesis.

Reducing equivalents in reductive syntheses

• Those tissues specializing in reductive syntheses generate reducing equivalents, such as NADPH, in the pentose phosphate pathway.
• Glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase are involved in NADPH generation.
• NADPH is required for hydroxylation (mixed function oxidase) reactions and 11-β-hydroxysteroid dehydrogenase-1.
• The latter enzyme facilitates the conversion of inactive cortisone to active cortisol in the liver, nervous system, and adipose tissue.

Pentose Phosphate Pathway & NADPH Generation

• The pentose phosphate pathway generates NADPH and produces precursors for nucleotide synthesis.
• NADPH is essential for reductive biosynthesis, like fatty acid and steroid synthesis, and for detoxification reactions.
• NADPH acts as a reducing agent in the detoxification of reactive oxygen species.

Non-oxidative Phase of the Pentose Phosphate Pathway

• The non-oxidative phase includes transketolase and transaldolase reactions.
• Transketolase uses thiamine diphosphate (Vitamin B1) as a cofactor.
• Transaldolase transfers a three-carbon dihydroxyacetone unit.

Regulation of The Pathway

• Xylulose 5-phosphate activates the protein phosphatase that regulates the 6-phosphofructo-2-kinase/fructose 2,6-bisphosphatase enzyme.
• This leads to increased formation of fructose 2,6-bisphosphate, which activates phosphofructokinase-1 and increases glycolytic flux.
• Xylulose 5-phosphate also activates the protein phosphatase involved in nuclear translocation and DNA binding of the carbohydrate response element-binding protein, boosting fatty acid synthesis in response to high-carbohydrate diets.

Tissues Actively Using NADPH

• The pentose phosphate pathway is actively functioning in tissues that require NADPH for reductive biosynthesis, including:
  • Liver
  • Adipose tissue
  • Adrenal cortex
  • Thyroid
  • Erythrocytes
  • Testis
  • Lactating mammary gland

Synthesis of Ribose

• Ribos is not circulated in the bloodstream.
• Tissues synthesize the ribose they need for nucleotide and nucleic acid synthesis through the pentose phosphate pathway.

Fructose Metabolism

• Fructose enters glycolysis at a different point than glucose, bypassing the regulatory step catalyzed by phosphofructokinase.
• Fructose metabolism in the liver can increase fatty acid synthesis, triacylglycerol esterification, and VLDL secretion.
• High fructose intake can contribute to hypertriacylglycerolemia, hypercholesterolemia and hyperuricemia.

Uronic Acid Pathway

• Converts glucose to glucuronic acid, ascorbic acid (except in humans), and pentoses.
• Glucuronic acid is a precursor for proteoglycans and conjugated glucuronides.
• It represents an alternative pathway for glucose oxidation, not producing ATP.

Galactose Metabolism

• Galactose can be converted to glucose through the Leloir pathway.
• Deficiencies in the Leloir pathway, such as the absence of galactokinase or galactose-1-phosphate uridyl transferase, lead to galactosemia.

Pentose Phosphate Pathway

• The pentose phosphate pathway is active in liver, adipose tissue, adrenal cortex, thyroid, erythrocytes, testis, and lactating mammary gland.
• The pathway is less active in non-lactating mammary gland and skeletal muscle.
• Tissues with high pathway activity utilize NADPH for reductive processes, including fatty acid and steroid synthesis, amino acid synthesis via glutamate dehydrogenase, and reduced glutathione.
• The synthesis of glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase can be induced by insulin in a fed state, leading to increased lipogenesis.
• NADPH is also used by NADPH oxidase in phagocytes and neutrophils for destruction of cells and bacteria using superoxide.

Ribose Synthesis

• Ribose synthesis occurs in virtually all tissues.
• Ribose is not a major component of blood; tissues must synthesize their own ribose for nucleotide and nucleic acid synthesis.
• This synthesis occurs primarily through the pentose phosphate pathway.

Galactose

• Galactose is derived from lactose, the sugar in milk, following intestinal hydrolysis.
• It is efficiently converted to glucose in the liver after transport by GLUT5.
• The majority of dietary galactose is metabolized in the liver.
• Galactose is required for the formation of lactose in lactation and as a constituent of glycolipids, proteoglycans, and glycoproteins.
• Galactokinase catalyzes the phosphorylation of galactose using ATP as a phosphate donor.
• Galactose-1-phosphate reacts with UDPGlc to form UDPGal and glucose-1-phosphate, catalyzed by galactose-1-phosphate uridyl transferase.
• UDPGal is then converted to UDPGlc by UDPGal 4-epimerase, utilizing NAD+ as a coenzyme.
• UDPGlc is incorporated into glycogen.

Glucose-6-phosphate Dehydrogenase Deficiency

• Several mutations in the gene for glucose-6-phosphate dehydrogenase lead to two primary consequences.
• This deficiency can cause red cell hemolysis, particularly under oxidative stress.
• Oxidative stress occurs from infection, drugs such as primaquine, sulfonamides, or from eating fava beans, leading to favism.

Amino Sugars

• Amino sugars, including glucosamine, galactosamine, mannosamine, and sialic acid, are important components of glycoproteins, glycosphingolipids, and glycosaminoglycans.
• N-acetylneuraminic acid (NeuAc) is the principal sialic acid found in human tissues.
• The synthesis of amino sugars involves several steps, including phosphorylation, acetylation, and epimerization.

Xylulose and Pentosuria

• Xylulose is a pentose sugar that is typically reduced to xylitol in the body, but a deficiency in xylulose reductase leads to pentosuria.
• Pentosuria does not have clinical consequences.
• However, xylulose is a reducing sugar and can produce false positive results with alkaline copper reagents used to test for urinary glucose.
• Certain drugs, like barbiturates, increase the production of xylulose through the uronic acid pathway.
• Increased consumption of fruits high in pentoses can also lead to pentosuria.

Fructose Metabolism

• Fructose enters glycolysis via fructokinase.
• The resulting fructose-1-phosphate can lead to phosphate sequestration and reduced ATP synthesis.
• This can cause hyperuricemia and gout.
• High oral doses of fructose can cause osmotic diarrhea.
• Fructose in the liver increases fatty acid and triacylglycerol synthesis as well as VLDL secretion, leading to hypertriacylglycerolemia and increased LDL cholesterol.

Disorders of Fructose Metabolism

• A deficiency in hepatic fructokinase leads to essential fructosuria, a benign condition with no symptoms.

The Polyol Pathway

• The polyol pathway, not present in the liver, is responsible for fructose production from glucose.
• The pathway is activated by high glucose concentrations in tissues that are not insulin sensitive.
• This includes the lens, peripheral nerves, and renal glomeruli, which can lead to sorbitol accumulation and osmotic damage.

The Uronic Acid Pathway

• The uronic acid pathway is responsible for the synthesis of glucuronic acid, which is used to conjugate endogenous and exogenous substances for excretion in bile and urine.
• Fructose bypasses the main regulatory step in glycolysis, catalyzed by phosphofructokinase.
• It stimulates liver glucose uptake, fatty acid synthesis, and hepatic triacylglycerol secretion.