Pentose PO4 pathway, Fructose, galactose metabolism

The Entner Doudoroff pathway, which is felt to have given rise through evolution to both the Pentose PO4 Pathway and Glycolysis, begins with ATP dependent glucose phosphotansferase producing Glucose 6 PO 4, but produce only one ATP. This pathway prevalent in anaerobes such as Pseudomonas, they do not have a Phosphofructokinase (PFK-1). This enzyme may the the last enzyme to evolve forming the glycolytic pathway.

For each mole of glucose 6 PO 4 metabolized to ribulose 5 PO 4, 2 moles of NADPH are produced. 6-Phosphogluconate dh is not only an oxidation step but it’s also a decarboxylation reaction. The pentose phosphate pathway (also called the phosphogluconate pathway and the hexose monophosphate shunt) is a biochemical pathway parallel to glycolysis that generates NADPH and pentoses. While it does involve oxidation of glucose, its primary role is anabolic rather than catabolic. There are two distinct phases in the pathway. The first is the oxidative phase, in which NADPH is generated, and the second is the non-oxidative synthesis of 5-carbon sugars. For most organisms, the pentose phosphate pathway takes place in the cytosol.

The primary results of the pathway are: The generation of reducing equivalents, in the form of NADPH, used in reductive biosynthesis reactions within cells (e.g. fatty acid synthesis). Production of ribose-5-phosphate (R5P), used in the synthesis of nucleotides and nucleic acids. Production of erythrose-4-phosphate (E4P), used in the synthesis of aromatic amino acids. Transketolase and transaldolase reactions are similar in that they transfer between carbon chains, transketolases 2 carbon units or transaldolases 3 carbon units.

Regulation; Glucose-6-phosphate dehydrogenase is the rate-controlling enzyme of this pathway. It is allosterically stimulated by NADP +. The ratio of NADPH:NADP + is normally about 100:1 in liver cytosol. This makes the cytosol a highly-reducing environment. An NADPH-utilizing pathway forms NADP +, which stimulates Glucose-6-phosphate dehydrogenase to produce more NADPH. This step is also inhibited by acetyl-CoA.

Epimerase interconverts the stereoisomers ribulose-5-phosphate and xylulose-5- phosphate. Isomerase converts the ketose ribulose-5-phosphate to the aldose ribose-5- phosphate. Both reactions involve deprotonation to form an endiolate intermediate, followed by specific reprotonation to yield the product. Both reactions are reversible. A major product of this pathway is PRPP, but more about that when we discuss Purine- Pyrimidine Synthesis. Ribose-phosphate diphosphokinase (or phosphoribosyl pyrophosphate synthetase) ATPAMP PRPP

Role of NADPH and glutathione in protecting cells against highly reactive oxygen derivatives. Reduced glutathione (GSH) protects the cell by destroying hydrogen peroxide and hydroxyl free radicals. Regeneration of GSH from its oxidized form (GSSG) requires the NADPH produced in the glucose 6-phosphate dehydrogenase reaction.

Muscle Metabolism of Fructose (Anaerobic Glycolysis) Large Amounts of Hexokinase

Two important kinetic properties distinguish glucokinase from the other hexokinases, allowing it to function in a special role as glucose sensor. Glucokinase has a lower affinity for glucose than the other hexokinases. Glucokinase changes conformation and/or function in parallel with rising glucose concentrations in the physiologically important range of 4–10 mmol/L (72–180 mg/dl). It is half-saturated at a glucose concentration of about 8 mmol/L (144 mg/dl). Glucokinase is not inhibited by its product, glucose-6-phosphate. This allows continued signal output (e.g., to trigger insulin release) amid significant amounts of its product.

In hepatocytes of various mammals, GKRP has always been found in molar excess of the amount of GK, but the GKRP:GK ratio varies according to diet, insulin sufficiency, and other factors. Free GKRP shuttles between the nucleus and the cytoplasm. It may be attached to the microfilament cytoskeleton. GKRP competes with glucose to bind with GK, but inactivates it when bound. In conditions of low glucose, GKRP then pulls the GK into the nucleus. Rising amounts of glucose coming into the hepatocyte prompt the GKRP to rapidly release GK to return to the cytoplasm.

Fructose is metabolized almost completely in the liver in humans, where it is directed toward replenishment of liver glycogen and triglyceride synthesis. Under one percent of ingested fructose is directly converted to plasma triglyceride. 29% - 54% of fructose is converted in liver to glucose, and about quarter of fructose is converted to lactate. 15% - 18% is converted to glycogen. Glucose and lactate are then used normally as energy to fuel cells all over the body.

In the liver, aldolase B can utilize both F-1,6-BP and F1P as substrates. Therefore, when presented with F1P the enzyme generates DHAP and glyceraldehyde. The DHAP is converted, by triose phosphate isomerase (TPI), to G3P and enters glycolysis. The glyceraldehyde can be phosphorylated to G3P by glyceraldehyde kinase or converted to DHAP through the concerted actions of alcohol dehydrogenase, glycerol kinase and glycerol phosphate dehydrogenase.

The protein encoded by this gene is a glycolytic enzyme that catalyzes the reversible conversion of fructose-1,6- bisphosphate to glyceraldehyde 3-phosphate and dihydroxyacetone phosphate. Three aldolase isozymes (A, B, and C), encoded by three different genes, are differentially expressed during development. Aldolase A is found in the developing embryo and is produced in even greater amounts in adult muscle. Aldolase A expression is repressed in adult liver, kidney and intestine and similar to aldolase C levels in brain and other nervous tissue. Aldolase A deficiency has been associated with myopathy and hemolytic anemia. Alternative splicing and alternative promoter usage results in multiple transcript variants.

Aldolase B also known as fructose-bisphosphate aldolase B or liver-type aldolase B is one of three isoenzymes (A, B, and C) of the class I fructose 1,6-bp aldolase enzyme, and plays a key role in both glycolysis and gluconeogenesis. The generic fructose 1,6-bp aldolase enzyme catalyzes the reversible cleavage of fructose 1,6-BP into GAP & DHAP as well as the reversible cleavage of fructose 1-p into glyceraldehyde and DHAP. In mammals, aldolase B is preferentially expressed in the liver, while aldolase A is expressed in muscle and erythrocytes and aldolase C is expressed in the brain. Slight differences in isozyme structure result in different activities for the two substrate molecules: FBP and fructose 1-p. Aldolase B exhibits no preference and thus catalyzes both reactions, while aldolases A and C prefer F 1,6bp.

The synthesis of glycogen in the liver proceeds from gluconeogenic precursors. Fructose is initially converted to DHAP and glyceraldehyde by fructokinase and aldolase B. The resultant glyceraldehyde then undergoes phosphorylation to glyceraldehyde-3-phosphate. Increased conc of DHAP and glyceraldehyde-3-phosphate in the liver drive the gluconeogenic pathway toward glucose-6-phosphate, glucose-1-phosphate and glycogen formation. It appears that fructose is a better substrate for glycogen synthesis than glucose and that glycogen replenishment takes precedence over triglyceride formation. Once liver glycogen is replenished, the intermediates of fructose metabolism are primarily directed toward triglyceride synthesis.

Fructose results in the insulin-independent induction of several important hepatic lipogenic enzymes including PK, NADP + -dependent malate dh, citrate lyase, acetyl CoA carboxylase, FA synthase, as well as pyruvate dh. Fr-1-PO4 then undergoes hydrolysis by Fr-1-PO4 aldolase (aldolase B) to form DHAP and glyceraldehyde; DHAP can either be isomerized to glyceraldehyde 3-PO4 by TIM or undergo reduction to glycerol 3-PO4 by glycerol 3-PO4 dh. The glyceraldehyde produced may also be converted to glyceraldehyde 3-PO4 by glyceraldehyde kinase or converted to glycerol 3-phosphate by glyceraldehyde 3-PO4 dh. The metabolism of fructose at this point yields intermediates in gluconeogenic pathway leading to glycogen synthesis, or can be oxidized to pyruvate and reduced to lactate, or be decarboxylated to acetyl CoA in the mitochondria and directed toward the synthesis of free FA, resulting finally in TG synthesis.

Lactose, which is converted to galactose and glucose, is the primary carbohydrate source for developing mammals, and in humans it constitutes 40 percent of the energy consumed during the nursing period. Why lactose evolved as the unique carbohydrate of milk is unclear, especially since most individuals can meet their galactose need by biosynthesis from glucose. Whatever the rationale for lactose in milk, the occurrence of galactose in glyco-proteins, complex polysaccharides, and lipids, particularly in nervous tissue, has suggested specific functions. The organoleptic and physical properties of galactose and, more specifically, the simultaneous occurrence of calcium and lactose in milk, may be significant evolutionary determinants.

Galactose-1-phosphate uridylyltransferase responsible for converting ingested galactose to glucose. Galactose-1-phosphate uridylyltransferase (GALT) catalyzes the second step of the Leloir pathway of galactose metabolism. UDP-glucose + galactose 1-phosphate= glucose 1- phosphate + UDP-galactose The expression of GALT is controlled by the actions of the FOXO3 gene. The enzyme UDP-glucose 4-epimerase also known as UDP-galactose 4- epimerase or GALE, is a homodimeric epimerase found in bacterial, fungal, plant, and mammalian cells. This enzyme performs the final step in the Leloir pathway of galactose metabolism, catalyzing the reversible conversion of UDP-galactose to UDP-glucose. GALE tightly binds nicotinamide adenine dinucleotide (NAD+), a co-factor required for catalytic activity. The Leloir pathway is a metabolic pathway for the catabolism of D-galactose. It is named after Luis Federico Leloir. In the first step, galactose mutarotase facilitates the conversion of β-D-galactose to α-D- galactose since this is the active form in the pathway. Next, α-D-galactose is phosphorylated by galactokinase to galactose 1-phosphate. In the third step, D-galactose-1-phosphate uridylyltransferase converts galactose 1-phosphate to UDP-galactose using UDP-glucose as the uridine diphosphate source. Finally, UDP-galactose 4-epimerase recycles the UDP-galactose to UDP-glucose for the transferase reaction. Additionally, phosphoglucomutase converts the D-glucose 1-phosphate to D-glucose 6- phosphate.

In the hepatocyte, insulin stimulates the utilization and storage of glucose as lipid and glycogen, while repressing glucose synthesis and release. This is accomplished through a coordinated regulation of enzyme synthesis and activity. Insulin stimulates the expression of genes encoding glycolytic and fatty- acid synthetic enzymes (in blue), while inhibiting the expression of those encoding gluconeogenic enzymes (in red). These effects are mediated by a series of transcription factors and co-factors, including sterol regulatory element-binding protein (SREBP)-1, hepatic nuclear factor (HNF)-4, the forkhead protein family (Fox) and PPARγ co-activator 1 (PGC1). The hormone also regulates the activities of some enzymes, such as glycogen synthase and citrate lyase (in green), through changes in phosphorylation state. GK, glucokinase; Glucose-6-P, glucose-6-phosphate; G-6-Pase, glucose-6- phosphatase; F-1,6-Pase, fructose-1,6-bisphosphatase; PEPCK, phosphoenolpyruvate carboxykinase; PFK, phosphofructokinase; PK, pyruvate kinase; ACC, acetyl-CoA carboxylase; FAS, fatty-acid synthase. Ins signaling & regulation of glucose metabolism Nature 2001