Lecture 28- Gluconeogenesis and the Pentose Phosphate Pathway

gluconeogenesis

-the synthesis of glucose

-occurs during times of fasting, extensive exercise, and when consuming a low-carb diet

-the primary purpose of gluconeogenesis is to facilitate a healthy glucose balance in the blood and generate nutrients for specific tissues

-gluconeogenesis happens primarily in the liver

-gluconeogenesis is not the exact reversal of glycolysis

glycolysis vs. gluconeogenesis

-glycolysis occurs entirely in the cytoplasm

-some reactions of gluconeogenesis occur in the cytoplasm, endoplasmic reticulum and mitochondria

-this allows these two reaction pathways to remain largely sequestered from each other

-the irreversible steps of glycolysis (hexokinase, PFK, and pyruvate kinase) are effectively bypassed in gluconeogenesis and use different enzymes

-furthermore, pyruvate (product of glycolysis) can be generated from other sources including lactate and amino acids

step 1 of gluconeogenesis

-involves the conversion of pyruvate to oxaloacetate by the enzyme pyruvate carboxylate; this reaction occurs in the mitochondria and consumes 1 molecule of ATP in the process

-this conversion uses a CO₂ molecule to add an additional carbon to pyruvate; the formation of a new carbon-carbon bond is an energetically demanding process, hence the use of ATP

pyruvate carboxylase

-is a unique enzyme that contains a biotin cofactor; this particular functionality is often used to carry carbon dioxide and transfer it to other molecules

the transfer of carbon dioxide to pyruvate

-first, a bicarbonate (HCO₃^-) molecule is phosphorylated using a phosphate from ATP

-second, the activated bicarbonate is attached to the biotin within the pyruvate carboxylase enzyme

-third, the carbon dioxide is added to the deprotonated pyruvate in the pyruvate carboxylase active site

once the oxaloacetate is generated

-the oxaloacetate is converted into phosphoenolpyruvate in the mitochondria by phosphoenolpyruvate carboxykinase

-this reaction involves a decarboxylation that is followed by a phosphorylation; the source of this phosphate is GTP

the carboxylation and decarboxylation of pyruvate

-reactions are energetically similar and have low standard free energies; this means they are constantly close to equilibrium

-only slight changes in the concentration of the reactants or products can shift these reaction; however, the consumption of ATP and GTP during this process will generally force the reaction to the right, generating more phosphoenolpyruvate

only 2 other reactions in gluconeogenesis differ from glycolysis

-each of these reactions involve hydrolysis of a phosphate-ester bond to generate a sugar hydroxyl group

-both of these reactions are catalyzed by phosphatases, and both are energetically favorable

antepenultimate step in gluconeogenesis

-the hydrolysis of fructose-1,6-biphosphate to fructose-6-phosphate

-this reaction occurs in the cytoplasm and is catalyzed by fructose-1,6-biphosphatase; this enzyme is allosterically regulated by AMP, which strongly inhibits this enzyme

-allosteric regulation allows this step to be a critical control point in gluconeogenesis; when concentration of AMP is high, it signals to the cell that energy stores are low and that glucose should be degraded, not synthesized

last step in gluconeogenesis

-involves the hydrolysis of glucose-6-phosphate to glucose

-this reaction is catalyzed by glucose-6-phosphatase, which is localized to the endoplasmic reticulum

-this reaction is energetically favorable and results in the formation of free glucose and a P_i molecule

-the free glucose can now be exported out of the cell and used for energy in other tissues

Pentose phosphate pathway (PPP)

-is an alternative metabolic pathway for the breakdown of glucose

-primarily used when the cell needs more NADPH (reducing power) than ATP

-in this pathway, glucose-6-phosphate is oxidized twice, generating 2 molecules of NADPH in the process

-following its oxidation and decarboxylation, the carbons of glucose-6-phosphate are rearranged to other metabolic intermediates

beginning of PPP: oxidative phase

-begins with a series of oxidation reactions on glucsose-6-phosphate that produce NADPH, CO₂ and a 5-carbon sugar (ribulose-5-phosphate)

second phase of PPP: non-oxidative phase

-involves the rearrangement of carbon atoms to generate metabolic intermediates that can be used for glycolysis, nucleic acid biosynthesis and amino acid catabolism

The first reaction of the PPP

-glucose-6-phosphate is oxidized to 6-phosphogluconate, generating a molecule of NADPH in the process; 6-phosphogluconate is a phosphorylated ionized acid that serves as a precursor to the next reaction in the pentose phosphate pathway (PPP)

-6-phosphogluconate is then oxidatively decarboxylated to ribulose-5-phosphate generating another molecule of NADPH

oxidatice decarboxylation

-is a process by which a molecule is further oxidized while releasing a carbon atom (CO₂)

by removing 1 carbon from a hexose

-the cell has generated considerable reducing power

-however, the end product of the oxidative phase of the PPP, rubulose-5-phosphate contains useful energy; therefore the cell does not want to waste this molecule

The second phase of the pentose phosphate pathway (PPP)

-involves the rearrangement of the carbon atoms to different metabolites that can be used for other pathways

-these steps are similar and primarily involve the transfer of two- or three-carbon units among the different metabolites; the goal is to end up with glycolytic intermediates that can be used in other pathways (fructose-6-phosphate and glyceraldehyde-3-phosphate)

-here, 3 molecules of ribulose-5-phosphate (15 carbons) are rearranged to generate 2 molecules of fructose-6-phosphate and 1 molecule of glyceraldehyde-3-phosphate (15 carbons)

the two- and three-carbon group transfer reactions in the PPP

-are catalyzed by specialized enzymes known as transketolases (2 carbon shifts) and transaldolases (three-carbon shift)

-these reactions are reversible, which allows the PPP to respond to the needs of an organism

The net reaction of the pentose phosphate pathway

6 glucose-6-phosphate + 12 NADP⁺ + 6H₂O → 6 ribose-5-phosphate + 6 CO₂ + 12 NADPH + 12 H⁺

The biochemical pathways of glycolysis and the pentose phosphate pathway

-are intimately related and can respond accordingly to the needs of an organism

-if the organism needs more NADPH than ribose-5-phosphate, the entire PPP is operative; on the other hand, if the organism needs more ribose-5-phosphate than NADPH, the non-oxidative phase works in reverse using metabolites from glycolysis