Gluconeogenesis and Glycogen Metabolism: Key Enzymes and Regulation

What is Gluconeogenesis?

The synthesis of glucose from non-carbohydrate precursors.

What are the primary non-carbohydrate precursors for gluconeogenesis?

Pyruvate, lactate, glycerol, and glucogenic amino acids.

Where is the primary site of gluconeogenesis in the body?

The liver (with a small amount occurring in the kidneys).

Why is gluconeogenesis not simply the reverse of glycolysis?

Because three steps in glycolysis are irreversible and must be bypassed by different enzymes.

Which three glycolytic enzymes are bypassed in gluconeogenesis?

Hexokinase, Phosphofructokinase-1 (PFK-1), and Pyruvate Kinase.

How many high-energy phosphate bonds are consumed to produce 1 glucose from 2 pyruvate?

Six (4 ATP and 2 GTP).

What is the net equation for gluconeogenesis?

$2 Pyruvate + 4 ATP + 2 GTP + 2 NADH + 2 H^{+} + 4 H_{2}O \rightarrow Glucose + 4 ADP + 2 GDP + 6 P_{i} + 2 NAD^{+}$.

What is the first bypass enzyme located in the mitochondrial matrix?

Pyruvate Carboxylase.

What reaction does Pyruvate Carboxylase catalyze?

$Pyruvate + ATP + HCO_{3}^{-} \rightarrow Oxaloacetate + ADP + P_{i}$.

What cofactor is essential for Pyruvate Carboxylase?

Biotin.

What is the role of Biotin in this reaction?

It acts as a carrier for activated $CO_{2}$.

What molecule is a required allosteric activator for Pyruvate Carboxylase?

Acetyl-CoA.

How does Oxaloacetate leave the mitochondria?

It is reduced to Malate, exported via a transporter, and then re-oxidized to Oxaloacetate in the cytosol.

What enzyme converts Oxaloacetate to Phosphoenolpyruvate (PEP)?

PEP Carboxykinase (PEPCK).

What high-energy molecule is the phosphate donor for the PEPCK reaction?

GTP.

Where is PEPCK located in the cell?

In the cytosol (and sometimes the mitochondria depending on the species).

What enzyme bypasses the PFK-1 reaction?

Fructose-1,6-bisphosphatase (FBPase-1).

What reaction does FBPase-1 catalyze?

$Fructose-1,6-bisphosphate + H_{2}O \rightarrow Fructose-6-phosphate + P_{i}$.

What inhibits FBPase-1?

AMP and Fructose-2,6-bisphosphate.

What enzyme bypasses the Hexokinase reaction?

Glucose-6-phosphatase.

Where is Glucose-6-phosphatase located within the liver cell?

In the membrane of the Endoplasmic Reticulum (ER).

Why is Glucose-6-phosphatase absent in muscle?

Muscle lacks this enzyme so it can retain glucose-6-phosphate for its own energy needs rather than exporting glucose to the blood.

What is Glycogen?

A highly branched polymer of glucose used for energy storage in animals.

What are the two types of glycosidic bonds in glycogen?

$\alpha(1\rightarrow4)$ for linear chains and $\alpha(1\rightarrow6)$ for branches.

Where are the primary stores of glycogen in the body?

Liver and skeletal muscle.

What enzyme releases Glucose-1-phosphate from glycogen?

Glycogen Phosphorylase.

What is the reaction type catalyzed by Glycogen Phosphorylase?

Phosphorolysis (breaking a bond using inorganic phosphate).

What is a 'Limit Branch'?

The 4 glucose units remaining on a branch that Glycogen Phosphorylase cannot cleave.

What enzyme handles the limit branches?

Debranching Enzyme.

What are the two activities of the Debranching Enzyme?

4-alpha-glucanotransferase and alpha(1->6) glucosidase.

What is the product of the alpha(1->6) glucosidase activity?

Free Glucose (not G1P).

What enzyme converts G1P to G6P?

Phosphoglucomutase.

What is the 'activated' form of glucose used for glycogen synthesis?

UDP-Glucose.

What enzyme forms UDP-Glucose from G1P and UTP?

UDP-Glucose Pyrophosphorylase.

What drives the UDP-glucose pyrophosphorylase reaction forward?

The rapid hydrolysis of the byproduct inorganic pyrophosphate ($PP_{i}$) to $2 P_{i}$.

What enzyme adds glucose units to the non-reducing end of a glycogen chain?

Glycogen Synthase.

Can Glycogen Synthase start a new chain from scratch?

No, it requires a primer.

What protein acts as the primer for glycogen synthesis?

Glycogenin.

What enzyme creates $\alpha(1\rightarrow6)$ branches?

Branching Enzyme (Amylo-(1,4->1,6)-transglycosylase).

How is Glycogen Phosphorylase regulated covalently?

It is active when phosphorylated (Phosphorylase a) and inactive when dephosphorylated (Phosphorylase b).

What enzyme phosphorylates Glycogen Phosphorylase?

Phosphorylase Kinase.

How is Glycogen Synthase regulated covalently?

It is active when dephosphorylated and inactive when phosphorylated.

What effect does Insulin have on glycogen synthesis?

It stimulates synthesis by activating Phosphoprotein Phosphatase 1 (PP1), which dephosphorylates Glycogen Synthase.

What effect does Glucagon have on liver glycogen?

It stimulates breakdown (glycogenolysis) to increase blood glucose.

What effect does Epinephrine (Adrenaline) have on muscle glycogen?

It stimulates rapid breakdown to provide energy for 'fight or flight.'

What is the second messenger for glucagon and epinephrine?

cyclic AMP (cAMP).

What enzyme produces cAMP?

Adenylate Cyclase.

What protein does cAMP activate?

Protein Kinase A (PKA).

What is Fructose-2,6-bisphosphate (F2,6P)?

A potent allosteric regulator of both glycolysis and gluconeogenesis.

How does F2,6P affect PFK-1?

It is a strong activator.

How does F2,6P affect FBPase-1?

It is a strong inhibitor.

What bifunctional enzyme controls the level of F2,6P?

PFK-2/FBPase-2.

In the liver, what happens to PFK-2/FBPase-2 when phosphorylated by PKA?

PFK-2 activity is inhibited, FBPase-2 activity is activated, and F2,6P levels drop.

What hormone triggers the phosphorylation of the bifunctional enzyme in the liver?

Glucagon.

What is the net result of low F2,6P in the liver?

Glycolysis is inhibited and Gluconeogenesis is stimulated.

What is the Cori Cycle?

The metabolic pathway in which lactate produced by anaerobic glycolysis in muscles moves to the liver and is converted back to glucose.

What is the Glucose-Alanine Cycle?

A cycle where muscle protein is degraded to alanine, which travels to the liver to be converted into glucose.

Why does the liver perform gluconeogenesis during fasting?

To maintain blood glucose levels for the brain and red blood cells.

What acts as the primary signal for 'low blood glucose'?

Glucagon.

What acts as the primary signal for 'high blood glucose'?

Insulin.

What is Von Gierke's Disease (GSD Type I)?

A deficiency in Glucose-6-phosphatase.

What are the symptoms of Von Gierke's Disease?

Severe hypoglycemia (low blood sugar) and enlarged liver (hepatomegaly).

What is McArdle's Disease (GSD Type V)?

A deficiency in muscle Glycogen Phosphorylase.

What are the symptoms of McArdle's Disease?

Exercise-induced muscle cramps and inability to perform high-intensity exercise.

What is Pompe's Disease (GSD Type II)?

A deficiency in lysosomal α(1→4) glucosidase.

What is Cori's Disease (GSD Type III)?

A deficiency in Debranching Enzyme.

Why is the hydrolysis of PPi important in metabolism?

It provides a large negative ΔG, making otherwise reversible reactions irreversible.

How many ATP/GTP are used to turn 2 Lactate into 1 Glucose?

6 ATP/GTP.

What is the role of Ca²⁺ in muscle glycogenolysis?

Ca²⁺ activates Phosphorylase Kinase, linking muscle contraction to energy release.

What enzyme breaks down cAMP?

Phosphodiesterase.

What effect does Caffeine have on glycogenolysis?

It inhibits phosphodiesterase, keeping cAMP levels high and prolonging the signal for glycogen breakdown.

What is the role of 'G-proteins' in this system?

They transduce the signal from the hormone receptor to Adenylate Cyclase.

Is glycogen synthesis exergonic or endergonic?

Overall it is endergonic, driven by the consumption of UTP.

Where is glycogen stored in the cell?

In large granules in the cytosol.

What is the purpose of glycogen branching?

Increases solubility and provides many non-reducing ends for rapid synthesis/breakdown.

What is the effect of high [AMP] on muscle Glycogen Phosphorylase?

It acts as an allosteric activator, signaling a need for energy.

What is the effect of [G6P] on liver Glycogen Synthase?

It acts as an allosteric activator.

Which hormone is a polypeptide produced by the α-cells of the pancreas?

Glucagon.

Which hormone is produced by the β-cells of the pancreas?

Insulin.

What is the glucose sensor in the liver?

Glycogen Phosphorylase a (binding of glucose causes it to be dephosphorylated).

How does PFK-1 activity change after a high-carb meal?

Increases due to insulin-stimulated rise in F2,6P.

What happens to gluconeogenesis during type 1 diabetes?

It is abnormally high because the lack of insulin prevents the suppression of gluconeogenic enzymes.

What is the precursor for glycerol in gluconeogenesis?

Dihydroxyacetone phosphate (DHAP).

Which amino acid is the most important 'carrier' of nitrogen to the liver?

Alanine.

Can fatty acids with even-numbered chains be used for gluconeogenesis?

No, because they are broken down to Acetyl-CoA, which cannot be converted to pyruvate in animals.

What is the term for a pathway that is a loop where energy is wasted?

A 'futility cycle' or 'substrate cycle.'

How do cells prevent a futility cycle between PFK and FBPase?

Reciprocal regulation (activator of one is an inhibitor of the other).

What is the role of Protein Phosphatase 1 (PP1)?

It removes phosphate groups from enzymes, generally favoring the storage (anabolic) state.

What inhibits PP1 during stress/fasting?

Phosphorylation of inhibitor proteins by PKA.

How many glucose residues are typically in a glycogen branch?

About 12-14.

What is the purpose of the ER location for Glucose-6-phosphatase?

It separates the step from the glycolytic enzymes in the cytosol.

What is the role of Malate Dehydrogenase in gluconeogenesis?

It allows the transport of oxaloacetate equivalents across the mitochondrial membrane.

What is the role of the enzyme Pyruvate Carboxylase in the TCA cycle?

It is an anaplerotic reaction that replenishes oxaloacetate.

What is the 'energy charge' of a cell performing gluconeogenesis?

High (high ATP/low AMP).

Which enzyme is considered the 'glucose-6-phosphate sensor'?

Glycogen Synthase.

What is the biological advantage of the phosphorolysis reaction?

It preserves the energy of the glycosidic bond by producing a phosphorylated sugar, saving an ATP.

How many non-reducing ends does a glycogen molecule have?

Many (equal to the number of branches + 1).

How many reducing ends does a glycogen molecule have?

Only one (attached to Glycogenin).

What are 'Glucogenic' amino acids?

Amino acids whose carbon skeletons can be converted to pyruvate or TCA intermediates.

What is the primary regulator that ensures the liver doesn't burn the glucose it just made?

Fructose-2,6-bisphosphate.