Metabolism: Gluconeogenesis and Regulation

400 Practice Flashcards based on Lecture Notes on Gluconeogenesis, covering pathways, enzymes, energy costs, and metabolic control.

What is the technical definition of gluconeogenesis?

The synthesis of glucose from non-carbohydrate precursors.

Where does the majority of gluconeogenesis occur in the human body?

Overwhelmingly in the liver (more than $90\%$).

What percentage of gluconeogenesis is typically handled by the kidneys?

About $10\%$, which can increase during long fasts.

Why is muscle unable to release free glucose into the blood during gluconeogenesis?

It lacks the final enzyme, glucose-6-phosphatase.

Under normal conditions, what is the brain's approximate daily glucose consumption?

About $120\,g/day$.

What percentage of the body's glucose at rest is consumed by the brain?

Roughly $60\%$.

Why are red blood cells constrained to burning glucose?

They have no mitochondria.

What is the product of glucose burning in red blood cells?

Lactate.

How much glycogen is typically stored in the liver?

About $100\,g$.

When does hepatic glycogen typically run out during a fast?

Somewhere between $12$ and $18\,hours$.

At what point in a fasting timeline does gluconeogenesis become the main source of glucose?

By about $18\,hours$.

What enter the metabolic picture days after a fast begins to reduce glucose demand?

Ketone bodies.

What does the 'neo' in gluconeogenesis stand for?

New.

Name the three primary precursors for gluconeogenesis.

Lactate, alanine, and glycerol.

Under what condition does muscle produce lactate?

When oxygen demand outruns supply and pyruvate gets reduced to regenerate $NAD^+$.

Where does alanine come from during a fast?

Muscle breaking down its own protein for energy.

How does muscle produce alanine from protein breakdown?

By packaging amino-acid nitrogen onto pyruvate.

What happens to the nitrogen from alanine once it reaches the liver?

It is stripped off for urea synthesis.

What molecule is hydrolyzed to release glycerol in fat tissue?

Triacylglycerols.

What are the two products of triacylglycerol hydrolysis?

Three fatty acids and one glycerol.

How many carbons do lactate, alanine, and glycerol each contain?

Three carbons.

What enzyme converts lactate back to pyruvate in the liver?

Lactate dehydrogenase.

Where does lactate enter the gluconeogenic pathway?

At the bottom, as pyruvate.

How does alanine enter the gluconeogenic pathway?

By transamination into pyruvate.

Where do most amino acids enter the pathway?

At the bottom as pyruvate, or as oxaloacetate/TCA intermediates.

Which precursor is considered an 'exception' because it skips the lower steps?

Glycerol.

What enzyme prepares glycerol for entry into the pathway?

Glycerol kinase.

To what glycolytic intermediate is glycerol eventually oxidized?

Dihydroxyacetone phosphate ($DHAP$).

Which precursor is the cheapest in terms of $ATP$ cost?

Glycerol.

Why can gluconeogenesis not just be the reverse of glycolysis?

Thermodynamics; three glycolytic steps involve large free-energy changes that are functionally irreversible.

What is the free-energy drop ($\Delta G$) for the hexokinase step in glycolysis?

About $17\,kJ/mol$.

What is the free-energy drop ($\Delta G$) for the phosphofructokinase-1 ($PFK-1$) step?

About $14\,kJ/mol$.

What is the largest energy barrier in glycolysis?

The pyruvate kinase step ($31\,kJ/mol$ drop).

How many bypass enzymes does gluconeogenesis use to overcome the three glycolytic barriers?

Four enzymes.

Which enzyme reverses the hexokinase step?

Glucose-6-phosphatase.

Which enzyme reverses the $PFK-1$ step?

Fructose-1,6-bisphosphatase.

Which bypass enzyme is the major regulatory enzyme for gluconeogenesis?

Fructose-1,6-bisphosphatase.

Which two enzymes are required to bypass the pyruvate kinase step?

Pyruvate carboxylase and $PEPCK$.

What is produced when pyruvate carboxylase adds $CO_2$ and $ATP$ to pyruvate?

Oxaloacetate.

What does $PEPCK$ stand for?

Phosphoenolpyruvate carboxykinase.

What molecules are required by $PEPCK$ to convert oxaloacetate to phosphoenolpyruvate ($PEP$)?

$GTP$ (as a phosphate source) and the decarboxylation of oxaloacetate.

What happens when oxygen supply lags during exercise in muscle?

Pyruvate gets reduced to lactate to regenerate $NAD^+$.

What is the structural reason muscle cannot release glucose into the blood?

Muscle lacks the enzyme glucose-6-phosphatase.

In what organelles does gluconeogenesis take place?

Mitochondrial matrix, cytosol, and endoplasmic reticulum ($ER$) lumen.

What is the byproduct of adding carbon dioxide to pyruvate?

Oxaloacetate.

What cofactor is covalently linked to a specific lysine residue on pyruvate carboxylase?

Biotin.

What molecule activates pyruvate carboxylase allosterically?

Acetyl-CoA.

What signal does high acetyl-CoA send to the mitochondria?

The TCA cycle is full; divert pyruvate toward glucose synthesis.

Where is pyruvate carboxylase located?

Mitochondrial matrix.

Where is $PEPCK$ located in humans?

Mostly cytosolic.

Why is oxaloacetate transport a problem?

Oxaloacetate is made in the matrix but used in the cytosol, and it cannot cross the inner mitochondrial membrane.

Into what molecule is oxaloacetate reduced to cross the mitochondrial membrane?

Malate.

Which enzyme reduces oxaloacetate to malate?

Mitochondrial malate dehydrogenase.

What is consumed during the reduction of oxaloacetate to malate?

$NADH$.

Once in the cytosol, how is malate converted back to oxaloacetate?

By cytosolic malate dehydrogenase, reducing $NAD^+$ to $NADH$.

Besides carbon transport, what else does the malate shuttle deliver to the cytosol?

Reducing power ($NADH$).

Why is cytosolic $NADH$ needed in gluconeogenesis?

For the reversal of the glyceraldehyde-3-phosphate dehydrogenase step.

What is the 'committed' and 'rate-limiting' step of gluconeogenesis?

The $PEPCK$ step.

How many $ATP$ equivalents are required per pyruvate to get to $PEP$?

Two ($1\,ATP$ and $1\,GTP$).

What is the reaction catalyzed by fructose-1,6-bisphosphatase ($FBPase-1$)?

Hydrolysis of fructose-1,6-bisphosphate and water to fructose-6-phosphate and inorganic phosphate ($P_i$).

True or False: $FBPase-1$ generates $ATP$ during its reaction.

False; it is a hydrolysis that releases energy as heat and entropy.

What molecule allosterically inhibits $FBPase-1$ and activates $PFK-1$?

Fructose-2,6-bisphosphate ($F-2,6-BP$).

What is a 'futile cycle' in metabolism?

Running opposing pathways simultaneously, resulting in the net hydrolysis of $ATP$ with no chemical work done.

Where is the active site of glucose-6-phosphatase located?

Facing into the lumen of the endoplasmic reticulum ($ER$).

Which transporter exports free glucose from the liver into the blood?

$GLUT2$.

How many reversible glycolytic enzymes are used in gluconeogenesis?

Seven enzymes.

List the seven reversible enzymes used in gluconeogenesis.

Enolase, phosphoglycerate mutase, phosphoglycerate kinase, glyceraldehyde-3-phosphate dehydrogenase, triose phosphate isomerase, aldolase, and phosphoglucose isomerase.

What is the total energy cost to make one glucose from two pyruvates?

Six $ATP$ equivalents ($4\,ATP$ and $2\,GTP$).

How many $ATP$ are spent at the phosphoglycerate kinase step during gluconeogenesis?

Two $ATP$ per glucose (one per pyruvate).

How much $NADH$ is consumed during the creation of one glucose?

Two $NADH$.

Where are the costs of gluconeogenesis concentrated in the pathway?

At the bottom ($4$ out of $6\,ATP$ equivalents are spent between pyruvate and $PEP$).

What is the thermodynamic cost of running gluconeogenesis compared to the gain from glycolysis?

The difference of four $ATP$ ($6$ in versus $2$ out).

Why does glycerol cost less to convert to glucose than lactate?

It enters at $DHAP$, skipping the expensive lower steps ($PC$ and $PEPCK$).

What is the cost of converting two glycerols into one glucose?

Two $ATP$ (one per glycerol kinase reaction).

Which hormone signaling activates protein kinase A ($PKA$)?

Glucagon.

What enzyme does $PKA$ phosphorylate to control gluconeogenesis?

The bifunctional enzyme $PFK-2 / FBPase-2$.

What happens to the level of $F-2,6-BP$ when the bifunctional enzyme is phosphorylated?

$F-2,6-BP$ levels fall (phosphatase domain is on).

What happens to the level of $F-2,6-BP$ when the bifunctional enzyme is dephosphorylated?

$F-2,6-BP$ levels rise (kinase domain is on).

Which enzyme dephosphorylates the bifunctional enzyme?

Protein phosphatase 1 ($PP1$).

Hormonally, what activates $PP1$?

Insulin.

What is the effect of high $F-2,6-BP$ on $PFK-1$?

It increases its affinity for fructose-6-phosphate and activates it.

What is the effect of citrate on $FBPase-1$?

It acts as an activator.

What is the effect of $AMP$ on $FBPase-1$?

It acts as an inhibitor.

Why does $AMP$ inhibit gluconeogenesis?

It signals a fuel-starved state with low energy charge.

What effect does acetyl-CoA have on pyruvate kinase?

None mentioned directly, but it activates pyruvate carboxylase; however, alanine and $ATP$ inhibit pyruvate kinase.

What hormone increases the expression of the $PEPCK$ gene via $CREB$?

Glucagon.

How do glucagon and insulin compare in terms of their speed of regulation?

Allosteric/covalent control is fast (minutes); transcriptional control is slow (hours).

What is the 'Cori cycle'?

The interorgan partnership where muscle moves lactate to the liver for conversion to glucose, which returns to the muscle.

Why is the Cori cycle 'net negative' in energy?

Muscle generates $2\,ATP$ but the liver spends $6\,ATP$, for a net loss of $4\,ATP$ equivalents.

Where does the liver get the $ATP$ to fuel the Cori cycle?

From the oxidation of fatty acids ($beta$-oxidation).

What is the clinical basis of fatigue in prolonged exercise?

The liver running out of $ATP$ to sustain the Cori cycle, leading to lactate buildup.

What 'accounting problem' does the alanine cycle solve?

Nitrogen handling during muscle protein breakdown.

What is the diagnostic threshold for fasting blood glucose in diabetes?

Above $126\,mg/dL$.

What is the primary action of the drug Metformin?

To suppress hepatic gluconeogenesis.

What is 'protein-sparing adaptation'?

The brain switching to ketone bodies after day 3 of fasting to reduce glucose demand and slow muscle protein breakdown.

Which enzyme catalyzes the reaction: $Pyruvate + CO_2 + ATP + H_2O \rightarrow Oxaloacetate + ADP + P_i$?

Pyruvate carboxylase.

Which enzyme catalyzes: $Oxaloacetate + GTP \rightarrow PEP + GDP + CO_2$?

Phosphoenolpyruvate carboxykinase ($PEPCK$).

What is the role of $G-protein-coupled$ receptors in this pathway?

They bind glucagon to initiate the $cAMP/PKA$ cascade.

Which metabolite is the universal 'fuel-low alarm'?

$AMP$.

What specific serine phosphorylation event on $PFK-2 / FBPase-2$ triggers the shift toward gluconeogenesis?

Phosphorylation by $PKA$.