Gluconeogenesis, Proteolysis and Ketone Body Synthesis (late fasting stage)

Question 1

Summarise the substrates available for de novo gluconeogenesis

Answer 1

Primary substrate:

• ## Glycerol: It is derived from adipose tissue as a result of lipolysis = breakdown of triglycerides = produces 30g of glucose per day [enters gluconeogenesis as dihydroxyacetone phosphate]
• Lactate = transported to the liver, where it is converted back to pyruvate and used in gluconeogenesis to produce glucose. [enters gluconeogenesis as pyruvate]
• Amino Acids = amino acid carbon skeletons can enter gluconeogenesis at different points, depending on their specific structures and metabolic pathways. [Enter at various places in gluconeogenesis,]

Question 2

Explain why using lactate as a substrate does not increase the circulating glucose pool via gluconeogenesis

Answer 2

The lactate fueled gluconeogenesis is just recycling
–> Cori Cycle converts lactate into glucose in the liver
= the glucose produced is often returned to the same tissues that generated the lactate in the first place
= no net increase in the overall circulating glucose pool.
–> localized energy source

Question 3

Describe the overall structure and strategy of gluconeogenesis

Answer 3

essentially, the reversal of glycolysis

3 Bypassed Rate-Limiting Steps:
1) hexokinase bypass
2) Phosphofructokinase
3) Pyruvate kinase

–> the complete pathway, including the final conversion of glucose-6-phosphate to glucose, is predominantly found in the liver.
–> occurs in the cytoplasm

However, the enzyme pyruvate carboxylase, which converts pyruvate to oxaloacetate, is located in the mitochondria. This step bridges glycolysis (which occurs in the cytoplasm) with gluconeogenesis.

Question 4

Hexokinase Bypass
The glucose trapping step

Answer 4

the direct reversal of the hexokinase step to convert G 6-P to glucose in gluconeogenesis involves the hydrolysis of ATP = energetically costly

the cell bypasses this step by utilizing glucose-6-phosphatase,
G-6-Phosphotase catalyzes the dephosphorylation of G 6-P to glucose.

–> occurs in the endoplasmic reticulum and not the cytoplasm.
.

Question 5

Phosphofructokinase (PFK) Bypass:
Phosphofructokinase
* The classic rate limiting step

Answer 5

the direct reversal of the PFK step to convert fructose-6-phosphate to fructose-1,6-bisphosphate in gluconeogenesis expends energy = energetically costly

the cell bypasses this step by utilizing fructose-1,6-bisphosphatase,
fructose-1,6-bisphosphatase catalyzes the dephosphorylation of fructose-6-phosphate to fructose-1,6-bisphosphate.

.

Question 6

Pyruvate kinase
* The final and energy releasing
step

Answer 6

pyruvate kinase catalyzes the conversion of phosphoenolpyruvate (PEP) –> pyruvate, releasing a molecule of ATP in the process.

Reversal reactoin = unnecessary consumption of ATP

Instead of using pyruvate kinase, gluconeogenesis employs pyruvate carboxylase. Pyruvate carboxylase converts pyruvate –> oxaloacetate in the mitochondria –> transported to the cytoplasm –> decarboxylated to form PEP, catalyzed by phosphoenolpyruvate carboxykinase (PEPCK).

Question 7

what happens after late stage starvation

Answer 7

blood glucose levels drop below 5 mM = insulin secretion stops = stimulating lipolysis, the breakdown of fats into fatty acids.

–> contributes to widespread proteolysis.
Proteins in tissues, particularly in muscles = release amino acids into the bloodstream.

Question 8

amino acid processing

Answer 8

amino acids remove amine groups and left with **α-keto acids **
acceptors (pyruvate, α-ketoglutarate, Oxaloacetate) take these amine groups to make new amino acids (Alanine, glutamate and aspartate)
– Pyruvate –> alanine
– α-ketoglutarate  glutamate
– Oxaloacetate  aspartate
Resulting **α-keto acids ** helps the body make new glucose (a type of sugar) when needed in gluconeogenesis.

Question 9

fate of amino acid groups

Answer 9

To use amino acids effectively, we need to remove amine group - (NH2) = known as deamination –> we’re left with the “carbon skeleton,”
–> crucial for making new things, like glucose (sugar) when our body needs it.
–> when we remove the amine group, it turns into ammonia (NH3) = too much ammonia is harmful

liver turns ammonia = urea (something safe) –> channeled into the urea cycle

the amino acids aspartate and glutamate contribute their amine groups to the formation of urea.

the urea cycle requires a considerable amount of energy in the form of ATP = to convert amine groups into urea.

Urea is a water-soluble compound that is much less toxic than ammonia. It is then excreted in the urine, effectively removing excess nitrogen from the body.

Question 10

not all AA can be used to make glucose

Answer 10

not all can be substrates of gluconeogenesis
cetyl CoA-Exclusive Amino Acids (Blue Ones):

Certain amino acids can only be converted into acetyl CoA = not usable as substrates for gluconeogenesis = for ATP production or fatty acid synthesis.

Amino acids that can be converted into pyruvate or Krebs Cycle intermediates can make glucose [direct precursors for glucose]

–> Almost half of the amino acids are not usable as substrates for gluconeogenesis

Question 11

inefficiency of amino acids = glucose

Answer 11

• Energy-intensive: a significant ATP investment.
• Limited usability: Not all amino acids are suitable for gluconeogenesis
• Inefficient ratio: Typically, 2g of protein is needed for 1g of glucose, leading to a strain on protein reserves –> finite protein storage.

Question 12

Understand how ketone bodies are formed (Acetoacetate)

Answer 12

Fatty acids undergo β-oxidation = make acetyl CoA molecules.

Excess acetyl CoA, instead of entering the Krebs Cycle, initiates ketogenesis.

Two molecules of acetyl CoA combine (form acetoacetyl-CoA)
Another acetyl CoA molecule is added to acetoacetyl-CoA, creating 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) using enzyme HMG-CoA synthase.

HMG-CoA is then cleaved to release acetoacetate, a ketone body using the enzyme HMG-CoA lyase

Question 13

how ketone bodies help to address the shortfall on glucose demanded by the brain during long-term starvation

Answer 13

Acetoacetate can be converted to two other ketone bodies: β-hydroxybutyrate and acetone –> released into the bloodstream and transported to various tissues, including the brain.
brain undergoes metabolic adaptations to utilize ketone bodies as an alternative energy source

acetoacetate and β-hydroxybutyrate are split to form acetyl-CoA.

–> acetyl-CoA can then enter the Krebs Cycle, providing an immediate source of fuel for energy production.

–> acetyl-CoA inhibits pyruvate dehydrogenase (PDH). This inhibition further reduces the breakdown of glucose-derived pyruvate into acetyl-CoA, helping to spare glucose.

Question 14

Predict the source of inefficiencies in energy metabolism induced by the ketotic state

Answer 14

1) loss of ketone bodies in the urine.
Ketone bodies, particularly acetoacetate, are water-soluble –> excreted in urine when their concentration exceeds the body’s utilization capacity
–> a loss of potential energy as ketones are eliminated from the body instead of being fully utilized for energy production.

2) Spontaneous Decarboxylation:
ketone bodies, particularly acetoacetate –> can undergo non-enzymatic reactions, leading to the loss of carbon dioxide (decarboxylation) and the formation of acetone
–> acetone is volatile and can be exhaled through the breath
= in the loss of carbon atoms
= contributes to the inefficiency of ketone body metabolism.

Question 15

consequences of late stage starvation

Answer 15

• Late-stage starvation leads to increased breakdown of proteins from various tissues.
• Muscles, even those not actively used, may undergo degradation.
• The process reaches equilibrium, balancing protein breakdown and synthesis.
• Loss of body proteins, crucial for structural and functional integrity, occurs.
• Severe consequences include weakness, compromised immune function, and increased susceptibility to infections.
• The ultimate cause of death in late-stage starvation is the depletion of essential proteins, leading to organ failure.

Question 16

Explain the role played by glucagon in extended starvation

Answer 16

• promotes gluconeogenesis.
• breakdown of stored glycogen in the liver, releasing glucose into the bloodstream.
• stimulates lipolysis, breaking down stored fats into fatty acids and glycerol.
• contributes to the production of ketone bodies from fatty acids in the liver, providing an alternative energy source, particularly for the brain
• promotes the utilization of fats for energy, preserving proteins and preventing excessive breakdown of muscle tissues.
• ensures a continuous supply of glucose to meet the energy demands of vital tissues, particularly the brain, in the absence of sufficient dietary carbohydrates.