Week 6: Gluconeogenesis and Nitrogen Fuel Balance Flashcards

1
Q

What are the characteristics of DKA?

A

Hyperglycemia (420 mg/dL, normal is 65-110 mg/dL)

Low bicarbonate

Acidosis (pH lower than 7.35-7.45 range)

Ketonemia and ketonuria (ketones in blood and urine)

Water defecit (glucose in blood pulls water out of cells)

Hypokalemia

High blood urea/nitrogen

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2
Q

What are the starting materials for gluconeogenesis? Why does this occur in DKA, and where does it mostly happen?

A

Starting materials can be derived from lactic acid or AAs from muscle. The energy derive from fatty acid oxidation, which generates ketone bodies (hence, DKA). This occurs because the B-cells of the pancreas no longer produce as much insulin (exhaustive overproduction due to growing insensitivity over time). Gluconeogenesis happens in the liver (90%) and kidneys (10%).

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3
Q

What is the first bypass of gluconeogenesis, and where does it occur? What enzymes and substrates are involved? What does this generate for the reverse GAPDH reaction?

A

Pyruvate is first converted to OAA by pyruvate carboxylase in the mitochondria, then to malate for transport to the cytosol. It is then converted back into OAA, and then into PEP by PEP carboxylase. This also generates NADH for the reverse GAPDH reaction.

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4
Q

What are the three enzymes that participate in the first, second, and third bypass of the TCA cycle during gluconeogenesis, and what corresponding enzymes do they bypass?

A

(1) Glucose-6-phosphatase bypasses hexokinase
(2) FBPase-1 bypasses PFK-1
(3) Pyruvate decarboxylase bypasses pyruvate kinase

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5
Q

How is OAA formed in the first bypass reaction?

A

Bicarbonate and pyruvate combine via pyruvate carboxylase, using an ATP to form OAA

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6
Q

How is pyruvate carboxylase activated in forming OAA? What is the significance of this reaction?

A

Low [TCAI] causes Acetyl-CoA buildup, and AcCoA activates carboxylate kinase. This reaction is significant because the oxidation of pyruvate to OAA is a major anapleurotic reaction in skeletal muscle

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7
Q

What enzyme facilitates the conversion of OAA to PEP, and what is required for the reaction?

A

GTP is required to form PEP from OAA, and PEP carboxykinase mediates the step that releases a CO2 molecule as well as a GDP. This reaction is highly entropically favored because it creates a gas molecule from a solid, and increases the molecular diversity in the space.

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8
Q

How are reducing agents produced during vigorous exercise, and what are they used for?

A

During vigorous exercise, lactate is converted to pyruvate by lactate dehydrogenase (reverse reaction of the normal reaction that generates NAD+), and formation of pyruvate creates NADH. This will be used in the (reverse) GAPDH step that forms GAP from 1,3-BPG, so reducing equivalents from the mitochondria will not be needed.

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9
Q

What are the two paths to convert pyruvate to PEP? Explain them both, and whether processes are happening in the cytosol or mitosol.

A

See photo

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10
Q

Explain the similarities and differences in the PFK-2 and FBPase-2 enzymes, how they relate, and the role F 2,6BP plays in the process.

A

When glucagon is not present and the enzyme is dephosphorylated, PFK-2 is active and catalyzes formation of Fructose 2,6-BP. This activates PFK-1 and helps drive glycolysis forward. At the same time, FBPase-2, which is a co-subunit on the same enzyme is activated by glucagon, which helps phosphorylate the FBPase-2 portion of the enzyme to it’s active form. During low [F 2,6-BP], glycolysis is inhibited and gluconeogenesis is stimulated. PFK-2 forms F 2,6-BP during the well-fed state, and FBPase-2 cleaves it to F 6-P during starvation, making more available for gluconeogenesis.

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11
Q

How is pyruvate kinase controlled in glycolysis, which normally forms pyruvate from PEP? (Hint: negatively regulated by two major molecules)

A

It is inhibited by phosphorylation promoted by glucagon, and by alanine.

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12
Q

What are some ways we can make intermediates in the TCA cycle?

A

Conversion of pyruvate to OAA by pyruvate carboxylase, or degradation of amino acids like alanine to yield TCA intermediates. The more OAA available in the mitochondria, the greater the flux of AcCoA from fatty acids into the TCA cycle.

Use of amino acids, pyruvate, and lactic acid for gluconeogeenesis must ALL proceed through OAA, which can only be synthesized in the mitochondria.

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13
Q

What happens when OAA is depleted by high activity of gluconeogenesis?

A

This slows down the entry of AcCoA into the TCA cycle in the liver mitochondria. Instead, more AcCoA is used in the liver mitochondria for ketogenesis. As such, blood levels of ketone bodies increase to make up for lower levels of blood [glucose]

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14
Q

What is the function of FA oxidation versus gluconeogenesis?

A

FA oxidation produces Acetyl-CoA, whereas gluconeogenesis produces all OTHER TCA intermediates.

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15
Q

Explain the two kinds of amino acids that cen enter the TCA cycle as intermediates

A

Glucogenic AA catabolism creates pyruvate and/or TCA intermediates

Ketogenic AA catabolism creates Acetyl-CoA, which is limited by the intermediates that can be formed. No matter how much Ac-CoA you have, you need intermediates to re-form glucose. This preserves proteins and prevents degradation of muscles and other critical tissues.

Green = glucogenic, Red = ketogenic

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16
Q

What prevents the major catabolism of proteins during starvation?

A

As amino acids are deaminated, the excess nitrogen is absorbed by a-ketoglutarate to form glutamate. As a-ketoglutarate intermediates are removed from the cycle in this process, low availability of TCA intermediates limits protein catabolism.

17
Q

What element of TAG serves as starting material for gluconeogenesis?

A

The glycerol moiety of TAGs becomes a gluconeogenesis intermediate.

18
Q

Explain how the process by which glucose breakdown in muscle helps regenerate the a-ketoglutarate molecule, or how what process helps form pyruvate from an AA in the muscle.

A

Breakdown of glucose to pyruvate in the muscle can “absorb” the NH3 group from glutamate by attaching the NH3 to pyruvate, forming alanine and regenerating a-ketoglutarate. Alanine can move to the liver and be regenerated into pyruvate for gluconeogenesis. In that regeneration process, a-ketoglutarate is converted to glutamate, which then forms glutamine (2 NH3 groups), which drops off an NH3 to form urea, and reforms glutamate. Glutamate can be deaminated to form a-ketoglutarate, or can be reformed into glutamine to drop off another NH3 group.

19
Q

What kind of enzyme catalyzes the deamination of amino acids, and what vitamin does it require? Explain the process.

A

Vitamin B6, or pyridoxine, is a part of a cofactor for the aminotransferases that abstract an NH3 group from an AA, leaving the carbon skeleton to form TCA intermediates and forming glutamate from a-ketoglutarate.

20
Q

What enzyme catalyzes the re-formation of a-ketoglutarate? What is it stimulated and inhibited by?

A

Glutamate dehydrogenase forms NADH OR NADPH (UNIQUE), and is activated by ADP and inactivated by GTP. It abstracts the NH3 from glutamate and uses a water molecule to re-form a-ketoglutarate.

21
Q

Explain/draw the formation of urea from glutamate, including the enzymes and intermediates involved.

A

Glutamate is formed into y-Glutamyl phosphate by glutamine synthetase, which adds a phosphate group from ATP. Then, glutamine synthetase adds another NH3 group that replaces the added phosphate, forming glutamine. Glutaminase converts glutamine back into glutamate in liver mitochondria, using water to hydrolyze an NH4 group, which, with another NH4 group and carbon dioxide, will form urea.