303: Gluconeogenesis

Importance of glucose

• Essential fuel source of organs/tissues

• Precursor for glycogen (storage in liver and muscle)

What organ needs the most glucose?

The brain

Where does most glucose come from for body’s use?

Glycogen stores in liver

Use of glycogen by skeletal muscle

ONLY skeletal muscle can use its glycogen stores, no transport

When glucose stores deplete (low glycogen)…

more glucose is needed via gluconeogenesis

Where does gluconeogenesis occur?

Liver mainly, but also kidney cortex

Location of gluconeogenesis

Mostly cytoplasm, step 1 occurs in mitochondria, and glucose is generated in ER

Where is glucose generated?

Endoplasmic reticulum (transported to cytoplasm)

Glycolysis vs. Gluconeogenesis

• Catabolism (oxidation) vs. anabolism (reduction)

• End product pyruvate vs. end product glucose

• Generate ATP vs. Use ATP (and GTP)

End products of glycolysis

1. 2 ATP

2. 2 NADH

3. 2 Pyruvate

Reactants in Gluconeogenesis

1. 4 ATP

2. 2 GTP

3. 2 Pyruvate

4. 2 NADH

Unique reactant of gluconeogenesis

GTP (2 used up)

Is gluconeogenesis reverse glycolysis?

NO, metabolic pathways only run in one direction, gluconeogenesis is completely separate and uses different enzymes

3 Irreversible reactions of glycolysis:

1. HK/GK adding first phosphate → G-6-P

2. PFK-1 adding 2nd phosphate → F-1,6-BP

3. Pyruvate Kinase transferring from PEP → ATP + Pyruvate

Last 7 steps of gluconeogenesis are driven by…

Mass action

Enzymes that bypass pyruvate kinase:

1. Pyruvate Carboxylase

2. PEPCK (Phosphoenolpyruvate Carboxykinase)

Pyruvate Carboxylase

REQUIRES ATP - coupling

Generates oxaloacetate in mitochondrial matrix (NOT IN CYTOPLASM)

Attaches carbon to pyruvate (carboxy-)

Which steps are NOT in cytoplasm?

Step 1 - generation of oxaloaceteate by pyruvate carboxylase in matrix

Last step - generation of glucose in ER by G-6-P

How does oxaloacetate get into cytoplasm?

Malate-aspartate shuttle (IN LIVER mitochondria)

Malate-Aspartate Shuttle

Oxaloacetate in matrix → Malate via NADH → Malate in cytoplasm → Oxaloacetate via NAD⁺

PEPCK

USES GTP to get PEP from oxaloacetate
IRREVERSIBLE

Releases CO₂ by decarboxylation

Cost of bypassing Pyruvate Kinase

ATP and GTP

PEP → F-1,6-BP

Uses 2 ATP and 2 NADH (opposite of glycolysis)

Glyceraldehyde…ase (NADH) and phosphoenol….ase (ATP)

Bypassing PFK-1

Fructose-1,6,Bisphosphatase

PRIMARY regulation of gluconeogenesis

Multisubunit enzyme

Bypassing GK (liver) /HK (kidney)

Glucose-6-Phosphatase (only found in liver/kidney)

Results in GLUCOSE

Where is G-6-Pase found?

ONLY in liver/kidney cortex on endoplasmic reticulum membrane

G-6-Pase

Complex of enzyme and 3 translocases (to transport glucose from ER → cytoplasm)

Where is G-6-Pase activity?

ER lumen

G-6-Pase Translocases

• Moves phosphate to cytoplasm

• Moves glucose to cytoplasm

• Brings G-6-P into lumen from membrane

von Gierke Disease

Glycogen Storage Disease resulting in enlarged liver, hypoglycemia due to body not being able to make G-6-Pase enzyme

Gluceoneogenesis Precursors

1. Alanine (Pyruvate)

2. Amino Acids (Pyruvate and oxaloacetate)

3. Lactate (Pyruvate)

4. Glycerol

Amino acids are precursors for gluconeogenesis except…

Leucine and lysine

Low ATP favors…

Glycolysis

High ATP favors…

Gluconeogenesis

Enzymes regulated in Gluconeogenesis

1. Pyruvate Carboxylase (Pyruvate → oxaloacetate)

2. PEPCK (Oxaloacetate → PEP)

3. Fructose-1,6-bisphosphatase (F-1,6-BP → F-6-P)

4. Glucose-6-phosphatase (G-6-P → Glucose)

Kidney Gluconeogenesis

Kidney reabsorbs precursors from blood like lactate, glutamine, glycerol

Extrahepatic PEPCK

Adipose tissue, does not have G-6-Pase so PEP is converted to glycerol-3-phosphate for glyceroneogenesis (TAGs)

PEPCK is usually LOW in skeletal muscle, but if it is HIGH…

Mice weigh less, use fatty acids for energy, little lactic acid