Gluconeogenesis & Oxidative Phosphorylation

Question 1

Metabolic role of gluconeogenesis

Answer 1

• liver is the most important site for gluconeogenesis
• kidney has the capability for gluconeogenesis and is responsible for roughly 20% of whole body glucose production under conditions of prolonged starvation.

Question 2

General process of gluconeogenesis

Answer 2

• Gluconeogenesis is the process whereby glucose is synthesized from non-carbohydrate precursors
• takes place mostly in the cell cytosol, although pyruvate carboxylase, one of its enzymes, is in mitochondria.

Question 3

Major gluconeogenic precursors & generation

Answer 3

• lactate
  
  • formed @ skeletal m. during exercise
  • formed @ RBCs (no mitochondria)
  • lactate ==> pyruvate ==> gluconeogen
• amino acids
  
  • alanine, when trans-aminated ==> pyruvate ==> gluconeogen
  • glutamine, when trans-aminated ==> alpha-ketoglutarate ==> gluconeogen
  • AA ==> TCA cycle ==> ocaloacetate ==> gluconeogen
• glycerol
  
  • hydrolysis of triglycerides ==> glycerol ==> gluconeogen

Question 4

Can fatty acids be converted to glucose?

Answer 4

• NO – no in mammals, anyway
• FA ==> TCA cycle as acetyl CoA ==> CO₂ + electron carriers; leaves no net carbons to contribute to glucose synthesis
• fatty acids DO provide energy for gluconeogenesis
  
  • FA oxidation is critical as it generates the ATP necessary for gluconeogenesis which is an energy requiring pathway

Question 5

General proces of gluconeogenesis

Answer 5

• gluconeogenesis = ~ reverse of glycolysis
• EXCEPT certain steps must be bypassed:
  
  • a. Pyruvate kinase/PEPCK (PEP + ADP ==> pyruvate + ATP)
  • b. Phosphofructokinase (F6P + ATP ==> F1,6BP)
  • c. Hexokinase (Gluc + ATP ==> G6P)

Question 6

Gluconeogenesis bypass rxn #1

Answer 6

• overall: pyruvate ==> oxaloacetate ==> phosphoenol pyruvate

1. pyruvate ==> mitochondria
2. pyruvate + HCO₃^- + ATP ==> oxoaloacetate (OAA) + ADP + Pi
   
   1. ​mediated by Pyruvate carboxylase
   2. pyruvate caroxylase = regulated enzyme
      
      1. acetyl CoA = positive effector
3. OAA + NADH + H⁺ ==> malate + NAD^{+ ​}
   
   1. mediated by malate dehydrogenase (mitochondrial)
4. malate ==> cytosol ==> OAA
5. OAA + GTP <==> phosphenolpyruvate + CO₂ + GDP
   
   1. phosphenol pyruvate carboxykinase (PEPCK; cytosol)

Question 7

Pathway and rxn involving pyruvate carboxylase?

Answer 7

• pathway: gluconeogenesis
• rxn: pyruvate + HCO3- + ATP ==> oxoaloacetate (OAA) + ADP + Pi
• regulation
  
  • acetyl CoA = positive effector

Question 8

Pathway and rxn involving malate dehydrogenase (mitochondrial)?

Answer 8

• pathway: gluconeogenesis OR TCA cycle
• rxn: OAA + NADH + H+ ==> malate + NAD+ ​

Question 9

Pathway and rxn involving PEPCK (cytosol)

Answer 9

• PEPCK = phosphoenol pyruvate carboxykinase
• pathway: gluconeogenesis
• rxn: OAA + GTP <==> phosphenolpyruvate + CO2 + GDP

Question 10

Gluconeogenesis bypass rxn #2

Answer 10

• fructose-1,6-bisphosphate + H₂O ==> F6P + P_i
• catalyzed by fructose 1,6 bisphosphatase
  
  • key regulator = fructose 2,6 bisphosphate
  • also regulated by phosphorylation

Question 11

regulation of PFK vs. F-1,6-bisphosphatase (schematic)

Answer 11

Question 12

Gluconeogenesis bypass rxn #3

Answer 12

• G6P + H2O ==> glucose + Pi
• mediated by Glucose-6-phosphatase
  
  • found in ER membrane @ hepatocytes, kidney cells

1. G6P ==> ER lumen ==> glucose ==> transported out

Question 13

Mechanisms for generation of energy via oxidative phosphorylation

Answer 13

• 4 multi protein complexes establish a proton gradient across the inner mitochondrial membrane
• [H+] is higher in the intermembrane space. H+ then drives ATP synthase/Complex V to create ATP from ADP.
• Complex II is not shown in the picture below, but it has the same function as complex I except it oxidizes FADH2.

Question 14

Components of electron transport chain + locations

Answer 14

• Complex I - IV are embedded in the inner membrane and develop the H+ gradient in the intermembrane space.
  
  • Complex I uses NADH
  • Complex II uses FADH2
  • Complex III + IV use Fe2+along with CoQ and Cytochrome C respectively
• _ATP synthas_e, otherwise known as Complex V, is also embedded in the inner membrane

Question 15

Substrates for oxidative phosphorylation

Answer 15

• FADH2
• NADH
• H+
• O2
• ADP, Pi
• electrons!

Question 16

Consequences of defects in electron transport chain

Answer 16

• inborn defects ==> problems after exercise e.g.:
  
  • retinal dysfunction
  • encephalopathy
  • Muscle myopathies
• possible connections to obesity and type 2 diabetes
• also…
  
  • heart failure, alzheimer’s, hypoglycemia