Oxidative phosphorylation and Gluconeogenesis Flashcards
electron transport chain
- consists of 4 (I-IV) large multi-protein complexes located in the inner mitochondrial membrane
- electron flow from NADH and FADH2 to O2.
- ATP formation is driven by a proton gradient across inner mitochondrial membrane by transfer of electrons to ETC in “chemiosmotic coupling”
establishing a proton gradient in ETC
-established by complexes I, III, and IV which pump H+ ions from the mitochondrial matrix to the intermembranous space.
Complex V
ATP synthase
-H+ ion gradient is used to drive ATP synthesis from ADP by enzyme ATP synthase
substrates and products for oxidative phosphorylation
Substrates: NADH, FADH2, O2, Pi, and ADP
Products: NAD, FAD, H2O, and ATP
oligomycin
drug that inhibits ATP synthesis (NADH and FADH2 back up, and reduce flux thru TCA cycle– cell must rely on glycolysis for energy =inefficient)
Carbon monoxide poisoning
- inability of hemoglobin to release oxygen
- so ETC can’t run even though “pO2” remains high.
uncoupling proteins
-allow proton gradient across inner mitochondrial membrane to dissipate without coupling to ATP generation resulting in loss of chem energy as heat. (brown adipose tissue uses this)
NADH and FADH2 yields
NADH: 2.5 ATP
FADH2: 1.5 ATP
What controls rate of oxidative phosphorylation?
level of ADP (“respiratory control”– O2 consumption depends on availability of ADP)
- low ADP (high ATP) decreases flow of e- (decreases O2 consumption)
- High ADP (low ATP) increases flow of e- (increases O2 consumption)
Overnutrition and consequences
- inability to couple excessive fuel with ATP synthesis is assoc with development of oxygen radicals –> cellular injury, aging, apoptosis.
- exercise: increase in oxidative capacity (less free rad buildup) due to increase in muscle mitochondria content
-hypocaloric diets might limit the generation of free radicals
PGC1alpha
peroxisome proliferater-activated receptor gamma co-activator 1alpha
- molecular mediator of mitochondrial proliferation
- ? drug target to combat metabolic diseases.
Location of gluconeogensis
Cytosol (but pyruvate carboxylase is in mitochondria)
Gluconeogenesis: glucose is synthesized from non-carbohydrate precursors
Sites of gluconeogenesis
Liver Kidney (up to 20% of glucose produc in prolonged starvation)
When does gluconeogenesis occur?
fasting
vigorous exercise
a low carb/high protein diet
under conditions of stress when counter-regulatory hormones are high
in states of insulin resistance and type 2 diabetes
Main sources of carbon skeletons for gluconeogenesis
- lactate (Cori cycle)
- amino acids (alanine and glutamine, especially)
- glycerol (from hydrolysis of triglycerides)
Can fatty acids convert to glucose?
- there is no net conversion of fatty acids to glucose in mammals (the 2 C that enter TCA as acetyl CoA from beta oxidation leave TCA as CO2—> no C left for glucose synthesis)
- fatty acids provide ENERGY for gluconeogenesis (generates ATP thru oxidation)
Pyruvate to PEP (gluconeogenesis) (bypass reaction I)
- pyruvate to phosphoenol pyruvate by way of oxaloacetate (OAA)
- pathway predominates when pyruvate or aa alanine is major precursor
- CO2 is activated and transferred by pyruvate carboxylase to its biotin prosthetic group.
- The enzyme transfers CO2 to pyruvate generating oxaloacetate
- Oxaloacetate cannot cross the mitochondrial membrane into cytosol so it is reduced to malate that can
- in cytosol, malate is reoxidized to oxaloacetate which is converted to phosphoenol pyruvate by PEP carboxykinase
(OAA to malate: catalyzed by malate dehydrogenase (mitochondrial)
Malate to OAA: malate dehydrogenase (cytosol)
Biotin deficiency
leads to buildup of pyruvate which is converted to lactic acid and leads to LACTIC ACIDOSIS
First regulated enzyme in gluconeogenesis
pyruvate carboxylase
Needs acetyl CoA as positive effector (provided by fat oxidation during fasting)
2 key enzymes in overall conversion of pyruvate to PEP
pyruvate carboxylase phosphoenol carboxykinase (PEP carboxykinase)
Overall need ATP and GTP inputs. (ATP source: oxidation of fat)
Bypass reaction II in gluconeogenesis
F1,6BP to F6P using fructose 1,6 bisphosphatase
Key regulator of fructose 1,6 bisphosphatase is fructose 2,6 bisphosphate
Gluconeogenesis is promoted when insulin is low and glucagon is high (bifunctional enzyme regulated by phosphorylation)
- high glucagon/insulin ratio–>cAMP–> PKA
- Favors phosphorylated form of PFK-2/FBP-2 complex (phosphorylated PFK-2 is inactive, whereas FPB-2 is active– impedes formation of fructose 2,6 BP. Decreased F2,6BP leads to decreased inhibition of fructose bisphosphatase 1 FBP-1–> gluconeogenesis
Bypass Rxn III in gluconeogenesis
G6P to glucose using glucose-6-phosphatase.
-enzyme only found in ER in liver and kidney cells.
(G6P goes into lumen of ER, and glucose formed is transported outside the cell)
Von Gierke’s disease
AT
- deficiency of G6 phosphatase in liver
- glycogen is normal, but affected indiv have severe fasting hypoglycemia, ketosis, lactic acidosis, enlarged liver and kidneys.
