Glycolysis and TCA Cycle Flashcards
3 Pathways Glucose-6-P is used in
Gluconeogenesis
Glycolysis
PPP
Glucose Transporters
Transport via conformational change
Hexokinase vs Glucokinase
Glucokinase: Km = 10 mM, NOT inhibited by glucose 6-phosphate. Present in liver and in pancreas b cells.
Hexokinase: Km= 0.2 mM, inhibited by glucose 6-phosphate. Present in most cells.
3 Regulatory Steps of Glycolysis
(+) Insulin ; (-) Glucagon
Hexokinase: inhibited by glucose-6-phosphate; least important regulated step
PFK-1: most important regulatory step; requires ATP; committing step; ** UNIQUE to Glycolysis**
Pyruvate Kinase
Glucose-6-P
Adding the phosphate traps the glucose in the cell
** inhibits Hexokinase (most tissues)
** does NOT inhibit Glucokinase (liver): enables glycogen synthesis when glucose is abundant
Glycolysis: Net Rxn
2 Pyruvate + 2 ATP + 2 NADH
Total of 4 ATP, however 2 used early on-> net of 2 ATP
Anaerobic Glycolysis
Occurs when: low O2, when respiration process is impaired (pyruvate dehydrogenase)
Pyruvate is reduced to lactate, regenerating NAD+ to keep the glycolytic pathway going for minimal ATP production
Hexokinase
- present in all tissues
- provides glucose-6-P for production of ATP when tissue [glucose] is low
- inhibited by it’s product (feedback inhibition)
Glucokinase
- LIVER enzyme
- glucose sensor: functions best when [glucose] is high
- High Km, high Vmax: allows liver to effectively remove glucose delivered by portal blood
- regulated by insulin
GKRP
Glucokinase Regulatory Protein (liver)
- regulates glucokinase; reversibly binds GK and translocates to nucleus, inhibiting cycle
- in presence of fructose-6P, GK moved to nucleus and bound tightly by GKRP
- when blood glucose is high, GK renters cytoplasm and catalyzes synthesis of G-6P
PFK-1
Principal site of regulation of glycolysis
Activators:
- AMP: signifies low energy charge; stimulates glycolysis at PFK1
- F-2,6,BP: signifies high blood [glucose]
Inhibitors:
- ATP: high energy charge
- Citrate
- H+ (lactate)
- FA: indicators of adequate nutrition
Pyruvate Kinase
Regulatory step in glycolysis
Activator: F-1,6-BP (“feed-forward”)
Inhibitors:
- ATP: high energy charge
- Alanine
- Hormonal: activation of protein kinase A
Fructose 2,6-bisphosphate synthesis is stimulated by
insulin
- insluin signals the well-fed state and stimulates glycolysis, primarily by enhancing PFK-2 catalyzed synthesis of F-2,6-BP, an allosteric activator of PFK-1
Maturity onset of diabetes of the young (Type II)
caused by mutations that decrease glucokinase activity
Arsenic poisioning
Arsenic resembles Pi and competes with Pi as a substrate for glyceraldehyde 3-P dehydrogenase; less ATP generated as a result
Pyruvate Dehydrogenase Deficiency
second most common cause of hemolytic anemia
Lactic Acidosis
In low O2 conditions, anaerobic glycolysis serves as a backup and generates ATP when ETC is not possible; LACTIC ACID created decreases blood pH to dangerous levels–> requires Cori cycle to uptake lactate from the blood by liver followed by gluconeogenesis
The Cori Cycle
Lactate produced in glycolysis during muscle exertion is transported to the liver for resynthesis of glucose by gluconeogenesis
Fructose Metabolism (liver)
- enters lower in glycolysis cycle
- ** bypasses regulation of PFK**
- does not stimulate the satiety-promotion substance leptin (you don’t feel full)
3 Stages of Cellular Respiration
1) Carbon from AA, CHO, and Lipids -> Acetyl CoA
2) TCA: oxidation of Carbon to CO2
3) ETC: reduced electron carriers are reoxidized; coupled to ATP synthesis; CHEMIOSMOTIC NOT substrate level phosphorylation
Pyruvate Dehydrogenase Complex Net Rxn
pyruvate -> AcetylCoA + NADH + CO2
IRREVERSIBLE: Acetyl COA CANNOT regenerate glucose (no going backwards)
Pyruvate shuttled into mitochondrial matrix for rxn
Pyruvate Dehydrogenase Complex Enzymes
- decarboxylase : lose C (as CO2)
- transacetylase: CoA -> Acetyl CoA
- dehydrogenase: NAD+ -> NADH
E1: pyruvate dehydrogenase/decarboxylase: PDC
catalyzes pyruvate to acetyl (releases CO2)
COFACTOR: thiamine pyrophosphate (TPP, Vitamin B1) -> no thiamine = lactic acidosis
E2: dihydrolipoyl transacetylase: PDC
attaches CoA to acetyl
COFACTOR: lipoic acid & coenzyme A
E3: dihydrolipoyl dehydrogenase: PDC
reduces NAD+ to NADH
COFACTOR: NAD+ & FAD
Pyruvate Dehydrogenase
- Active form NOT P
- when high ATP, phosphorylated to deactivate (occurs when high NADH and acetylCoA)
dephosphorylation occurs when high Mg and Ca
Deficiency in E1 Component PDH
E1: pyruvate dehydrogenase
congenital lactic acidosis
- pyruvate can’t be converted into acetylCoA and thus is shunted to lactic acid; fix with diet and thiamine supplement
Arsenic Poisoning PDC
inhibits PDC
Isocitrate dehydrogenase mutation
defect in gene encoding TCA cycle; leads to cancers (i.e. glioblastomas)
Sites for TCA Control
- isocitrate dehydrogenase
- a-ketoglutarate dehydrogenase
- malate dehydrogenase
- citrate synthesis
Glycolysis control vs. TCA control
Glycolysis Control: kinase rxns
TCA Control: Dehydrogenase rxns
TCA Net Rxn
(1 GTP + 3 NADH + 1 FADH2) x 2 (because 2 acetylCoA/glucose)
Total: 2 GTP + 6 NADH + 2 FADH2
Complete Oxidation of Glucose to CO2
36 ATP
-FA oxidation produces less energy?
Conversion of pyruvate -> AcetylCoA and CO2 requires
Lipoic Acid
Insulin vs Glucagon P
Insulin Dephosphorylates
Glucagon Phosphorylates
PFK II
2 Domains: Kinase; Phosphatase
Fructose-6-P -> Fructose-2,6-BiP Stimulates PFK 1
Fructose-2,6-Bisphosphatase
Fructose-2,6-Bisphosphate -> Fructose-6-Phosphate inhibits PFK1 and glucokinase
Glucagon/Insulin PFKII
Glucagon Ps: inactivating PFK II ; activating F26BiPase
Insulin DePs: activating PFK II ; inactivating F26BiPase
Most powerful regulator of PFK I
Fructose-2,6-BisP
- made by PFK II from F6P and stimulates PFK I
Fed State
Blood glucose levels high -> insulin secretion -> promotes:
- glycolysis
- glycogenesis
- lipogenesis
- protein synthesis
Fasting State
Blood glucose levels fall -> glucagon secretion -> promotes:
- glycogenolysis
- gluconeogenesis
- lipolysis
- ketogenesis
