Glycolysis and Gluconeogenesis Flashcards
What does phosphorylation of glucose accomplish?
Traps glucose in the cell and raises energy of glucose to prepare for following rxns
Why isn’t glucose phosphorylation the “committed step” in glycolysis?
Possible to take different paths other than glycolysis (glucose-6-phosphate = metabolic cross roads)
What are potential fates of Glc-6-P? Do you think Glc-6-P would be directed to the pentose phosphate pathway if the ADP/ATP ratio was high? Why or why not?
Potential fates:
Glucose 1-phosphate - to glycogen (energy storage)
Glucose 6-phosphate to fructose 6-phosphate - to glycolysis
6-phosphoglucono-ð-lactone to ribose 5-phosphate - to nucleotide biosynthesis
ANSWER QUESTION
Would hexokinase I or glucokinase (hexokinase IV) be more active at lower glucose concentrations?
Low Km for glucose (high affinity), active at low levels of glucose
Glucokinase - larger Km (lower affinity) and needs higher conc to be active
Why is Glucokinase predominantly in liver cells?
Liver cells = where glucose storage occurs (glucokinase can deal w/ sudden incr in conc glucose)
What reactions require ATP in glycolysis? Why is this considered a good “investment”?
Phosphorylation rxns in prep phase use 2 ATP
WHY GOOD INVESTMENT
Why is the PFK1 step the considered commitment step? Why are the highly exergonic reactions the most regulated steps?
Commitment step b/c can only proceed in one direction
WHY EXERGONIC REGULATED
Why aren’t the exergonic reactions identical for both glycolysis and gluconeogenesis?
How does ATP levels affect Phosphofructokinase 1 (PFK1)?
High levels inhibit PFK1 - no longer need ATP, stops glycolysis from continuing
Low levels activate PFK1
How do AMP levels affect Fructose-1,6-bisphosphatase (F1,6BPase)?
FBPase needed for gluconeogenesis
High levels of AMP inhibit FBPase b/c AMP signals need for ATP (AMP activates PFK1)
Low levels of AMP activate FBPase
How does Fructose-2,6-BP affect PFK1?
Allosteric activator of PFK1
Incr F2,6BP, incr PFK1, incr glycolysis
Where does Fructose-2,6-Bisphosphate (F2,6BP) come from?
Generated from PFK2 enzyme
How does F2,6BP affect the direction of carbohydrate metabolism?
What is F2,6BP’s impact on homeostatic regulation of glycolysis?
Why does F2,6BP predominate in only gluconeogenic tissues?
What happens to ATP regulation of PFK1 in the presence of F2,6BP?
ATP no longer able to efficiently inhibit, ATP conc negligible for regulation of PFK-1
What are the 2 activities of the enzyme Phosphofructokinase 2 (PFK2)?
Bifunctional - kinase domain and phosphatase domain (which is active depends on hormone regulation)
Active kinase (PFK-2) - incr in F-2,6-BP, incr glycolysis
Active phosphatase (FBP-2) - decr in F-2,6-BP, incr gluconeogenesis
How does glucagon and insulin signaling affect PFK2’s activities? What proteins are involved in this regulation?
Liver wants to inhibit PFK-2 when blood sugar is low (needs gluconeogenesis); PFK-2 normally generates F-2,6-BP which inhibits FBPase-1 (used in gluconeogenesis)
PROTEINS
What is the product of PFK2’s phosphatase activity? Where does that product go?
Substrate level phosphorylation
Formation of ATP by transferring a phosphate group directly from substrate directly to ADP
What reactions in glycolysis perform substrate level phosphorylation?
Phosphoglycerate kinase and pyruvate kinase
How is pyruvate kinase regulated? What regulation is in all cells? Why is there a special regulation mechanism in the liver?
Allosteric regulation - ATP, acetyl-CoA, long-chain fatty acids (neg regulators)
WHY SPECIAL REG?
What are the potential fates of pyruvate? What happens to pyruvate in anaerobic conditions?
Energy needs - Used in TCA/citric acid cycle
Low oxygen - pyruvate used for anaerobic fermentation
How does lactate dehydrogenase increase the rate of glycolysis?
Lactate dehydrogenase catalyzes conversion of lactate to pyruvate
Allows reoxidization of NADH to NAD+ which allows glycolysis/TCA to continue after slowing down from NADH buildup
Prolonged muscle activity causes ATP demand to exceed oxygen supply - muscle cells become anerobic, pyruvate and NADH can’t go to TCA cycle, builds up and slows glycolysis
What’s the point of the Cori cycle?
Couples glucose fermentation in exercising muscle w/ gluconeogenesis in liver
Prevents lactic acidosis (excessive accumulation of lactate) in muscle under anaerobic conditions
How is pyruvate converted to PEP in gluconeogenesis? Why is pyruvate to PEP done in two steps?
CO2 group added to pyruvate w/ ATP, makes oxaloacetate
Oxaloacetate decarboxylated and phosphorylated, makes PEP
WHY 2 STEPS
Why is biotin important and what does it do?
Why is oxaloacetate a useful intermediate for gluconeogenesis—where else can it come from?
Glycogen
What is it made of? Why does it exist and in which tissues? What enzyme makes glycogen? How is it regulated? What are its substrates and products? Why would energy be required for glycogen synthesis,a nd how does energy status of the cell affect it? How is its activity regulated by insulin and glucagon?
How does glucagon affect glycogen synthase?
Glucagon promotes GS phosphorylation (inactivation)
Activates cAMP-dependent protein kinase A (PKA) and other kinases to inactivate GS and PP1
If the phosphorylated form of a glycolytic enzyme is less active, which hormone would direct this? How?
Which hormone would increase the activity of Glycogen synthase? How?
Insulin wants glycogen synthesis
Wants to prevent GS phosphorylation/inactivation by glycogen synthase kinase 3 (GSK3)
Activates phosphoprotein phosphatase 1 (PP1) to reverse any phosphorylation of GSK3
Glc-6-P that is produced by either gluconeogenesis or glycogen catabolism is converted to Glucose by a phosphatase that is inside of the ER. Why does it make sense for a liver cell to have this in the ER rather than in the cytoplasm?
What does fermentation in animals produce? Why does low oxygen drive this?
Why is the NADH/NAD+ ratio important?
What is aerobic glycolysis?
Where/when is it observed? Why don’t these cells use O2 when it is around to produce ATP? What else are they producing?
What is the end product of glucose metabolism if the TCA cycle is active?
3 NADH, 1 ATP, 1 FADH
Why would rapidly dividing cells prefer to keep the TCA cycle minimally active?
Rapidly dividing cells rely primarily on glycolysis for energy production (Warburg effect), TCA cycle moves on from glycolysis - cells want to stay in glycolysis
How do insulin and glucagon affect PFK1 and F-1,6-Bisphosphatease activities?
What is Protein Kinase A? How is it activated and how is its activity regulated by insulin vs glucagon?
Protein kinase A - key regulatory enzyme regulated by cyclic-AMP
Insulin activates PKA
Glucagon activates cAMP-dependent PKA and other kinases to inactivate glycogen synthase and PP1
How is Pyruvate kinase affected by insulin and glucagon, and why is this regulation mostly/only in the liver?
Glucagon inhibits pyruvate kinase, signals liver to activate gluconeogenesis
WHY LIVER
