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144 Cards in this Set
- Front
- Back
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What is glycolysis? What are its 2 possible products? By what pathways?
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Metabolism of sugar to:
-2 Pyruvate (aerobic) -2 Lactate (anaerobic) |
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What is the net energy yield per glucose molecule in glycolysis?
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2 ATP
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What is the great advantage of glycolysis?
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Can run at high speed, and
Can run in absence of oxygen at expense of flooding tissues w/ lactate. |
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What in general terms is gluconeogenesis?
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Replenishing the glucose consumed by glycolysis.
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What is the purpose of gluconeogenesis in the liver?
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To maintain blood glucose levels
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What is the purpose of gluconeogenesis in muscle?
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To replenish muscle glycogen consumed in earlier bursts of muscle activity.
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How far back in reverse of glycolysis does muscle gluconeogenesis proceed? Why?
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Only back to G6P - muscle does not have G6Phosphatase.
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What exactly is produced by glycolysis?
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-2 Pyruvate
-2 ATP -2 NADH |
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Where does pyruvate go after glycolysis?
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If O2 is available, to TCA.
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In what cell compartment does glycolysis take place?
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Cytosol
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Where does gluconeogenesis occur?
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Partly in cytosol
Partly in mitochondria |
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What do isomerases do?
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Change aldehydes to carbonyls and vice versa
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What do mutases do?
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Move phosphates
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Is glucose the only sugar substrate for glycolysis?
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No:
-Fructose enters directly -Others (galactose) enter after conversion. |
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Why is glucose the ideal substrate for rapid energy production vs FA's which have more energy content?
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-It is more soluble
-Can be metabolized when O2 is limiting |
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What tissue does gluconeogenesis mainly occur in?
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LIVER
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Why doesn't as much gluconeogenesis happen in muscle?
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They don't have the enzymes necessary to generate free glucose
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What is "gluconeogenesis" (not really) used for in muscle?
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To replenish GLYCOGEN.
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What are the 2 main stages of glycolysis?
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1. Making two 3-carbon molecules from one 6-carbon glucose.
2. Using the 2 products of stage 1 to generate 2 ATP total |
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Why are only 2 ATP generated in glycolysis?
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-2 invested in stage one
-2 produced per 3-C molecule (4) 4-2=2 |
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What begins glycolysis
-Historically -Formally |
Historic: includes glucose trapping via hexo/glucokinase
Formal: PFK-1 catalyzed reaction (rate determining step of pthwy) |
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What are the 2 irreversible steps in glycolysis?
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1. PFK-1 (commitment step_
2. Pyruvate kinase (last step) |
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How many enzymes are involved in glycolysis?
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9 (not including the glucose trapping enzyme hexokinase)
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What are the 9 enzymes?
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PPATGPPEP
Pumpkin Pie Apple Tart Grateful Phor Plenty of Enzyme Pathways |
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PPATGPPEP stands for:
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1. Phosphoglucose isomerase
2. Phosphofructokinase 3. Aldolase 4. Triose phosphate isomerase 5. G3P dehydrogenase 6. Phosphoglycerate kinase 7. Phosphoglycerate mutase 8. Enolase 9. Pyruvate kinase |
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What are the enzymes for the irreversible steps of glycolysis?
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1. 'Pie' - phosphofructokinase
9. 'Pathways' - Pyruvate kinase |
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What is the first step of glycolysis?
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An isomerization to convert G6P to F6P
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How is the isomerization done?
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Via simple acid-base catalysis.
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What enzyme is very important in glycolysis? Why?
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PFK-1
B/c it catalyzes the rate-determing step of commitment. |
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What happens in the reaction catalyzed by PFK1?
What is the product? |
-Another phosphate is donated from ATP to Fructose6Phosphate
-Generates F-1,6-Bisphosphate |
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What is the major difference between glucose and fructose (achieved by an ISOMERASE)?
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Isomerases convert aldehydes to ketones (glu->fru).
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What is the quatern. structure of PFK1?
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Tetramer
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What is involved in the PFK reaction?
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ATP hydrolysis
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At what carbon is Fructose 6-P phosphorylated by PFK1?
What is the anomeric carbon? |
Carbon 1
Carbon 2 is anomeric |
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What type of regulation is PFK1 subject to?
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Allosteric
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How is PFK1 exist?
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In one of 2 conformations:
R-relaxed, active T-taut, inactive |
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What conformation is preferentially bound by
-Activators? -Inhibitors? |
Activators - bind relaxed
Inhibitors - bind taut |
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What is the main allosteric inhibitor of PFK1?
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ATP - acts as a general signal of the cell's energy content; if high, indicates no need for E.
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How does ATP alter PFK1?
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By altering its Km for F6P:
-High ATP = High Km of F6P -Low ATP = Low Km... |
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How does ATP alter PFK's activity as Fructose-6-P conc. increases on a graph?
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Sigmoidally - only as F6P substrate increases does the inhibitory effect of ATP get overcome.
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What is the effect on PFK1 activity when both AMP+ATP are present?
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AMP increases it - not as much as if ATP were not there, but it makes PFK1 active even at low F6P concentrations. (graph)
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What are the 3 allosteric inhibitors of PFK1?
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1. ATP
2. Citrate 3. Low pH (acid) |
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Why is citrate a PFK inhibitor?
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B/c it is TCA cycle intermediate.
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Why is Acid an inhibitor of PFK?
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It occurs when Active glycolysis generates Lactic Acid (pyruvate not converted to CO2).
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What are the 3 energy metabolite allosteric activators of PFK1? How do they work?
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AMP, ADP, and Pi;
Bind allosteric ATP site, so they compete with ATP. |
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What is a non-energy metabolite allosteric activator of PFK1?
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Fructose 2,6-Bisphosphate
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How does F26BP alter the activity of PFK1?
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By decreasing its Km; Thus PFK1 operates more efficiently even at low F1,6BP concentrations.
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How exactly does F2,6BP decrease the Km of PFK1?
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Binding increases PFK1's affinity for substrate (F6P) and decreases ATP's inhibition.
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Why is Fructose 2,6BP so important?
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B/c it is NOT a metabolic intermediate, but a longer term regulator; it sets the rate of glycolysis. MOST IMPORTANT!!!
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What is F26BP itself regulated by?
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Insulin and glucagon levels in the liver.
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How is the regulatory molecule F26BP made? What enzyme?
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By conversion of a little bit of F6P by the enzyme PFK2
KINASE |
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What enzyme breaks down F26BP when activation is not desired?
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Fructose Bisphosphatase-2 (FBPase-2)
PHOSPHATASE |
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How are PFK2 and FBPase-2 related?
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They are both the same enzyme, just bifunctional - reciprocal kinase /phosphatase activities.
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To what 2 types of regulation is this bifunctional enzyme subject?
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1. Allosteric (via F6P)
2. Covalent |
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Where does covalent regulation of PFK2/FBPase-2 take place? How?
(for the liver enzyme) |
-At a single serine residue in the gene regulatory domain;
-Gets Phosphorylated by a Protein Kinase. -UNIQUE FOR THE LIVER ENZYME |
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So what are the 3 domains of the PFK2/FBPase-2 gene?
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1. Phosphatase
2. Kinase 3. Regulatory |
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What directs covalent regulation?
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Insulin vs. Glucagon levels in the liver.
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How does phosphorylation of the Ser on the regulatory domain of the bifunctional enzyme alter glycolysis?
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Phosphoryltn = FBPase2
(no glycolysis b/c no PFK1) DEPhosphorylation = PFK2 (Glycolysis b/c PFK1 = active) |
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What does high Glucagon stimulate?
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Increased cAMP; Active cDPK; Dephosphorylation so PFK2 is inactive and glycolysis WON'T occur - THE LIVER NEEDS TO RELEASE GLUCOSE AND MAKE IT; not break it down.
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What concurrently results when Glucagon stimulates FBPase2 formation by phosphorylating the regulatory domain?
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Stimulation of gluconeogenesis.
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What step takes place after PFK produces Fructose 1,6-BP?
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It is cleaved into DHAP and G3P (GAP) by Aldolase
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What type of reaction is catalyzed by Aldolase?
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The reverse of an aldol condensation - produces an aldehyde (GAP) and ketone (DHAP)
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Which product of aldolase continues with glycolysis?
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Only GAP
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How is DHAP converted to GAP - by what enzyme?
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Triose phosphate isomerase (TIM)
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What would be the result of no TIM were present?
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Only 1/2 of each glu molecule would proceed thru glycolysis - no net production of ATP.
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What is the 1st step of the energy-producing part of glycolysis? Why is it special?
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Conversion of GAP-> 1,3-BPG
-First oxido-reduction -Generates hi-energy PO4 bond |
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What 2 things account for the energy derived in this conversion?
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-Hi energy phosphate bond
-NADH produced |
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How does G3P-dehydrogenase allow the hi-energy PO4 bond to be made?
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Has a Cys in active site; oxidizes aldehyde of GAP to C=O; Phosphate attachment yields MIXED ACID ANHYDRIDE
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What is the use of a mixed acid anhydride?
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It drives the formation of ATP via substrate level phosphorylation in the next step.
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So why is the GAP-> 1,3BPG step special?
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1. First redox reaction
2. Generates hi-energy bond |
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What inactivates GAP dehydrogenase? (2 things)
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1. Heavy metals - inactivate the thiol in active site.
2. Arsenate |
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How does Arsenate inhibit GAP dehydrogenase?
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By mimicking Pyrophosphate; prevents forming the hi energy bond, and ATP generating step is skipped.
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What enzyme catalyzes the step that harvests the energy of the mixed acid anhydride 1,3-BPG?
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Phosphoglycerate kinase
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What are the products of this harvest?
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ATP + 3-Phosphoglycerate
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What happens after the first substrate level phosphorylation?
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Mutation of 3-phosphoglycerate; PO4 moves to Carbon 2
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What enzyme catalyzes this mutation?
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Phosphoglycerate mutase
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What step occurs after 2-phosphoglycerate is formed?
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Dehydration - removal of an OH to form a double bond
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What catalyzes this dehydration step 8, and what is the product?
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Enolase - produces PEP (phosphoenolpyruvate)
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Why is PEP so peppy?
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Because it has a PHOSPHATE coming right off a double bond; that's very unstable.
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What is the use of the phosph-enol of PEP?
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Provides energy for another substrate level phosphorylation.
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What is the final product of glycolysis?
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Pyruvate
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What enzyme catalyzes formation of pyruvate?
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Pyruvate kinase.
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What is an alternative use of 1,3-BPG rather than the first substrate level phosphorylation?
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Can be converted by RBCs to 2,3-DPG via BPG mutase.
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What regulates the concentration of 2,3-DPG in rbcs?
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Rates of Mutase and Phosphatase
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What does 2,3-BPG phosphatase form?
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3-phosphoglycerate for re-entry into glycolysis.
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Is the 2nd substrate-level phosphorylation reversible?
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no
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What regulates pyruvate kinase?
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F1,6BP - it is an allosteric feedforward activator.
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Why is it good that F16BP is an activator of PK?
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Because it allows the 2 regulatory steps to be coupled.
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What are 2 feedback inhibitors of PK?
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-ATP
-Alanine |
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Why is ALANINE a feedback inhibitor of PK?
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-High levels signal protein degradation; meaning blood glucose and glycogen levels are depleted; now the liver should be doing gluconeogenesis.
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How are Alanine and Pyruvate related to each other?
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Structurally; can be interconverted via an aminotransferase.
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What type of regulation is LIVER pk subject to?
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Covalent - cADPK can phosphorylate it.
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What do you think happens to glycolysis when cADPK phosphorylates PK?
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cADPK would be stimulated by glucagon which sends a signal to stop breaking down sugar in the liver, and instead run gluconeogenesis to replenish the blood glucose.
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So phosphorylated PK is ____ and
unphosphorylated PK is ____? |
Phosphorylated=inactive
Unphosphoryltd=active |
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What enzyme dephosphorylates PK to reactivate it?
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Phosphoprotein phosphatase
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How does covalent regulation of PK relate to that of PFK1?
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Phosphorylated bifunctional enzyme has FBPase1 activity; F26BP levels decrease and PFK1 is not active.
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During ANAEROBIC glycolysis how is NAD+ regenerated?
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By converting Pyruvate to lactate (concomitant oxidation of NADH)
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During AEROBIC glycolysis how is NAD+ regenerated?
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By oxidative phosphorylation in mitochondria.
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2 Shuttles regenerate NAD+ for aerobic glycolysis:
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1. Glycerol phosphate shuttle
2. Malate Aspartate shuttle |
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How does the Glycerol phosphate shuttle work?
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G3P dehydrogenase on inner mito membrane oxidizes G3P (diffused thru outer mito memb) by reducing FAD to release FADH2 into mitosol.
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What gets produced from this?
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DHAP - into the mito intermembrane space; diffuses back to cytosol
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What happens to the FADH2 generated in the mitosol?
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It goes on to ETC
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What is the drawback to relying on the Glycerol phosphate shuttle for NAD+ replenishing?
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Oxidative phosphorylation only produces 1.5 ATP/pair of reducign equivalents transferred from NADH to FADH2 utlimately.
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How does the Malate-OAA shuttle work?
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1. OAA is reduced to Malate by cytosolic malate dehydrogenase - produces NAD+ in cytosol.
2. Malate exhcanges for a-ketoglutarate to get into mito matrix. 3. Malate transfers e- to NAD+ by mitochondrial malate dehydrogenase + OAA |
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How does OAA get back out of the mito matrix to cytosol?
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Converts to 4-Carbon Asp which can cross; Glu brings amino group back into matrix.
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What determines how fructose enters glysolysis?
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Tissue type:
Other than liver: hexokinase directly phosphorylates fructose Liver: glucose outcompetes fructose for the active site of glucokinase. |
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What pathway takes fructose into glycolysis in the liver?
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Fructose-1-phosphate pathway; requires ATP, Fructokinase, Fructose 1-phosphate aldolase, Triose Kinase, and G3P.
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What is required for the F1P pathway in liver? (substrate adn 3 enzymes)
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-Fructose (duh) + ATP
-Fructokinase -Fructose 1-phosphate aldolase -Triose Kinase |
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What is a good/bad thing about fructose entering glycolysis?
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It bypasses the regulatory step of PFK1 b/c it enters as G3P.
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Why is it bad for Fructose to enter glycolysis if high?
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It ties up all the phosphate and depletes ATP.
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How does Galactose enter glycolysis?
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As glucose-6-phosphate
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What is the key enzyme that converts galactose to glucose?
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An epimerase - flips one hydroxyl to make glucose.
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So how does Glucagon exert its effect?
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Via cAMP-depenent protein kinase
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What are the sites of action of cAMP-dependent protein kinase in regulating glycolysis? What signal does it send?
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-Glucokinase
-PFK2 -Pyruvate kinase All are inhibited by glucagon |
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What are the majority of enzyme defects in glycolysis related to?
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Pyruvate kinase
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How is enzyme deficiency in glycolysis mainly manifested?
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As HEMOLYTIC ANEMIA - RBCs can't mantain adequete ATP to maintain their membranes.
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What 4 molecules can glucose be MADE from?
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-Lactate
-Pyruvate -Glycerol (lipid backbone) -a-ketoacids |
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What is the difference between glucogenic and ketogenic amino acids?
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Glucogenic - can become glucose
Ketogenic - only lead to AcCoA; can't become glucose. (Leu/Lys) |
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What are the 4 enzymes unique to gluconeogenesis b/c they are involved in irreversible steps?
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1. Pyruvate carboxylase
2. PEP Carboxykinase 3. FBPASE-1 4. G6Pase |
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What is the first step in gluconeogenesis?
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Generation of PEP from pyruvate.
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Where does pyruvate come from?
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The mitochondrion
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What enzymes are needed to bypass the irreversible step of Pyruvate->PEP?
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-Pyruvate carboxyylase
-PEP Carboxykinase |
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What is BIOTIN?
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a coenzyme derived from the vitamin Biotin (aka Vit H)
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Which enzyme has the biotin coenzyme? Where is this enzyme?
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Pyruvate Carboxylase - located in mito matrix.
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What reaction does Pyruvate Carboxylas catalyse?
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Pyruvate + CO2 (AT) -> OAA (ADP+Pi)
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What are the 2 steps of the Pyruvate Carboxylase reaction?
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1. Activate enzyme via CO2 addition *Requires ATP
2. Lose Enzyme, Add Pyruvate to CO2; makes OAA |
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How does mitochondrial OAA get into the cytosol?
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Mito Malate dehydrogenase converts OAA to Malate by oxidizng NADH -> NAD; malate can diffuse.
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What happens to malate in cytosol?
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Reconverted to OAA (produces NADH)
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What acts on OAA in the cytosol?
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PEP Carboxykinase - uses a GTP to make high energy PEP and lose CO2.
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The pathway using malate as the shuttle for OAA from mito to cyto is probably wrong; what is right?
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OAA + Glu -> Asp + a-KG (mito)
-Asp/a-KG diffuse to cytosol- Asp + a-KG -> OAA + Glu -Glu takes NH3 back to mitosol- |
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Why is the latter shuttling pathway probly correct?
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It doesn't generate NAD+ in the mitosol or consume NAD+ in the cytosol.
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What happens after PEP is formed in the cytosol?
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The next steps of glycolysis are simply reversible, up to PFK1
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What allows for glycolysis to simply be reversed? (2 things)
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-Law of mass action - the elevated levels of PEP push reactions backward.
-Liver Pyruvate Kinase is inactivated by PKA during gluconeogenesis |
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What enzyme catalyzes the step in place of PFK1 (which can't go backwards)?
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F1,6BPase - a critical site of gluconeogenesis regulation!!!
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What allosterically inhibits F1,6-BPASE?
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AMP - signals low energy; want glycolysis
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What also inhibits F1,6BPase?
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F2,6-BP
-Inhibitor of gluconeogenesis -Activatory of glycolysis |
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What is the last irreversible step of glycolysis that must be overcome?
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Conversion of G6P to free glucose + Phosphate
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Where is the enzyme for reversing hexo/glucokinase?
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Inside ER lumen: Glucose-6-phosphatase (G6Pase)
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What tissue is rich in G6Pase?
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Liver (not muscle). B/c glucose levels fluctuate a lot more in liver than in muscle.
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What happens to skeletal muscle in strenuous exercise?
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It doesn't have enough O2 to run aerobic glycolysis -> produces lactate.
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What happens to the lactate generated from exercising muscle?
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Goes to blood, then liver.
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What does the liver do with lactate?
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Converts it to pyruvate then glucose
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How much ATP is required for the liver to make 1 glucose?
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6
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What is this cycle (Muscle producing lactate sent to liver producing glucose sent back to muscle) called?
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The Cori Cycle
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