Chapter 15 (Cont.)-16

Question 1

1. What happens when glucose levels are high?
   
   1. What is upregulated?
   2. What is slowed down?
   3. What is done with glucose?
   4. What is this process mediated by?
2. What happens when glucose levels are low?
   
   1. What is upregulated?
   2. What is slowed down?
   3. What is done with glucose?
   4. What is this process mediated by?
3. What are the main control points in glycolysis?

Answer 1

1. Glucose levels are high:
   
   1. Glycolysis
   2. Gluconeogenesis
   3. Converted into fat and stored as glycogen.
   4. Mediated by Insulin (activates a phosphatase PPA2).
2. Glucose levels are low:
   
   1. Gluconeogenesis
   2. Glycolysis
   3. Glucose is mobilized from glycogen.
   4. Mediated by Glucoagon (activates cAMP kinase.
3. Hexokinase, PFK-1, Pyruvate kinase

Question 2

1. How many isozymes of hexokinase are there?
2. What is hexokinase II?
3. What is hexokinase IV?

Answer 2

1. 4 isozymes (different enzymes that catalyze the same reaction; they typically share similar sequences but may have different kinetic properties). In humans with different properties.
2. Hexokinase II (muscle/brain): High affinity for Glc (K1/2 = 0.1 mM*), allosteric inhibition by Glc 6P.
3. Hexokinase IV (liver): Low affinity for Glc (K1/2 = 10 mM). Location of HKIV is regulated by [Glc] and [Frc 6P]. No inhibition by Glc-6-P. STEADY STATE CONC OF BLOOD GLUCOSE 5 mM.

Question 3

1. What does PFK-1 catalyze?
2. What is an allosteric inhibitor of PFK-1? What reverses this inhibition?
3. Why does citrate inhibit PFK-1?
4. What happens when the energy charge is high ([ATP] high, [AMP] low)?
5. What happens when the energy charge is low ([ATP] low, [AMP] high)?
6. What is this process called?

Answer 3

1. Catalyzes irreversible step that commits
   Glc 6P to degradation (vs. glycogen
   production and PP pathway). Controls fraction of Glc-6P that enters glycolysis.
2. ATP, AMP reverses this inhibition.
3. C.A.C. produces more energy so, more pyruvate is not needed.
4. Glycolysis is off, gluconeogenesis is on.
5. Glycolysis is on, gluconeogenesis is off.
6. Reciprocal Regulation of PFK-1 and FBPase-1.

Question 4

1. What is Fructose-2,6 bisphosphate?
2. What does it do?
3. What controls its amount?
4. What control the cellular concentration of Frc 2,6-bisP
5. What carries out the activities of PFK-2 and FBPase-2?
6. How does it affect PFK-1 & FBPase-1?

Answer 4

1. A very potent activator of PFK.
2. Increases the affinity for Fruc-6-P. Diminishes the inhibitory effect of ATP. An allosteric activator of PFK that shifts the conformation from the R-state to the T-state.
3. Under hormonal control.
4. Cellular concentration is set by its rates of synthesis and break-down.
5. The same bifunctional protein: Phosphofructokinase II
6. Increases activity of PFK-1, decreases the activity of FBPase-1

Question 5

1. What does Xylulose 5 - Phosphate do?

Answer 5

1. Regulates PFK, by further activting PFK. Thus, even further increasing glycolysis and the PPP.

Question 6

1. How is pyruvate kinase allosterically regulated?
2. How is pryuvate kinase covalently regulated?
3. What are the main control points of gluconeogenesis?

Answer 6

1. Allosterically Regulated:
   
   1. ATP inhibits (high energy state-no glycolysis).
   2. Fruc 1,6-bisphosphate activates (keeps the pace of glycolysis).
2. Covalently Regulated: L-type pyruvate kinase (liver isozyme) is reversibly phosphorylated when glucose level is low-this inactivates the enzyme.
3. Main control points:
   
   1. Pyruvate Carboxylase
   2. PFK-1/FBPase

Question 7

1. What are the two alternative fates for Pyruvate?
2. How does Aceytl-CoA stimulate glucose synthesis?

Answer 7

1. Points:
   
   1. Pyruvate can be a source of new glucose.
      
      1. Store energy as glycogen.
      2. Generate NADPH via pentose phosphate pathway.
   2. Pyruvate can be a source of acetyl-CoA.
      
      1. Store energy as body fat.
      2. Make ATP via citric acid cycle
2. Stimulates glucose synthesis by activating pyruvate kinase.

Question 8

1. What are some advantages of using glycogen instead of glucose?
2. What are some advantages of using glycogen instead of fats?

Glycogen Break-Down (1)

1. What does Glycogen phosphorylase do?
2. What does phosphorolysis do?
3. What is the product?

Answer 8

1. Lower osmolarity, no thermodynamic penalty.
2. Glc more versatile, anaerobic energy source, fast mobilization.
3. Cleaves off Glc units from the non-reducing end.
4. Preserves energy of glycosidic bonds and prevents Glc loss by diffusion.
5. Glu 1P

Question 9

Glycogen Break-Down (2)

1. What can Glycogen Phosphorylase not do?
2. When Glc chain is 4 units long, debranching enzyme, what does Glycogen Phosphorylse do?

Answer 9

1. Cannot cleave (α1→6) glycosidic bonds of branch points.
2. (1) Tranfers Glc₃ unit to another non-reducing end. (2) Hydrolyzes (α1→6) glycosidic bond of branch point.

Question 10

Glycogen Break-Down (3)

1. What enzyme converts Glc 1P ⇔ Glc 6P?
2. What happens to Glc 6P in the muscle?
3. What happens to it in the liver?

Answer 10

1. Phosphoglucomutase
2. In muscle: Glc 6P directly enters glycolysis.
3. In liver: Glc 6P is converted into Glc by Glc 6-phosphatase and exported into blood

Question 11

Glycogen Synthesis (1): UDP-Glucose

1. Where does glycogen synthesis mainly occur?
2. What enzyme forms UDP-glucose?
3. What is the advantage of this reaction?

Glycogen Synthesis (2): Chain extension

1. What does Glycogen Synthase do?

Glycogen Synthesis (3): Branching

1. How are new branches formed?

Answer 11

1. Mainly in the liver and muscle. Glc 6P ⇒ Glc 1P via phosphoglucomutase.
2. UDP-glucose phosphorylase
3. Formation is metabolically irreversible.
4. Adds (UDP-) glucose units to non-reducing ends of glycogen chains.
5. Branching enzyme introduces new branch points by transferring 6-7 Glc units from non-reducing end to an internal Glc generating a

(a1→6) glycosidic bond

Question 12

Glycogen Synthesis (4): Priming

1. What is the limit when adding UDP-Glc?
2. What is formation of new glycogen molecules cataylzed by?

Answer 12

1. Glycogen synthase can only add UDP-Glc to existing glycogen chains.
2. Formation of new glycogen molecules is catalyzed by glycogenin.
   
   1. Tyr-194 sidechain of glycogenin is glycosylated using UDP-Glc.
   2. 7 more UDP-Glc are added → Tyr-Glc8.
   3. Terminal Glc serves as priming site for glycogen synthase.
   4. Glycogenin remains in core of glycogen molecule.

Question 13

1. How are Glycogen Synthase and Glycogen Phosphorylase reciprocally regulated by hormone-induced phosphorylation/dephosphorylation?
2. How is phosphorylase activated?

Answer 13

1. Regulation:
   
   1. Phosphorylation of both enzymes stimulates glycogen break-down and inhibits glycogen synthesis.
   2. Dephosphorylation of both enzymes inhibits glycogen break-down and stimulates glycogen synthesis.
2. Activated by a Regulation Cascade Triggered by Glucagon or Epinephrine.

Question 14

Cellular respiration

1. What is consumed and produced in this process?
2. How does it compare to glycolysis?
3. Aside from capturing energy from glucose, what else can it capture energy from?
4. What are the 3 major stages of respiration?

Answer 14

1. Process in which cells consume O2 and produce CO2.
2. Provides more energy (ATP) from glucose than glycolysis.
3. Also captures energy stored in lipids and amino acids. Evolutionary origin: developed about 2.5 billion years ago. Used by animals, plants, and many microorganisms.
4. Three major stages:
   
   1. acetyl CoA production
   2. acetyl CoA oxidation
   3. electron transfer and oxidative phosphorylation

Question 15

Stage 1 of Respiration

1. Do amino acids and fatty acids have to be enzyme mediated?
2. How is Pyruvate made into Acetyl-CoA?
   
   1. What is the CoA attached to?
   2. What is released?
   3. What is produced?

Answer 15

1. No, amino acids and fatty acids can be directly made into Acetyl-CoA.
2. Pyruvate has to undergo a reaction mediated by the enzyme pyruvate dehydrogenase complex.
   
   1. A thiolester
   2. CO₂
   3. ATP, NADH, FADH₂

Question 16

Respiration Stage 2

1. What is this stage called?
2. What does it generate?
3. What do NADH and FADH₂ act as?

Repiration Stage 3

1. What occurs in this stage?
2. What is produced?

Answer 16

Respiration Stage 2

1. Acetyl-CoA oxidation, aka The Citric Acid Cycle
2. One GTP, NADH and FADH₂
3. Both act as reduced electron carriers.

Repiration Stage 3

1. Electron transfer and oxidative phosphorylation
2. A lot of ATP, this is the stage where oxygen is used. H₂O is produced via the respiratory (electron-transfer) chain.

Question 17

1. Where does the Citric Acid Cycle occur?
2. Where does oxidative phosphorylation occur?
3. How many coenzymes does stage one of respiration require?
4. What does the Pyruvate Dehydrogenase Complex consist of?
5. What does channeling minimizes?
6. What is the complex regulated?

Answer 17

1. Occur in the mitochondrial matrix
2. In the inner membrane
3. Requires 5 coenzymes
4. Three enzymes
   
   1. pyruvate dehydrogenase, TPP prosthetic group
   2. dihydrolipoyl transacetylase, Lipoamide
   3. dihydrolipoyl dehydrogenase, FAD
   4. Irreversible, NADH is produced
5. Side reactions
6. ATP

Question 19

5 steps in the oxidative decarboxylation of Pyruvate

1. Enzyme 1:
   
   1. Step 1?
   2. Step 2?
2. Enzyme 2:
   
   1. Step 3?
3. Enzyme 3:
   
   1. Step 4?
   2. Step 5?
4. What are the two co-substrates that are used?

Answer 19

1. Enzyme 1:
   
   1. Decarboxylation of pyruvate to an aldehyde
   2. Oxidation of the aldehyde to a carboxylic acid. Electrons reduce lipoamide and form a thioester.
2. Enzyme 2:
   
   1. Formation of acteyl-CoA (product 1)
3. Enzyme 3:
   
   1. Reoxidation of the lipoamide cofactor.
   2. Regeneration of the oxidized FAD cofactor. Forming NADH (product 2).
4. NAD⁺ and CoA-SH

Question 20

1. What happens when there is a thiamine deficiency?
2. What does it increase?

Answer 20

1. Affects the three multi-enzyme complexes that contain enzymes with TPP cofactor:
   1. pyruvate dehydrogenase complex (PDC)
   2. a-ketoglutarate dehydrogenase complex
   3. transketolase
2. Increase in pyruvate and a-ketoglutarate concentration.