Chapter 14 (Cont.)-15

Question 1

1. What is the other pathway glucose can continue down, aside from oxidation via glycolysis?
2. What is produced?
3. What is the product the precursor for?

Answer 1

1. Oxidation via the pentose phosphate pathway.
2. Ribose 5-phosphate
3. Nucleotide biosynthesis

Question 2

1. What are the two phases of the pentose phosphate pathway?
2. What does the oxidative phase use? What does it produce?
3. What does the nonoxidative phase do?
4. Applications of the PPP:
   
   1. In erythrocytes?
   2. In adipose tissue?
   3. In rapidly dividing tissue?
   4. In photosynthetic tissues in plants?

Answer 2

1. Oxidative and Nonoxidative phases
2. Uses glucose to produce:
   
   1. ribose → DNA, RNA, etc.
   2. NADPH → biosynthesis, oxidative stress
3. Nonoxidative phase interconverts sugars with differing carbon numbers → photosynthesis.
4. Applications of the PPP:
   
   1. PPP delivers NADPH to combate oxidative damage due to oxygen transport.
   2. PPP delivers NADPH required for fatty acid synthesis.
   3. PPP delivers ribose-5P for DNA synthesis.
   4. Use PPP to regenerate the CO2 acceptor ribulose 1,5-bisP from glyceraldehyde 3P.

Question 3

Oxidation Phase (1): Oxidation of Glucose 6P

1. What is the principle behind this reaction?
2. What happens to G6PD?
3. What does the enzyme lactonase do?

Oxidation Phase (2): Decarboxylation of 6-Phosphogluconate

1. What does 6-Phosphogluconate dehydrogenase do?
2. What does Pentose isomerase do?

Answer 3

1. Aldehyde is oxidized to carboxylic acid.
2. Transfers 2 e^- from glucose 6P to NADP⁺
3. Hydrolyzes 6-Phosphoglucono-δ-lactone to 6-Phosphogluconate.
4. Oxidizes C₃ hydroxyl to keto group → unstable b-keto acid spontaneously decarboxylates. Transfers 2 e^- to NADP⁺.
5. Converts ketopentose ribulose 5P into aldopentose ribose 5P.

Question 4

Non-oxidative phase:

1. When is ribose 5P recycled to glucose 6P?
2. What two carbon units mediate the transfer reactions when ribose 5P is recycled?
3. Overall, what is glucose oxidized to?
4. What is the summary of the Pentose Phosphate Pathway?

Answer 4

1. When demand for NAPDH higher than demand for ribose 5P then ribose 5P is recycled to glucose 6P.
2. Recycling is achieved by a series of transfer reactions involving 2 carbon units (transketolase) and 3 carbon units (transaldolase).
3. Glucose 6P is completely oxidized to yield 6 CO₂ and 12 NADPH.
4. Picture:

Question 5

1. What is our daily demand for glucose?
2. What is our daily supply?
3. What do neurons use?
4. How much oxygen is used by the brain?
5. What does the brain depend on?
6. What is the energy used for?

Answer 5

1. 160 g (120 g for the brain).
2. 20 g body fluids, 190 g glycogen
3. Neurons use almost exclusively glucose.
4. 20% of total oxygen consumption used by the brain.
5. There is little glycogen storage, so the brain depends on glucose in the blood.
6. Energy used to maintain and create electrical potential across neuronal membranes (important for action potential).

Question 6

1. Can all sugars be used to eventually make energy?
2. What can mammals not convert fatty acids to?
3. Where does glycolysis mainly occur?
4. Where does gluconeogenesis mainly occur?
5. What does gluconeogenesis maintain?
6. What molecule does it usually start with?
   
   1. If it starts with lactate, how is it converted to pyruvate?
   2. What enzyme is used?

Answer 6

1. Yes, no matter what the sugar is, it can be converted to energy.
2. Cannot convert fatty acids into sugars.
3. Mainly occurs in the muscles and the brain.
4. Mainly occurs in the liver.
5. Maintains steady glucose level in the blood for muscles and brain.
6. Usually starts with pyruvate or lactate.
   
   1. lactate + NAD⁺ ⇔ pyruvate + NADH
   2. LDH

Question 7

1. What are glucogenic intermediates?
2. What can and cannot be converted in mammals?
3. What is gluconeogenesis?
   
   1. What does it bypass?

Answer 7

1. Glucogenic Intermediates
   
   1. Citrate Acid Cycle Intermediates
      
      1. All intermediates that can undergo oxidation to oxaloacetate.
      2. Glucogenic amino acids (Ala, Gln).
2. There is no net conversion of fatty acid into glucose (no pathway that converts Acetyl-CoA into pyruvate); but glycerol can be converted.
3. Anabolic synthesis of glucose (C6) from pyruvate (or other carbon sources). “Reversal” of glycolysis.
   
   1. Bypass three irreversible steps of glycolysis.

Question 8

Production of PEP:

1. Where do these reactions occur?
2. What does the Malate shuttle do?
   
   1. What is required for GapDH to function?
3. What two steps usually occur during the production of PEP?

Answer 8

1. Complex series of reactions that involve both mitochondrion and cytosol.
2. Transfers e^- from mitochondrion to cytosol.
   
   1. NADH is required for GapDH function.
3. Two steps
   
   1. Pyruvate is converted into a high energy intermediate (oxaloacetate) upon ATP consumption. By pyruvate carboxylase.
   2. Oxaloacetate is decarboxylated
      to yield PEP; phosphate group on PEP is transferred from GTP. By PEP carboxykinase.
   3. 2 high energy phosphate equivalents are required for PEP production.

Question 9

Fructose 1,6bisP → Fructose 6P

1. Why is H₂O used in this reaction and not ADP?
2. What is the reaction?
   
   1. What enzyme is it catalyzed by?

Glucose 6P → Glucose

1. Why is there no reversal of glycolysis possible in this reaction?
2. What is the reaction?
   
   1. What enzyme catalyzes the reaction?
   2. Where is glucose released?
3. What is the overall summary of gluconeogenesis?

Answer 9

Fructose 1,6bisP → Fructose 6P

1. Phosphate transfer potential of Frc 1,6bisP is insufficient to transfer phosphate group to ADP ⇒ no reversal of glycolysis possible.
2. Fructose 1,6bisP + H₂O → Fructose 6P + P_i
3. Hydrolysis is catalyzed by fructose 1,6-bisphosphatase-1.

Glucose 6P → Glucose

1. Phosphate transfer potential of Glucose 6P is insufficient to transfer phosphate group to ADP ⇒ no reversal of glycolysis possible.
2. Glucose 6P + H₂O → Glucose + P_i
   
   1. Hydrolysis is catalyzed by glucose 6-phosphatase in liver (ER resident).
   2. Glucose is subsequently released in the blood.

Question 10

1. What are some of the dedicated purposes that pathways have?
2. Pathways can be represented as a map. What can we learn from the map?

Answer 10

1. Purposes:
   
   1. Extraction of energy
   2. Storage of fuels
   3. Synthesis of important building blocks
   4. Elimination of waste materials
2. What we can learn:
   
   1. Follow the fate of metabolites and building blocks.
   2. Identify enzymes that act on these metabolites.
   3. Identify points and agents of regulation.
   4. Identify sources of metabolic diseases

Question 11

1. How is homeostasis maintain?
2. In a steady state,…?
3. What state are pathways in?
4. What happens after perturbation occurs?
5. What are some situations in which levels of metabolites must be altered very rapidly?
6. What are two ways we can change the specific activity of an enzyme?
7. How can we change the concentration of the enzyme?

Answer 11

1. Organisms maintain homeostasis by keeping the concentrations of most metabolites at steady state.
2. In steady state, the rate of synthesis of a metabolite equals the rate of breakdown of this metabolite.
3. Pathways are at steady state unless perturbed.
4. After perturbation a NEW steady state will be established.
5. Situations:
   
   1. Need to increase the capacity of glycolysis during action.
   2. Need to reduce the capacity of glycolysis after the action.
   3. Need to increase the capacity of gluconeogenesis after successful action.
6. Binding of an inhibitor or activator. Covalent modification.
7. Changing the enzyme location. Changing the steady-state level of enzyme (translation, protein degradation). Changing the stready-state level of the mRNA encoding the enzyme (transcription, mRNA degradation).

Question 12

1. Irreversible steps often serve as regulation points in metabolic pathways. What is an example of one of these steps?
2. When ATP is being depleted what would happen?

Answer 12

1. Phosphofructokinase-1:
   
   1. Committed step in glycolysis (point of no return). Allows unidirectional control (e.g. does not affect gluconeogenesis). Partially serves as pacemaker (limits the total flow through pathway). Integrates various metabolic signals for regulation.
2. ATP depletion would slow down ATP-dependent reactions and change their thermodynamics (i.e., they would become less favorable) → Cells must try to maintain a stable [ATP]:[AMP] ratio. Pathways that generate or consume ATP must respond to changes in ATP levels.

Question 13

Guidelines for Metabolic Regulation:

1. What are the three guidelines?

Answer 13

1. Guidelines:
   
   1. Fuel Efficiency: Use ATP responsibly (e.g. don’t run glycolysis and gluconeogenesis in parallel).
   2. Partitioning: Divert the flow from a common intermediate such that the product in demand is favored.
   3. Slow down pathway when product accumulates (i.e. there is little demand for product).

Question 14

1. Why is glycolysis not the most efficient way of producing energy?

Answer 14

1. Because only a small amount of energy contained in a glucose molecule is captured by glycolysis. Full oxidation of glucose yields about -2,840 kJ/mol, compared to the -146 kJ/mol using glycoylsis.