Question

1. what are the 2 stages of glycolysis we discussed? how many atp does each step use and/or produce?
2. what is the net result of 1 molecule of glucose entering glycolysis?
3. what are the main enzymes of regulation in glycolysis? justify this in terms of energy consumption/production.
4. gluconeogenesis accomplishes the reverse of glycolysis- synthesis of glucose from pyruvate. in gluconeogenesis, which steps of glycolysis are not directly reversed? why? which enzymes replaces these steps in gluconeogenesis?

Explanation:

For Question 5:

Glycolysis is split into two core stages. The energy investment stage uses ATP to prime glucose for breakdown, while the energy payoff stage generates ATP and reducing power.

For Question 6:

The net result accounts for ATP used in the investment stage and produced in the payoff stage, plus the reduced electron carriers and end product of glycolysis.

For Question 7:

The regulatory enzymes control key irreversible, energy-dependent or energy-generating steps of glycolysis, matching regulation to cellular energy needs.

For Question 8:

Glycolysis has irreversible, energy-releasing steps that require unique enzymes in gluconeogenesis to bypass them, as reversing these steps would be energetically unfavorable.

Answer:

1. Question 5

• The two stages are:

1. Energy Investment Stage: Uses 2 molecules of ATP (1 ATP to convert glucose to glucose-6-phosphate, 1 ATP to convert fructose-6-phosphate to fructose-1,6-bisphosphate). No ATP is produced here.
2. Energy Payoff Stage: Produces 4 molecules of ATP (via substrate-level phosphorylation) and 2 NADH.
3. Question 6

For 1 molecule of glucose entering glycolysis, the net result is: 2 pyruvate molecules, 2 NADH molecules, and 2 ATP molecules (4 ATP produced minus 2 ATP invested).

1. Question 7

The main regulatory enzymes are hexokinase, phosphofructokinase-1 (PFK-1), and pyruvate kinase:

• Hexokinase: Regulates the first irreversible, ATP-consuming step; it is inhibited by glucose-6-phosphate to prevent over-investment of ATP when glucose-6-phosphate accumulates.
• PFK-1: The key rate-limiting enzyme; it is inhibited by ATP and citrate (high energy signals) and activated by AMP/ADP (low energy signals), matching glycolysis rate to cellular energy demand.
• Pyruvate kinase: Regulates the final irreversible, ATP-producing step; it is activated by fructose-1,6-bisphosphate (feeds forward to match payoff to investment) and inhibited by ATP, preventing unnecessary ATP production when energy is abundant.

1. Question 8

• The irreversible steps of glycolysis that are not directly reversed in gluconeogenesis are:

1. Glucose → Glucose-6-phosphate (catalyzed by hexokinase)
2. Fructose-6-phosphate → Fructose-1,6-bisphosphate (catalyzed by PFK-1)
3. Phosphoenolpyruvate → Pyruvate (catalyzed by pyruvate kinase)

• These steps cannot be directly reversed because they are highly exergonic (release large amounts of energy) and have a very negative free energy change, making their reversal energetically unfavorable under cellular conditions.
• The replacement enzymes in gluconeogenesis are:

1. Glucose-6-phosphatase (reverses hexokinase step)
2. Fructose-1,6-bisphosphatase (reverses PFK-1 step)
3. Pyruvate carboxylase and phosphoenolpyruvate carboxykinase (PEPCK) (together reverse the pyruvate kinase step, first converting pyruvate to oxaloacetate, then to phosphoenolpyruvate)