Study Notes on Glycolysis

Glycolysis Fundamental Aspects
- Glycolysis is a metabolic pathway that converts glucose into pyruvate, yielding energy and reducing power in the form of ATP and NADH.
- It occurs in two main phases: the preparatory (investment) phase and the payoff phase.

Investment Phase
- ATP is consumed to allow glucose to undergo activation. This phase consumes 2 ATP molecules.
- Energy (ΔG) is required due to the activation barrier of the reaction.
Key Steps
1. Phosphorylation of Glucose:
- Enzyme involved: Hexokinase
- Substrate: Glucose
- Product: Glucose-6-phosphate (G6P)
- ATP donates a phosphoryl group to glucose, resulting in G6P, which is not transported out of the cell.
- Reaction:
  $ext{Glucose} + ext{ATP} \rightarrow ext{G6P} + ext{ADP}$ .
1. Isomerization of G6P to F6P:
- Enzyme: Phosphohexose Isomerase
- Reversible reaction, converting G6P to Fructose-6-phosphate (F6P).
- Mechanism of isomerization without adding or removing prefix groups, just rearranging structures.
1. Second Phosphorylation:
- Enzyme: Phosphofructokinase-1 (PFK-1)
- Irreversible step, commits glucose to glycolysis.
- Function: Transfers a phospho group to form Fructose-1,6-bisphosphate (F1,6BP) from ATP.
- Reaction:
  $ext{F6P} + ext{ATP} \rightarrow ext{F1,6BP} + ext{ADP}$ .
1. Cleavage of F1,6BP:
- Enzyme: Aldolase
- Reaction splits the six-carbon molecule into two three-carbon molecules: Glyceraldehyde-3-phosphate (G3P) and Dihydroxyacetone phosphate (DHAP).
- It is a reversible reaction.
1. Isomerization of DHAP to G3P:
- Enzyme: Triose Phosphate Isomerase
- Converts DHAP to G3P so that all molecules can enter the payoff phase.

ATP Generation Phase
- Gain of net 2 ATP after subtracting the 2 ATP consumed in the preparatory phase.
- Key Steps
1. Oxidation of G3P:
- Enzyme: Glyceraldehyde-3-phosphate Dehydrogenase
- Converts G3P into 1,3-Bisphosphoglycerate (1,3BPG), producing NADH from NAD+ (reduction).
- Reaction:
  $ext{G3P} + ext{NAD}^+ + ext{P}_i \rightarrow ext{1,3BPG} + ext{NADH}$ .
1. ATP Formation from 1,3BPG:
- Enzyme: Phosphoglycerate Kinase
- Transfers a phosphate from 1,3BPG to ADP, yielding ATP through substrate-level phosphorylation.
- Reaction:
  $ext{1,3BPG} + ext{ADP} \rightarrow ext{3-Phosphoglycerate} (3PG) + ext{ATP}$ .
1. Isomerization of 3PG:
- Enzyme: Phosphoglycerate Mutase
- Converts 3PG to 2-Phosphoglycerate (2PG).
1. Dehydration of 2PG:
- Enzyme: Enolase
- Water is removed from 2PG to yield Phosphoenolpyruvate (PEP).
1. ATP Generation from PEP:
- Enzyme: Pyruvate Kinase
- Transfers a phosphate from PEP to ADP, producing pyruvate and ATP.
- Reaction:
  $ext{PEP} + ext{ADP} \rightarrow ext{Pyruvate} + ext{ATP}$ .

Overall Reaction:
- Start with 1 Glucose → end with 2 Pyruvate, yielding a net gain of 2 ATP and 2 NADH.
- The full equation is:
  $ext{Glucose} + 2 ext{NAD}^+ + 2 ext{ADP} + 2 ext{P}_{i} \rightarrow 2 ext{Pyruvate} + 2 ext{NADH} + 2 ext{ATP}$

Key Regulatory Enzyme:
- Phosphofructokinase-1 (PFK-1) is crucial for controlling the rate of glycolysis, activated when AMP levels are high (indicating low energy) and inhibited by high ATP levels.
Energetics:
- Glycolysis is an exergonic process with a negative ΔG; energetics must be conserved for further metabolic processes.
Fate of Pyruvate:
- With oxygen: enters TCA cycle as Acetyl CoA.
- Without oxygen: converted to either lactic acid or ethanol, depending on the organism (e.g., lactic acid in mammals, ethanol in yeast).

Pathway Entry Points:
- Sugars other than glucose, such as fructose and galactose, can enter glycolysis through various conversion steps and isomerization processes, making glycolysis accessible to a variety of carbohydrates.
Phosphorylation's Role:
- Phosphorylation plays a pivotal role in trapping glucose in the cell and regulating metabolism through signaling pathways.
Significance in Examination:
- Exam questions may cover the overall reaction, enzyme functions at each stage, ATP and reducing equivalents produced, and the importance of regulation in glycolysis.