Glycolysis: Practice Questions and Key Concepts

Glycolysis: Practice Questions and Key Concepts

Overview of Glycolysis

Glycolysis is a central metabolic pathway that converts glucose into pyruvate, generating ATP and NADH in the process. It is a key pathway in cellular respiration and occurs in the cytoplasm of cells.

• Definition: Glycolysis is the enzymatic breakdown of glucose (a six-carbon sugar) to two molecules of pyruvate (a three-carbon compound).

• Importance: Provides energy (ATP), reducing power (NADH), and intermediates for other metabolic pathways.

Key Steps and Products of Glycolysis

• Net Reaction: The conversion of 1 mole of fructose-1,6-bisphosphate to 2 moles of pyruvate yields:

  • 2 moles NADH

  • 4 moles ATP (gross), but net gain is 2 ATP per glucose due to initial investment

  • 2 moles pyruvate

• Equation:

$\text{Fructose-1,6-bisphosphate} \rightarrow 2 \text{Pyruvate} + 2 \text{NADH} + 4 \text{ATP}$

• Example: In glycolysis, glucose is split into two three-carbon molecules, each processed to yield energy and reducing equivalents.

Labeling Experiments in Glycolysis

Radioactive labeling (e.g., with 14C) is used to trace the fate of carbon atoms through glycolysis and fermentation.

• Example: If glucose labeled with 14C at C-1 and C-6 is metabolized, the fate of the label in pyruvate or ethanol can be predicted based on the pathway's chemistry.

Fate of Carbons in Fermentation

• Alcoholic Fermentation: In yeast, pyruvate is converted to ethanol and CO2. The fate of labeled carbons can be traced to specific positions in the products.

• Key Enzymes: Pyruvate decarboxylase and alcohol dehydrogenase.

ATP Generation in Glycolysis

ATP is generated at two substrate-level phosphorylation steps in glycolysis:

• Phosphoglycerate kinase reaction:

  • 1,3-bisphosphoglycerate + ADP → 3-phosphoglycerate + ATP

• Pyruvate kinase reaction:

  • Phosphoenolpyruvate + ADP → Pyruvate + ATP

• Equation:

$\text{1,3-bisphosphoglycerate} + \text{ADP} \rightarrow \text{3-phosphoglycerate} + \text{ATP}$ $\text{Phosphoenolpyruvate} + \text{ADP} \rightarrow \text{Pyruvate} + \text{ATP}$

Enzyme Functions in Glycolysis

• Enolase: Catalyzes the conversion of 2-phosphoglycerate to phosphoenolpyruvate, releasing water.

• Aldolase: Cleaves fructose-1,6-bisphosphate into two three-carbon sugars.

• Triose phosphate isomerase: Interconverts dihydroxyacetone phosphate and glyceraldehyde-3-phosphate.

Summary Table: ATP-Producing Steps in Glycolysis

Step	Enzyme	Substrate	Product	ATP Produced
1	Phosphoglycerate kinase	1,3-bisphosphoglycerate + ADP	3-phosphoglycerate + ATP	1 per triose
2	Pyruvate kinase	Phosphoenolpyruvate + ADP	Pyruvate + ATP	1 per triose

Key Terms and Definitions

• Substrate-level phosphorylation: Direct transfer of a phosphate group to ADP to form ATP.

• NADH: Nicotinamide adenine dinucleotide (reduced form), an electron carrier produced in glycolysis.

• Pyruvate: The end product of glycolysis, which can enter the citric acid cycle or fermentation pathways.

Additional info:

• Glycolysis is regulated at key enzymatic steps, including hexokinase, phosphofructokinase, and pyruvate kinase.

• Fermentation regenerates NAD+ to allow glycolysis to continue under anaerobic conditions.