Question

Name some of the key metabolic intermediates that are seen in more than one pathway.

Short answer

Key metabolic intermediates are Acetyl-CoA, Pyruvate, and Glucose-6-phosphate.

Step-by-step solution

Identify the definition

Define what metabolic intermediates are. Metabolic intermediates are compounds that are formed and consumed during the chemical reactions in metabolic pathways.

Analyze metabolic pathways

Examine major metabolic pathways such as glycolysis, the citric acid cycle, and the pentose phosphate pathway to identify intermediates.

Identify common intermediates

List compounds that appear in multiple pathways. Important metabolic intermediates include Acetyl-CoA, Pyruvate, and Glucose-6-phosphate.

Key concepts

glycolysis

Glycolysis is the process of breaking down glucose into pyruvate, producing ATP and NADH in the process. This pathway occurs in the cytoplasm and is the first step in cellular respiration.
During glycolysis, one glucose molecule (a six-carbon compound) is split into two molecules of pyruvate (each a three-carbon compound).

Key Steps:
1. Glucose is phosphorylated by ATP to form glucose-6-phosphate.
2. Glucose-6-phosphate is converted to fructose-6-phosphate.
3. Another ATP is used to phosphorylate fructose-6-phosphate to fructose-1,6-bisphosphate.
4. Fructose-1,6-bisphosphate splits into two three-carbon molecules, which are converted to glyceraldehyde-3-phosphate.
5. Glyceraldehyde-3-phosphate undergoes a series of reactions, producing 2 ATP, 2 NADH, and 2 pyruvate molecules.

Pyruvate, formed during glycolysis, is a pivotal metabolic intermediate. It can enter the citric acid cycle (Krebs cycle) if oxygen is present or be converted into lactate in anaerobic conditions.

Importance of Glycolysis:

• Primary source of energy (ATP) for cells, especially when oxygen is low or absent.
• Provides intermediates for other pathways like amino acid synthesis.
• Generates products that feed into the citric acid cycle and other metabolic pathways.

citric acid cycle

The citric acid cycle, also known as the Krebs cycle, is a series of chemical reactions used by all aerobic organisms to generate energy from carbohydrates, fats, and proteins.
This cycle occurs in the mitochondria and plays a vital role in both energy production and providing building blocks for biosynthesis.

Key Steps:
1. Acetyl-CoA combines with oxaloacetate to form citrate.
2. Citrate is converted into its isomer, isocitrate.
3. Isocitrate undergoes dehydrogenation to form alpha-ketoglutarate, producing NADH.
4. Alpha-ketoglutarate is further oxidized to succinyl-CoA, generating more NADH.
5. Succinyl-CoA is converted to succinate, producing GTP (or ATP).
6. Succinate is oxidized to fumarate.
7. Fumarate is hydrated to malate.
8. Malate is oxidized back to oxaloacetate, producing the final NADH in the cycle.

Acetyl-CoA, serving as a substrate for the citric acid cycle, is a key metabolic intermediate.

Significance of the Citric Acid Cycle:

• Generates high-energy molecules (NADH, FADH2) that drive ATP production in the mitochondrial electron transport chain.
• Provides intermediates for the synthesis of amino acids, nucleotide bases, and other essential biomolecules.
• Plays a central role in metabolic integration and regulation.

pentose phosphate pathway

The pentose phosphate pathway (PPP) is another crucial metabolic pathway that runs parallel to glycolysis.
This pathway provides reducing power and precursors for nucleotide and amino acid biosynthesis.
The PPP occurs in the cytoplasm and can be divided into oxidative and non-oxidative phases.

Key Steps:
Oxidative Phase:
1. Glucose-6-phosphate is dehydrogenated to 6-phosphoglucono-𝛿-lactone, producing NADPH.
2. 6-Phosphoglucono-𝛿-lactone is hydrolyzed to 6-phosphogluconate.
3. 6-Phosphogluconate is then oxidatively decarboxylated to ribulose-5-phosphate, generating a second NADPH.
Non-Oxidative Phase:
4. Ribulose-5-phosphate can be converted to ribose-5-phosphate for nucleotide synthesis or to xylulose-5-phosphate.
5. Ribose-5-phosphate and xylulose-5-phosphate undergo a series of transketolase and transaldolase reactions, generating intermediates for glycolysis.

Glucose-6-phosphate, a key intermediate, can funnel into both glycolysis and the pentose phosphate pathway.

Importance of the Pentose Phosphate Pathway:

• Produces NADPH, required for reductive biosynthetic reactions and protecting cells from oxidative damage.
• Generates ribose-5-phosphate, necessary for nucleotide and nucleic acid synthesis.
• Provides a means to interconvert sugars of different chain lengths, useful in various biosynthetic processes.