chapter 14 Study Notes on Glycolysis, Gluconeogenesis, and Pentose Phosphate Pathway

Energy is vital for powering thoughts, movements, cellular repair, and creating protective agents in the body.
Energy conversion is a constant process that is efficiently managed, happening billions of times per second.

The focus is on cellular energy management, specifically how cells manage energy and synthesize essential molecules.
Insights are gathered from Chapter 14 of "Principles of Biochemistry, 8th Edition" by Nelson and Cox, covering:
- Glycolysis
- Gluconeogenesis
- Pentose Phosphate Pathway

Definition: Glycolysis means "sweet splitting" in Greek; it is a nearly universal metabolic pathway for breaking down glucose.

Glycolysis was the first metabolic pathway to be fully understood, with pioneering work by Edward Buchner in 1897 through yeast extracts.
His discovery revolutionized the understanding of fermentation outside living cells, leading to foundational insights in biochemistry, including:
- Enzyme purification
- The role of NAD+ and ATP.

Key Role: For many cells (e.g., red blood cells, brain), glycolysis is the sole source of metabolic energy.
The pathway has been conserved through evolution, ensuring that enzymes from various organisms (e.g., yeast, plants) are remarkably similar, demonstrating a universal language of energy.

Glycolysis occurs in two main stages:
1. Preparatory Phase: Investment of energy
2. Payoff Phase: Generation of energy

Two ATP molecules are spent to phosphorylate glucose, rendering it more reactive and trapping it within the cell.
The phosphorylated glucose undergoes cleavage to produce two three-carbon molecules (glyceraldehyde-3-phosphate).

The three-carbon molecules are oxidized, and inorganic phosphate is added to form high-energy compounds.
Substrate-Level Phosphorylation: The high-energy compounds transfer a phosphate group to ADP, directly generating ATP—occurring twice in the process, yielding two ATP molecules.
Additional rearrangements produce phosphoenolpyruvate (PEP), which donates its phosphate group to ADP, generating two more ATP molecules.
Net Yield: For one molecule of glucose, glycolysis produces:
- Net 2 ATP (4 produced - 2 invested)
- 2 NADH (an electron carrier holding high-energy electrons that can produce more ATP later, given the presence of oxygen).

Glycolysis has a large negative free energy change, making it essentially irreversible in cells.
The total energy release from glucose is initially low, with pyruvate still being energy-rich.
Under aerobic conditions, pyruvate enters the mitochondria for the citric acid cycle and oxidative phosphorylation, yielding approximately 30-32 ATP per glucose.
Under anaerobic conditions, pyruvate undergoes fermentation instead.

Cells utilize various carbohydrates, not just glucose, for energy through glycolysis.
Glycogen and Starch: Byproducts of these breakdowns convert to glucose-1-phosphate, further entering glycolysis and yielding a net gain of three ATP per glucose unit from glycogen.
Dietary Carbohydrates: Food sugars are broken down into monosaccharides that enter the glycolytic pathway:
- Lactose Intolerance: Lactose (disaccharide) must be broken down by lactase into glucose and galactose; deficiency of this enzyme leads to undigested lactose fermentation in the intestines, causing discomfort.
- Galactose and Fructose: Each sugar has distinct pathways for conversion into intermediates that enter glycolysis.

Galactose is phosphorylated and converted into glucose-1-phosphate, subsequently isomerizing to glucose-6-phosphate for glycolysis.
Enzyme deficiencies lead to galactosemias, highlighting important health implications.

Fructose phosphorylation differs in muscle and liver. In muscle, it directly forms fructose-6-phosphate. In the liver, it forms glyceraldehyde-3-phosphate, bypassing early regulatory steps and having metabolic implications.

Pyruvate can follow three main fates based on the oxygen availability:
1. In aerobic conditions: Enters the mitochondria for oxidation in the citric acid cycle.
2. In anaerobic conditions: Fermentation occurs, either lodging processes leading to lactic acid or ethanol production.

Lactic Acid Fermentation: Meat used in muscle cells during oxygen scarcity; pyruvate recycles electrons from NADH, forming lactate and allowing glycolysis to continue.
Ethanol Fermentation: Occurs mainly in yeast, where pyruvate is decarboxylated to acetaldehyde, which subsequently gets reduced to ethanol, regenerating NAD+.

The pentose phosphate pathway generates NADPH for reductive biosynthesis but is sensitive to G6PD deficiency, which can lead to conditions like favism.

Defined as the synthesis of glucose from non-carbohydrate precursors under conditions such as fasting or excessive energy demands.
Primarily occurs in the liver, using sources like lactate and some amino acids to produce glucose.
Gluconeogenesis is distinct from glycolysis; it circumvents irreversible reactions via specialized enzymes—allowing for independent regulation of the two pathways.

Metabolic pathways are tightly regulated to avoid wastage in energy and resources; key regulatory enzymes often have reciprocal control.
Key Amplifying Factors:
- High energy states inhibit glycolysis while stimulating gluconeogenesis and vice versa, driven by levels of ATP, ADP, and AMP influencing enzyme activity.
- Fructose-2,6-bisphosphate plays a major regulatory role in switching metabolism between glycolysis and gluconeogenesis based on glucose availability.

G6P is pivotal in routing into glycolysis or pentose phosphate pathway (PPP).
Functions of PPP:
1. Producing NADPH for biosynthesis and cellular protection.
2. Generating ribose-5-phosphate necessary for nucleotide synthesis.

The interconnected nature of metabolism highlights a highly regulated system maintaining energy balance and resource allocation in human biology.
Glycolysis, gluconeogenesis, and the pentose phosphate pathway not only showcase energy flow and storage but also present metabolic flexibility to meet changing cellular demands and stresses.