Metabolic pathway

Metabolic Pathways

In the realm of biochemistry, metabolic pathways refer to a series of interconnected chemical reactions taking place within a cell. These reactions, steered by enzymes and often necessitating certain dietary minerals, vitamins, and other cofactors, modify principal chemicals. Given the plethora of chemicals — commonly termed metabolites — involved, these pathways can be intricate. Multiple such pathways function simultaneously in a cell, collectively forming the metabolic network. These pathways are instrumental in ensuring an organism's homeostasis.

Classification

Metabolic pathways are primarily classified into two categories:

• Catabolic Pathways: These involve breaking down molecules and often yield energy.
• Anabolic Pathways: These pathways are concerned with the synthesis of molecules, often utilizing energy.

These pathways are interdependent, often culminating in the creation of new biomolecules.

Functional Perspective

When a molecule, referred to as a substrate, joins a metabolic pathway, it undergoes sequential modifications to transform into another product. Depending on the cellular needs and the molecule's availability, this product might:

• Be utilized immediately
• Kickstart another metabolic pathway, known as a flux generating step
• Be retained by the cell

The rate of a specific metabolic process might be influenced by the concentration of the intermediates and end-products of both anabolic and catabolic pathways.

Overview and Directionality

Each metabolic pathway is essentially a sequence of biochemical reactions connected via their intermediates. This means that a product of one reaction becomes the substrate for the succeeding one. While all chemical reactions are inherently reversible, a cell's conditions often make it more thermodynamically favorable for reactions to proceed in a specific direction.

For instance, while one pathway might synthesize a particular amino acid, the degradation of this amino acid might be facilitated by a distinct pathway. However, exceptions exist. A case in point is the metabolism of glucose: while glycolysis breaks down glucose, some reactions within this pathway are reversible, aiding in glucose's re-synthesis (gluconeogenesis).

Glycolysis: A Historical Perspective

Glycolysis holds the distinction of being the earliest discovered metabolic pathway. A snapshot of its process is as follows:

• As glucose makes its entry into a cell, it undergoes immediate phosphorylation by ATP, turning into glucose 6-phosphate in a non-reversible first step.
• During periods of surplus lipid or protein energy sources, certain glycolysis reactions might function in reverse, producing glucose 6-phosphate. This is then stored either as glycogen or starch.
• Feedback inhibition often regulates metabolic pathways.

Cyclic Metabolic Pathways

Some metabolic pathways operate in a 'cyclic' manner. Here, each component of the cycle acts as a substrate for the succeeding reaction within the cycle. The Krebs Cycle exemplifies this.

Regulation and Separation in Eukaryotes

In eukaryotes, anabolic and catabolic pathways often function separately. This separation can be:

• Physical: Via compartmentalization within organelles.
• Biochemical: Through the necessity of distinct enzymes and cofactors.

Major metabolic pathways

Glucuronate metabolism

Pentose interconversion

Inositol metabolism

Cellulose and sucrose
metabolism

Starch and glycogen
metabolism

Other sugar
metabolism

Pentose phosphate pathway

Glycolysis and Gluconeogenesis

Amino sugars metabolism

Small amino acid synthesis

Branched amino acid
synthesis

Purine biosynthesis

Histidine metabolism

Aromatic amino
acid synthesis

Pyruvate
decarboxylation

Fermentation

Fatty acid
metabolism

Urea cycle

Aspartate amino acid
group synthesis

Porphyrins and
corrinoids
metabolism

Citric acid cycle

Glutamate amino
acid group
synthesis

Pyrimidine biosynthesis

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Cellular respiration

Main article: Cellular respiration

Several distinct but linked metabolic pathways are used by cells to transfer the energy released by breakdown of fuel molecules into ATP and other small molecules used for energy (e.g. GTP, NADPH, FADH).

These pathways occur within all living organisms in some form:

1. Glycolysis
2. Aerobic respiration and/or Anaerobic respiration
3. Citric acid cycle / Krebs cycle (not in most obligate anaerobic organisms)
4. Oxidative phosphorylation (not in obligate anaerobic organisms)

Synthesis of energetic compounds from non-living matter:

• Photosynthesis (plants, algae, cyanobacteria)

See also

• Metabolism
• Metabolic network
• Metabolic network modelling
• Metabolic engineering

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  Net reactions for glycolysis, oxidative decarboxylation of pyruvate, and Krebs cycle

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  Gluconeogenesis pathway

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  Amphibolic properties of the citric acid cycle