Metabolic pathway

Metabolic Pathways[edit | edit source]

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[edit | edit source]

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[edit | edit source]

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[edit | edit source]

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[edit | edit source]

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[edit | edit source]

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[edit | edit source]

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[edit | edit source]

All pathway labels on this image are links, simply click to access the article.
A high resolution labeled version of this image is available here.

Cellular respiration[edit | edit source]

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:

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

Synthesis of energetic compounds from non-living matter:

Photosynthesis (plants, algae, cyanobacteria)

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Metabolic pathway