METABOLIC ROUTES: TYPES AND MAIN ROUTES - BIOLOGY - 2025

A metabolic pathway is a set of chemical reactions, catalyzed by enzymes. In this process, a molecule X is transformed into a molecule Y, by means of intermediate metabolites. Metabolic pathways take place in the cellular environment.

Outside of the cell, these reactions would take too long, and some might not occur. Therefore, each step requires the presence of catalyst proteins called enzymes. The role of these molecules is to accelerate the speed of each reaction within the pathway by several orders of magnitude.

Main metabolic routes

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Physiologically, the metabolic pathways are connected to each other. That is, they are not isolated within the cell. Many of the most important pathways share metabolites in common.

Consequently, the set of all chemical reactions that occur in cells is called metabolism. Each cell is characterized by exhibiting a specific metabolic performance, which is defined by the content of enzymes within, which in turn is genetically determined.

General characteristics of metabolic pathways

Within the cellular environment, a large number of chemical reactions occur. The set of these reactions is metabolism, and the main function of this process is to maintain the homeostasis of the body under normal conditions, and also under stress conditions.

Thus, there must be a balance of fluxes of these metabolites. Among the main characteristics of metabolic pathways we have the following:

Reactions are catalyzed by enzymes

Reaction catalyzed by cyclooxygenase enzymes (Source: Pancrat via Wikimedia Commons)

The protagonists of metabolic pathways are enzymes. They are responsible for integrating and analyzing the information on the metabolic state and are capable of modulating their activity based on the cellular requirements of the moment.

Metabolism is regulated by hormones

The metabolism is directed by a series of hormones, which are capable of coordinating metabolic reactions, considering the needs and performance of the body.

Compartmentalisation

There is a compartmentalization of metabolic pathways. That is, each pathway takes place in a specific subcellular compartment, call it cytoplasm, mitochondria, among others. Other routes can occur in several compartments simultaneously.

The compartmentalisation of the pathways aids in the regulation of the anabolic and catabolic pathways (see below).

Coordination of metabolic flow

The coordination of metabolism is achieved through the stability of the activity of the enzymes involved. It should be noted that anabolic pathways and their catabolic counterparts are not totally independent. In contrast, they are coordinated.

There are key enzymatic points within the metabolic pathways. With the rate of conversion of these enzymes, the entire flow of the pathway is regulated.

Types of metabolic pathways

In biochemistry, three main types of metabolic pathways are distinguished. This division is carried out following bioenergetic criteria: catabolic, anabolic and amphibole routes.

Catabolic routes

Catabolic pathways encompass oxidative degradation reactions. They are carried out in order to obtain energy and reducing power, which will be used later by the cell in other reactions.

Most of the organic molecules are not synthesized by the body. In contrast, we must consume it through food. In catabolic reactions, these molecules are degraded into the monomers that compose them, which can be used by cells.

Anabolic routes

Anabolic pathways comprise the chemical reactions of synthesis, taking small, simple molecules and transforming them into larger, more complex elements.

For these reactions to take place, energy needs to be available. Where does this energy come from? From the catabolic pathways, primarily in the form of ATP.

In this way, the metabolites produced by catabolic pathways (which are globally called "pool of metabolites") can be used in anabolic pathways in order to synthesize more complex molecules that the body needs at the moment.

Among this pool of metabolites, there are three key molecules of the process: pyruvate, acetyl coenzyme A and glycerol. These metabolites are responsible for connecting the metabolism of different biomolecules, such as lipids, carbohydrates, among others.

Amphibious routes

An amphibole pathway functions as either an anabolic or a catabolic pathway. That is, it is a mixed route.

The best known amphibole route is the Krebs cycle. This route has a fundamental role in the degradation of carbohydrates, lipids and amino acids. However, it also participates in the production of the precursors for synthetic routes.

For example, the Krebs cycle metabolites are the precursors of half of the amino acids that are used to build proteins.

Main metabolic pathways

In all cells that are part of living beings, a series of metabolic pathways are carried out. Some of these are shared by most of the organisms.

These metabolic pathways include the synthesis, degradation, and conversion of life-critical metabolites. This whole process is known as intermediate metabolism.

Cells permanently need organic and inorganic compounds, and also chemical energy, which is obtained mainly from the ATP molecule.

ATP (adenosine triphosphate) is the most important form of energy storage in all cells. And the energy gains and investments of metabolic pathways are often expressed in terms of ATP molecules.

The most important pathways that are present in the vast majority of living organisms will be discussed below.

Glycolysis or glycolysis

Figure 1: glycolysis vs gluconeogenesis. Reactions and enzymes involved.

Glycolysis is a route that involves the degradation of glucose up to two molecules of pyruvic acid, obtaining as a net gain two molecules of ATP. It is present in virtually all living organisms and is considered a fast way to obtain energy.

In general, it is usually divided into two stages. The first involves the passage of the glucose molecule into two glyceraldehyde molecules, inverting two ATP molecules. In the second phase, high-energy compounds are generated, and 4 ATP molecules and 2 pyruvate molecules are obtained as final products.

The route can continue in two different ways. If there is oxygen, the molecules will finish their oxidation in the respiratory chain. Or, in the absence of this, fermentation occurs.

Gluconeogenesis

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Gluconeogenesis is a pathway for the synthesis of glucose, starting from amino acids (with the exception of leucine and lysine), lactate, glycerol or any of the intermediates of the Krebs cycle.

Glucose is an essential substrate for certain tissues, such as the brain, red blood cells, and muscles. The glucose supply can be obtained through glycogen stores.

However, when these are depleted, the body must begin glucose synthesis in order to meet the demands of the tissues - primarily the nervous tissue.

This pathway occurs mainly in the liver. It is vital since, in fasting situations, the body can continue to obtain glucose.

The activation or not of the pathway is linked to the feeding of the organism. Animals that consume diets high in carbohydrates have low gluconeogenic rates, whereas diets low in glucose require significant gluconeogenic activity.

Glyoxylate cycle

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This cycle is unique to plants and certain types of bacteria. This pathway achieves the transformation of two-carbon acetyl units into four-carbon units - known as succinate. This last compound can produce energy and can also be used for the synthesis of glucose.

In humans, for example, it would be impossible to subsist on acetate alone. In our metabolism, acetyl coenzyme A cannot be converted to pyruvate, which is a precursor of the gluconeogenic pathway, because the reaction of the pyruvate dehydrogenase enzyme is irreversible.

The biochemical logic of the cycle is similar to that of the citric acid cycle, with the exception of the two decarboxylation stages. It occurs in very specific organelles of plants called glyoxysomes, and is particularly important in the seeds of some plants such as sunflowers.

Krebs cycle

Tricarboxylic acid cycle (Krebs cycle). Taken and edited from: Narayanese, WikiUserPedia, YassineMrabet, TotoBaggins (translated into Spanish by Alejandro Porto).

It is one of the routes considered as central in the metabolism of organic beings, since it unifies the metabolism of the most important molecules, including proteins, fats and carbohydrates.

It is a component of cellular respiration, and aims to release the energy stored in the acetyl coenzyme A molecule - the main precursor of the Krebs cycle. It is made up of ten enzymatic steps and, as we mentioned, the cycle works in both anabolic and catabolic pathways.

In eukaryotic organisms, the cycle takes place in the matrix of the mitochondria. In prokaryotes - which lack true subcellular compartments - the cycle takes place in the cytoplasmic region.

Electron transport chain

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The electron transport chain is made up of a series of transporters anchored in a membrane. The chain aims to generate energy in the form of ATP.

The chains are capable of creating an electrochemical gradient thanks to the flow of electrons, a crucial process for the synthesis of energy.

Fatty acid synthesis

Fatty acids are molecules that play very important roles in cells, they are mainly found as structural components of all biological membranes. For this reason, the synthesis of fatty acids is essential.

The entire synthesis process occurs in the cytosol of the cell. The central molecule of the process is called malonyl coenzyme A. It is responsible for providing the atoms that will form the carbon skeleton of the fatty acid in formation.

Beta oxidation of fatty acids

Beta oxidation is a process of degradation of fatty acids. This is accomplished through four steps: FAD oxidation, hydration, NAD + oxidation, and thiolysis. Previously, the fatty acid needs to be activated by the integration of coenzyme A.

The product of the mentioned reactions are units formed by a carbon pair in the form of acetyl coenzyme A. This molecule can enter the Krebs cycle.

The energy efficiency of this pathway depends on the length of the fatty acid chain. For palmitic acid, for example, which has 16 carbons, the net yield is 106 ATP molecules.

This pathway takes place in the mitochondria of eukaryotes. There is also another alternative route in a compartment called the peroxisome.

As most of the fatty acids are located in the cell cytosol, they must be transported to the compartment where they will be oxidized. Transport is dependent on cartinitan, and allows these molecules to enter the mitochondria.

Nucleotide metabolism

The synthesis of nucleotides is a key event in cellular metabolism, since these are the precursors of the molecules that form part of the genetic material, DNA and RNA, and of important energy molecules, such as ATP and GTP.

The precursors of nucleotide synthesis include different amino acids, ribose 5 phosphate, carbon dioxide and NH ₃. Recovery pathways are responsible for the recycling of free bases and nucleosides released from the breakdown of nucleic acids.

The formation of the purine ring takes place from ribose 5 phosphate, it becomes a purine nucleus and finally the nucleotide is obtained.

The pyrimidine ring is synthesized as orotic acid. Followed by binding to ribose 5 phosphate, it is transformed into pyrimidine nucleotides.

Fermentation

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Fermentations are oxygen-independent metabolic processes. They are of the catabolic type and the end product of the process is a metabolite that still has oxidation potential. There are different types of fermentations, but lactic fermentation takes place in our body.

Lactic fermentation takes place in the cell cytoplasm. It consists of the partial degradation of glucose in order to obtain metabolic energy. As a waste substance, lactic acid is produced.

After an intense session of anaerobic exercises, the muscle is not with adequate concentrations of oxygen and lactic fermentation occurs.

Some cells in the body are forced to ferment, as they lack mitochondria, as is the case with red blood cells.

In industry, fermentation processes are used with a high frequency, to produce a series of products for human consumption, such as bread, alcoholic beverages, yogurt, among others.

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