ANABOLISM: FUNCTIONS, PROCESSES, DIFFERENCES WITH CATABOLISM - BIOLOGY - 2025

The anabolism is a division of metabolism including formation reactions of large molecules from smaller ones. For this series of reactions to occur, a source of energy is necessary and, generally, it is ATP (adenosine triphosphate).

Anabolism, and its metabolic inverse, catabolism, are grouped into a series of reactions called metabolic pathways or pathways orchestrated and regulated primarily by hormones. Every small step is controlled so that a gradual transfer of energy occurs.

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Anabolic processes can take the basic units that make up biomolecules - amino acids, fatty acids, nucleotides, and sugar monomers - and generate more complicated compounds, such as proteins, lipids, nucleic acids, and carbohydrates as final energy producers.

Features

Metabolism is a term that encompasses all chemical reactions that occur within the body. The cell resembles a microscopic factory where synthesis and degradation reactions are constantly taking place.

The two goals of metabolism are: first, to use the chemical energy stored in food, and second, to replace structures or substances that no longer function in the body. These events occur according to the specific needs of each organism and are directed by chemical messengers called hormones.

Energy comes mainly from the fats and carbohydrates that we consume in food. In the event of a deficiency, the body can use protein to make up for the deficiency.

Also, regeneration processes are closely linked to anabolism. The regeneration of tissues is a sine qua non condition to maintain a healthy body and working properly. Anabolism is responsible for producing all the cellular compounds that keep them functioning.

There is a delicate balance in the cell between metabolic processes. Large molecules can be broken down to their smallest components by catabolic reactions and the reverse process - from small to large - can occur through anabolism.

Anabolic processes

Anabolism includes, in a general way, all the reactions catalyzed by enzymes (small protein molecules that accelerate the speed of chemical reactions by several orders of magnitude) responsible for the "construction" or synthesis of cellular components.

The overview of anabolic pathways includes the following steps: Simple molecules that participate as intermediates in the Krebs cycle are either aminated or chemically transformed into amino acids. These are later assembled into more complex molecules.

These processes require chemical energy, coming from catabolism. Among the most important anabolic processes are: fatty acid synthesis, cholesterol synthesis, nucleic acid synthesis (DNA and RNA), protein synthesis, glycogen synthesis and amino acid synthesis.

The role of these molecules in the body and their synthesis routes will be briefly described below:

Fatty acid synthesis

Lipids are highly heterogeneous biomolecules capable of generating a large amount of energy when oxidized, particularly triacylglycerol molecules.

Fatty acids are the archetypal lipids. They are made up of a head and a tail made of hydrocarbons. These can be unsaturated or saturated, depending on whether or not they have double bonds on the tail.

Lipids are the essential components of all biological membranes, in addition to participating as a reserve substance.

Fatty acids are synthesized in the cell's cytoplasm from a precursor molecule called malonyl-CoA, derived from acetyl-CoA and bicarbonate. This molecule donates three carbon atoms to start the growth of the fatty acid.

After the formation of malonil, the synthesis reaction continues in four essential steps:

-The condensation of acetyl-ACP with malonyl-ACP, a reaction that produces acetoacetyl-ACP and releases carbon dioxide as a waste substance.

-The second step is the reduction of acetoacetyl-ACP, by NADPH to D-3-hydroxybutyryl-ACP.

-A subsequent dehydration reaction occurs that converts the previous product (D-3-hydroxybutyryl-ACP) to crotonyl-ACP.

-Finally, crotonyl-ACP is reduced and the final product is butyryl-ACP.

Cholesterol synthesis

Cholesterol is a sterol with a typical 17-carbon sterans nucleus. It has different roles in physiology, since it functions as a precursor of a variety of molecules such as bile acids, different hormones (including sexual ones) and is essential for the synthesis of vitamin D.

Synthesis occurs in the cytoplasm of the cell, primarily in liver cells. This anabolic pathway has three phases: first the isoprene unit is formed, then the progressive assimilation of the units occurs to originate squalene, this passes to lanosterol and finally cholesterol is obtained.

The activity of the enzymes in this pathway is regulated mainly by the relative ratio of the hormones insulin: glucagon. As this ratio increases, the activity of the pathway increases proportionally.

Nucleotide synthesis

Nucleic acids are DNA and RNA, the first contains all the information necessary for the development and maintenance of living organisms, while the second complements the functions of DNA.

Both DNA and RNA are composed of long chains of polymers whose fundamental unit is nucleotides. Nucleotides, in turn, are made up of a sugar, a phosphate group, and a nitrogenous base. The precursor of purines and pyrimidines is ribose-5-phosphate.

Purines and pyrimidines are produced in the liver from precursors such as carbon dioxide, glycine, ammonia, among others.

Nucleic acid synthesis

Nucleotides must be joined into long DNA or RNA chains in order to fulfill their biological function. The process involves a series of enzymes that catalyze the reactions.

The enzyme in charge of copying DNA to generate more DNA molecules with identical sequences is DNA polymerase. This enzyme cannot initiate de novo synthesis, so a small piece of DNA or RNA called a primer must participate, which allows the formation of the chain.

This event requires the participation of additional enzymes. The helicase, for example, helps open the DNA double helix so that the polymerase can act and the topoisomerase is able to modify the topology of the DNA, either by tangling or unraveling it.

Similarly, RNA polymerase participates in the synthesis of RNA from a DNA molecule. Unlike the previous process, RNA synthesis does not require the mentioned primer.

Protein synthesis

Protein synthesis is a crucial event in all living organisms. Proteins carry out a wide variety of functions, such as transporting substances or playing the role of structural proteins.

According to the central “dogma” of biology, after DNA is copied into messenger RNA (as described in the previous section), it is in turn translated by ribosomes into a polymer of amino acids. In RNA, each triplet (three nucleotides) is interpreted as one of the twenty amino acids.

Synthesis occurs in the cytoplasm of the cell, where ribosomes are found. The process occurs in four phases: activation, initiation, elongation, and termination.

Activation consists of the binding of a particular amino acid to the corresponding transfer RNA. Initiation involves the binding of the ribosome to the 3 'terminal portion of messenger RNA, assisted by "initiation factors".

Elongation involves the addition of amino acids according to the RNA message. Finally, the process stops with a specific sequence in the messenger RNA, called termination condoms: UAA, UAG, or UGA.

Glycogen synthesis

Glycogen is a molecule made up of repeating glucose units. It acts as an energy reserve substance and is mostly abundant in the liver and muscle.

The synthesis route is called glycogenogenesis and requires the participation of the enzyme glycogen synthase, ATP and UTP. The pathway begins with the phosphorylation of glucose to glucose-6-phosphate and then to glucose-1-phosphate. The next step involves the addition of a UDP to yield UDP-glucose and inorganic phosphate.

The UDP-glucose molecule adds to the glucose chain via an alpha 1-4 bond, releasing the UDP nucleotide. In the event that branches occur, these are formed by alpha 1-6 bonds.

Synthesis of amino acids

Amino acids are units that make up proteins. In nature there are 20 types, each with unique physical and chemical properties that determine the final characteristics of the protein.

Not all organisms can synthesize all 20 types. For example, humans can only synthesize 11, the remaining 9 must be incorporated into the diet.

Each amino acid has its own pathway. However, they come from precursor molecules such as alpha-ketoglutarate, oxaloacetate, 3-phosphoglycerate, pyruvate, among others.

Regulation of anabolism

As we mentioned earlier, metabolism is regulated by substances called hormones, secreted by specialized tissues, either glandular or epithelial. These function as messengers and their chemical nature is quite heterogeneous.

For example, insulin is a hormone secreted by the pancreas and has a major effect on metabolism. After high carbohydrate meals, insulin works as a stimulant of the anabolic pathways.

Thus, the hormone is responsible for activating the processes that allow the synthesis of storage substances such as fats or glycogen.

There are periods of life where anabolic processes are predominant, such as childhood, adolescence, during pregnancy or during training focused on the growth of muscles.

Differences with catabolism

All the chemical processes and reactions that occur within our body - specifically within our cells - are globally known as metabolism. We can grow, develop, reproduce, and maintain body heat thanks to this highly controlled series of events.

Synthesis versus degradation

Metabolism involves the use of biomolecules (proteins, carbohydrates, lipids or fats, and nucleic acids) to maintain all the essential reactions of a living system.

Obtaining these molecules come from the food we eat every day and our body is able to "break down" them into smaller units during the digestion process.

For example, proteins (which can come from meat or eggs, for example) are broken down into their main components: amino acids. In the same way, we can process carbohydrates into smaller units of sugar, generally glucose, one of the carbohydrates most used by our body.

Our body is able to use these small units - amino acids, sugars, fatty acids, among others - to build new larger molecules in the configuration that our body needs.

The process of disintegration and obtaining energy is called catabolism, while the formation of new more complex molecules is anabolism. Thus, synthesis processes are associated with anabolism and degradation processes with catabolism.

As a mnemonic rule we can use the "c" in the word catabolism and relate it to the word "cut".

Use of energy

Anabolic processes require energy, while degradation processes produce this energy, mainly in the form of ATP - known as the energy currency of the cell.

This energy comes from catabolic processes. Let's imagine that we have a deck of cards, if we have all the cards stacked neatly and we throw them on the ground they do so spontaneously (analogous to catabolism).

However, in case we want to order them again we must apply energy to the system and collect them from the ground (analogous to anabolism).

In some cases the catabolic pathways need an "injection of energy" in their first steps to get the process started. For example, glycolysis or glycolysis is the breakdown of glucose. This pathway requires the use of two ATP molecules to get started.

Balance between anabolism and catabolism

To maintain a healthy and adequate metabolism it is necessary that there is a balance between the processes of anabolism and catabolism. In the event that the processes of anabolism exceed those of catabolism, the synthesis events are the ones that prevail. In contrast, when the body is receiving more energy than necessary, the catabolic pathways predominate.

When the body experiences adversity, call it illness or periods of prolonged fasting, the metabolism focuses on degradation pathways and enters a catabolic state.

Source: By Alejandro Porto, from Wikimedia Commons

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