Introduction:
Metabolism is defined as the series of chemical processes that occur within cells of living organisms that produce the substances and energy needed to sustain life.
The metabolic processes involve many interconnected cellular pathways that help in growth and reproduction and also to maintain the structures of living organisms.
Metabolites are the chemical substances that are involved in this process. This process might be linear (Glycolysis), cyclic (Krebs cycle), or spiral (Fatty acid synthesis).
Purposes:
Metabolism is the entire quantity of metabolic events involved in keeping cells alive in an organism. All living creatures require energy for several critical functions as well as the creation of new biological molecules.
Because of metabolic activity, organisms react to their surroundings. All chemical events that occur in living organisms, from digestion to the movement of chemicals from cell to cell, require energy which is release and produce due to the metabolism
Types:
Metabolism can be divided into a sequence of chemical events that include both the synthesis and degradation of complex macromolecules, known as anabolism and catabolism, respectively.
Metabolism is the accumulation of the essential activities that occur in a living being for existence. Metabolism is made up of catabolism and anabolism.
There are two types of metabolic process:
Catabolism
Catabolism refers to the series of enzyme-catalyzed reactions that break down or degrade relatively complex molecules in living cells.
It is the metabolic activity used to degrade molecules to produce simpler constituents and energy.
Krebs’s Cycle/Citric Acid Cycle/TCA Cycle, Glycolysis or sugar catabolism, Lipolysis or fatty acid catabolism, Oxidative deamination and transamination (protein catabolism), oxidative phosphorylation, muscle tissue breakdown or muscle catabolismis an example of catabolism.
The precise details of catabolic reactions vary from organism to organism and can be categorized based on their energy and carbon sources, which are listed below-
- Organotrophs get their energy from organic sources.
- Lithotrophs feed on inorganic materials.
- Phototrophs use sunshine to generate chemical energy.
Anabolism
The process through which the body uses the energy liberated by catabolism to build complex molecules is known as anabolism. Cellular structure is constructed from small, basic precursors that serve as building blocks, and these complicated molecules are then used to create those structures.
There are three stages in anabolism. They are
Production of precursors like isoprenoids, monosaccharides, nucleotides, and amino acids.
ATP energy is used to activate the abovementioned precursors into reactive forms.
Assemble the precursors to create complex compounds including proteins, lipids, nucleic acids, and polysaccharides.
Examples of anabolism include the formation and mineralization of bones, the growth of muscular mass, and photosynthesis. One example is the creation of glucose from carbon dioxide. Other examples are the creation of DNA strands from the building blocks of nucleic acids or the synthesis of proteins from amino acids (nucleotides).
Fig: Coupling of anabolic and catabolic pathways
Compartment of Metabolism:
In cells, all reactions take place in a single compartment that is isolated from other compartments by semipermeable membranes. They aid in the separation of even the most chemically diverse surroundings, allowing the course of chemical reactions to be optimized.
Individual enzymes catalyzing individual reactions usually have variable temperature and pH optimums, and if there was only one cellular compartment, a fraction of enzymes would most likely not work or their-catalyzed reactions would be inefficient. By dividing the cellular space, optimal conditions for individual enzymatically catalyzed reactions are created.
Compartmentalization has an impact on metabolic pathway regulation as well, making them more accurate and targeted, as well as less interfering with one another.
At the same time, the cell defends itself against the activities of lytic enzymes. For example, trapping the cellular digestion in lysosomes inhibits inappropriate auto-digestion of other organelles within the cell.
A common process that accompanies the disruption of some of the compartments (like spilling the content of lysosomes or mitochondria) are necrosis or activation of apoptosis (the process of programmed cell death).
Metabolic pathways:
There are a variety of different metabolic pathways. The most significant metabolic processes in humans are:
- Glycolysis – oxidation of glucose molecule.
- Citric acid cycle (Krebs’ cycle) – oxidation of acetyl-CoA oxidation in order to synthesis ATP and other several metabolites.
- Oxidative phosphorylation – synthesis of ATP through the transfer of electron generated through metabolism
- Pentose phosphate pathway – synthesis of pentoses and production of the reducing agents that will be needed for various biosynthetic reaction in a living system.
- Urea cycle – removal of excess ammonia from human body in less toxic forms
- Fatty acid β-oxidation – breakdown of fatty acids into acetyl-CoA, which will be eventually used by the Krebs’ cycle.
- Gluconeogenesis – biosynthesis of glucose molecule from simple precursors
Characteristics of metabolic pathway are:
(1) They are almost always irreversible.
(2) In eukaryotic cells, they occur in specific cellular sites (compartment).
(3) Metabolic pathway is regulated in several ways.
Regulation:
Intracellular signals: (Inside cell)
- • Availability of substrate, the rate of reaction is affected by substrate concentration.
- • Product inhibition (ability of the products to control the metabolism).
- • Allosteric control of an enzyme by a metabolic intermediary or co-enzyme
Intercellular communications: (Between cells)
- In multicellular organisms, extracellular signals such as growth factors and hormones that act from outside the cell influence the cellular concentration of an enzyme by altering the rate of its synthesis or degradation.
- Chemical signaling (hormones) from outside the cell:
- Second messenger: cAMP, cGMP, Ca\ phosphatidylinositol.
Hormonal control of metabolism
The basal metabolic rate of the body is controlled by the hormones thyroxine (T4) and triiodothyronine (T3). T3 and T4 are produced by the thyroid gland in response to the thyroid stimulating hormone (TSH), produced by the anterior pituitary.
It is generally documented that thyroid hormone level correlates with body weight and energy expenditure.
Metabolism and Microbiota:
The human gut includes trillions of microorganisms that play an important role in host biology, including the provision of vital nutrients from the diet.
The gastrointestinal (GI) tract is home to a diverse microbial ecosystem that includes bacteria, yeast, fungus, bacteriophages, and other viruses, as well as protozoa and archaea.
Food is a primary source of precursors for metabolite formation; in fact, diet affects the gut microbiota (GM) as nutrients derived from dietary consumption.
The gut microbial community provides major benefits to host physiology. A definite link between gut bacteria and host metabolism has now been established, with microbial-mediated gut hormone secretion playing a key role. Bacteria in the gut lumen create a variety of metabolites and have structural components that serve as signaling molecules to a variety of cell types.
Fig: Metabolism and Microbiota
References:
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- David, L., Nelson, D.L., Cox, M.M., Stiedemann, L., McGlynn Jr, M.E. and Fay, M.R., 2000. Lehninger principles of biochemistry.
- Satyanarayana, U., 2021. Biochemistry, 6e-E-book. Elsevier Health Sciences.
- Martin, A.M., Sun, E.W., Rogers, G.B. and Keating, D.J., 2019. The influence of the gut microbiome on host metabolism through the regulation of gut hormone release. Frontiers in Physiology, 10, p.428.