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Metabolism (Biotransformation)


Metabolism (Biotransformation)



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Metabolism is the sum of all chemical reactions occurring in organisms at the cellular level to sustain life. The main purpose of metabolism is to convert food to energy to run cellular processes, build macromolecule building blocks, i.e., proteins, lipids, nucleic acids and carbohydrates and eliminate nitrogenous wastes. Metabolism is catalyzed by enzymes, which are protein in nature. There are thousands of enzymes in an organism catalyzing a great variety of metabolic pathways (e.g., glycolysis, Krebs cycle), the sum of which is called metabolism, and the molecules that are produced during the process (intermediates or final products) are called metabolites. Metabolism allows organisms to grow, maintain their structures, respond to their environment, and reproduce. Metabolism is usually divided into two categories: catabolism or the breaking down of organic matter by cellular respiration, and anabolism or the building up of cellular components like proteins and nucleic acids. Usually, catabolism releases energy and anabolism consumes energy. These two processes are linked through cofactors, especially through widely distributed pyridine nucleotides in the form of NAD, NADP and adenine nucleotides in the form of ATP, ADP, and AMP. A primary catabolic process is cellular respiration where starting molecules such as glucose are oxidized in a stepwise fashion to pyruvate and the released energy is captured in the form of ATP and water and carbon dioxide are released as waste products of oxidation. The ATP is in turn used to drive anabolic processes such as amino acid and lipid biosynthesis.

In addition to converting food to energy, biosynthesizing cellular components, and eliminating nitrogenous waste, metabolism also covers biotransformation of foreign chemicals (xenobiotics) through a series of enzyme-catalyzed processes. Biotransformation alters the physicochemical properties of xenobiotics making them from accessible into cells, by enhancing their absorption across biological membranes, to eliminate into urine or bile, by enhancing hydrophilicity. The enzymes responsible for the xenobiotic biotransformation are often called drug-metabolizing enzymes. In the absence of biotransformation, xenobiotics we routinely exposed to, unintentionally or intentionally, for example, pharmaceuticals, industrial chemicals, pesticides, pollutants, cooked food containing pyrolysis products, alkaloids, plant metabolites of pesticides/chemicals, and toxins produced by plants, fungi and microbes will accumulate and eventually reach to toxic levels.

This article is part of the absorption, distribution, metabolism, and excretion or ADME series; the four main processes governing chemical (including drug) disposition in biological systems. Therefore, this article will summarize some fundamental principles of xenobiotic biotransformation (or metabolism) in mammals and major enzymes involved in this process.


This curated reference collection was last updated in 2019.


Medicine and Health Sciences | Pharmacy and Pharmaceutical Sciences


biomedical sciences, disease, human biology, human health, therapy


Metabolism (Biotransformation)
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