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Anabolism

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Anabolism (from Greek: ἁνά, "upward" and βάλλειν, "to throw", also called biosynthesis) is the set of metabolic pathways that construct molecules from smaller units.[1] These reactions require energy, known also as an endergonic process.[2] Metabolic processes are either "anabolic" or "catabolic".

Contents

PathwayEdit

Polymerization, an anabolic pathway used to build macromolecules such as nucleic acids, proteins, and polysaccharides, uses condensation reactions to join monomers.[3] Macromolecules are created from smaller molecules using enzymes and cofactors.

 
Use of ATP to drive the endergonic process of anabolism.

Energy SourceEdit

Anabolism is powered by catabolism, where large molecules are broken down into smaller parts and then used up in cellular respiration. Many anabolic processes are powered by the cleavage of adenosine triphosphate (ATP).[4] Anabolism usually involves reduction and decreases entropy, making it unfavorable without energy input.[5] The starting materials, called the precursor molecules, are joined together using the chemical energy made available from hydrolyzing ATP, reducing the cofactors NAD+, NADP+, and FAD, or performing other favorable side reactions.[6] Occasionally it can also be driven by entropy without energy input, in cases like the formation of the phospholipid bilayer of a cell, where hydrophobic interactions aggregate the molecules.[7]

CofactorsEdit

The reducing agents NADH, NADPH, and FADH2,[8] as well as metal ions,[9] act as cofactors at various steps in anabolic pathways.

SubstratesEdit

Substrates for anabolism are mostly intermediates taken from catabolic pathways.[10]

FunctionsEdit

Anabolic processes build organs and tissues. These processes produce growth and differentiation of cells and increase in body size, a process that involves synthesis of complex molecules. Examples of anabolic processes include the growth and mineralization of bone and increases in muscle mass. Endocrinologists have traditionally classified hormones as anabolic or catabolic, depending on which part of metabolism they stimulate. The classic anabolic hormones are the anabolic steroids, which stimulate protein synthesis and muscle growth, and insulin.

Photosynthetic Carbohydrate SynthesisEdit

Photosynthetic carbohydrate synthesis in plants and certain bacteria is an anabolic process that produces glucose, cellulose, starch, lipids, and proteins from CO2.[5] It uses the energy produced from the light-driven reactions of photosynthesis, and creates the precursors to these large molecules via carbon assimilation in the photosynthetic carbon reduction cycle, a.k.a. the Calvin cycle.[10]

 
Amino acid biosynthesis from intermediates of glycolysis and the citric acid cycle.

Amino Acid BiosynthesisEdit

All amino acids are formed from intermediates in the catabolic processes of glycolysis, the citric acid cycle, or the pentose phosphate pathway. From glycolysis, glucose 6-phosphate is a precursor for histidine; 3-phosphoglycerate is a precursor for glycine and cysteine; phosphoenol pyruvate, combined with the 3-phosphoglycerate-derivative erythrose 4-phosphate, forms tryptophan, phenylalanine, and tyrosine; and pyruvate is a precursor for alanine, valine, leucine, and isoleucine. From the citric acid cycle, α-ketoglutarate is converted into glutamate and subsequently glutamine, proline, and arginine; and oxaloacetate is converted into aspartate and subsequently asparagine, methionine, threonine, and lysine.[10]

Glycogen StorageEdit

During periods of high blood sugar, glucose 6-phosphate from glycolysis is diverted to the glycogen-storing pathway. It is changed to glucose-1-phosphate by phosphoglucomutase and then to UDP-glucose by UTP--glucose-1-phosphate uridylyltransferase. Glycogen synthase adds this UDP-glucose to a glycogen chain.[10]

GluconeogenesisEdit

Glucagon is traditionally a catabolic hormone, but also stimulates the anabolic process of gluconeogenesis by the liver, and to a lesser extent the kidney cortex and intestines, during starvation to prevent low blood sugar.[8]

RegulationEdit

Anabolism operates with separate enzymes from catalysis, which undergo irreversible steps at some point in their pathways. This allows the cell to regulate the rate of production and prevent an infinite loop from forming with catabolism.[10]

The balance between anabolism and catabolism is sensitive to ADP and ATP. Excess ATP causes cells to favor the anabolic pathway and slow catabolic activity, while excess ADP slows anabolism and favors catabolism.[10] These pathways are also regulated by circadian rhythms, with processes such as glycolysis fluctuating to match an animal's normal periods of activity throughout the day.[11]

ReferencesEdit

  1. ^ de Bolster, M. W. G. (1997). "Glossary of Terms Used in Bioinorganic Chemistry: Anabolism". International Union of Pure and Applied Chemistry. Archived from the original on 30 October 2007. Retrieved 2007-10-30.
  2. ^ Biology. Rice University: Openstax College. 2013.
  3. ^ Alberts, Bruce; Johnson, Alexander; Julian, Lewis; Raff, Martin; Roberts, Keith; Walter, Peter (2002). Molecular Biology of the Cell (5th ed.). CRC Press. Archived from the original on 6 June 2017. Retrieved 2018-11-01.
  4. ^ Nicholls D. G. and Ferguson S. J. (2002) Bioenergetics Academic press 3rd edition ISBN 0-12-518121-3.
  5. ^ a b Ahern, Kevin; Rajagopal, Indira (2013). Biochemistry Free and Easy (PDF) (2nd ed.). Oregon State University.
  6. ^ Donald., Voet, (2013). Fundamentals of biochemistry : life at the molecular level. Voet, Judith G., Pratt, Charlotte W. (Fourth ed.). Hoboken, NJ: Wiley. ISBN 9780470547847. OCLC 738349533.
  7. ^ Phospholipids : biochemical, pharmaceutical, and analytical considerations. Hanin, Israel,, Pepeu, Giancarlo,. New York. ISBN 9781475713640. OCLC 885405600.
  8. ^ a b Jakubowski, Henry (2002). Biochemistry Online. College of St. Benedict, St. John's University.
  9. ^ Alberts, Bruce; Johnson, Alexander; Lewis, Julian; Raff, Martin; Roberts, Keith; Walter, Peter (2002). Molecular Biology of the Cell (4th ed.). CRC Press.
  10. ^ a b c d e f Lehninger, Albert. Principles of Biochemistry. ISBN 978-1429234146.
  11. ^ Ramsey K. M., Marcheva B., Kohsaka A., Bass J. (2007). "The clockwork of metabolism". Annu. Rev. Nutr. 27: 219–40. doi:10.1146/annurev.nutr.27.061406.093546. PMID 17430084.