Anabolism is powered by catabolism, where large molecules are broken down into smaller parts and then used up incellular respiration. Many anabolic processes are powered by thecleavage of adenosine triphosphate (ATP).[5] Anabolism usually involvesreduction and decreasesentropy, making it unfavorable without energy input.[6] The starting materials, called the precursor molecules, are joined using thechemical energy made available from hydrolyzing ATP, reducing the cofactorsNAD+,NADP+, andFAD, or performing other favorable side reactions.[7] Occasionally it can also be driven byentropy without energy input, in cases like the formation of thephospholipid bilayer of a cell, where hydrophobic interactions aggregate the molecules.[8]
The reducing agentsNADH,NADPH, andFADH2,[9] as well as metal ions,[4] act as cofactors at various steps in anabolic pathways. NADH, NADPH, and FADH2 act aselectron carriers, while charged metal ions within enzymes stabilize chargedfunctional groups onsubstrates.
Anabolic processes buildorgans andtissues. These processes produce growth and differentiation of cells and increase in body size, a process that involvessynthesis of complexmolecules. Examples of anabolic processes include the growth and mineralization ofbone and increases inmuscle mass.
Endocrinologists have traditionally classifiedhormones as anabolic or catabolic, depending on which part of metabolism they stimulate. The classic anabolic hormones are theanabolic steroids, which stimulate protein synthesis and muscle growth, andinsulin.
Glucagon is traditionally a catabolic hormone, but also stimulates the anabolic process ofgluconeogenesis by the liver, and to a lesser extent the kidney cortex and intestines, during starvation to preventlow blood sugar.[9] It is the process of converting pyruvate into glucose. Pyruvate can come from the breakdown of glucose,lactate, amino acids, orglycerol.[11] The gluconeogenesis pathway has many reversible enzymatic processes in common with glycolysis, but it is not the process of glycolysis in reverse. It uses different irreversible enzymes to ensure the overall pathway runs in one direction only.[11]
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, also known as afutile cycle, from forming with catabolism.[10]
The balance between anabolism and catabolism is sensitive toADP and ATP, otherwise known as the energy charge of the cell. High amounts of ATP cause cells to favor the anabolic pathway and slow catabolic activity, while excess ADP slows anabolism and favors catabolism.[10] These pathways are also regulated bycircadian rhythms, with processes such asglycolysis fluctuating to match an animal's normal periods of activity throughout the day.[12]
^Voet D, Voet JG, Pratt CW (2013).Fundamentals of biochemistry : life at the molecular level (Fourth ed.). Hoboken, NJ: Wiley.ISBN978-0-470-54784-7.OCLC738349533.
^Hanin I, Pepeu G (2013-11-11).Phospholipids: biochemical, pharmaceutical, and analytical considerations. New York.ISBN978-1-4757-1364-0.OCLC885405600.{{cite book}}: CS1 maint: location missing publisher (link)
