Editing Metabolism (section)

===Coenzymes===
[[File:Adenosintriphosphat protoniert.svg|thumb|class=skin-invert|alt=Skeletal formula of adenosine triphosphate|Structure of [[adenosine triphosphate]] (ATP), a central intermediate in energy metabolism]]
{{main|Coenzyme}}
Metabolism involves a vast array of chemical reactions, but most fall under a few basic types of reactions that involve the transfer of [[functional group]]s of atoms and their bonds within molecules.<ref>{{cite journal | vauthors = Mitchell P | title = The Ninth Sir Hans Krebs Lecture. Compartmentation and communication in living systems. Ligand conduction: a general catalytic principle in chemical, osmotic and chemiosmotic reaction systems | journal = European Journal of Biochemistry | volume = 95 | issue = 1 | pages = 1–20 | date = March 1979 | pmid = 378655 | doi = 10.1111/j.1432-1033.1979.tb12934.x | doi-access = free }}</ref> This common chemistry allows cells to use a small set of metabolic intermediates to carry chemical groups between different reactions.<ref name="Wimmer-1978"/> These group-transfer intermediates are called [[coenzyme]]s. Each class of group-transfer reactions is carried out by a particular coenzyme, which is the [[Substrate (biochemistry)|substrate]] for a set of enzymes that produce it, and a set of enzymes that consume it. These coenzymes are therefore continuously made, consumed and then recycled.<ref name="Dimroth-2006">{{cite journal | vauthors = Dimroth P, von Ballmoos C, Meier T | title = Catalytic and mechanical cycles in F-ATP synthases. Fourth in the Cycles Review Series | journal = EMBO Reports | volume = 7 | issue = 3 | pages = 276–82 | date = March 2006 | pmid = 16607397 | pmc = 1456893 | doi = 10.1038/sj.embor.7400646 }}</ref>

One central coenzyme is [[adenosine triphosphate]] (ATP), the energy currency of cells. This [[nucleotide]] is used to transfer chemical energy between different chemical reactions. There is only a small amount of ATP in cells, but as it is continuously regenerated, the human body can use about its own weight in ATP per day.<ref name="Dimroth-2006"/> ATP acts as a bridge between [[catabolism]] and [[anabolism]]. Catabolism breaks down molecules, and anabolism puts them together. Catabolic reactions generate ATP, and anabolic reactions consume it. It also serves as a carrier of phosphate groups in [[phosphorylation]] reactions.<ref>{{cite journal | vauthors = Bonora M, Patergnani S, Rimessi A, De Marchi E, Suski JM, Bononi A, Giorgi C, Marchi S, Missiroli S, Poletti F, Wieckowski MR, Pinton P | display-authors = 6 | title = ATP synthesis and storage | journal = Purinergic Signalling | volume = 8 | issue = 3 | pages = 343–57 | date = September 2012 | pmid = 22528680 | pmc = 3360099 | doi = 10.1007/s11302-012-9305-8 }}</ref>

A [[vitamin]] is an organic compound needed in small quantities that cannot be made in cells. In [[human nutrition]], most vitamins function as coenzymes after modification; for example, all water-soluble vitamins are phosphorylated or are coupled to nucleotides when they are used in cells.<ref>{{cite journal| vauthors = Berg JM, Tymoczko JL, Stryer L |date=2002|title=Vitamins Are Often Precursors to Coenzymes|url=https://www.ncbi.nlm.nih.gov/books/NBK22549/|journal=Biochemistry. 5th Edition|language=en|access-date=9 June 2020|archive-date=15 December 2020|archive-url=https://web.archive.org/web/20201215232601/https://www.ncbi.nlm.nih.gov/books/NBK22549/|url-status=live}}</ref> [[Nicotinamide adenine dinucleotide]] (NAD<sup>+</sup>), a derivative of vitamin B<sub>3</sub> ([[Niacin (nutrient)|niacin]]), is an important coenzyme that acts as a hydrogen acceptor. Hundreds of separate types of [[dehydrogenase]]s remove electrons from their substrates and [[redox|reduce]] NAD<sup>+</sup> into NADH. This reduced form of the coenzyme is then a substrate for any of the [[reductase]]s in the cell that need to transfer hydrogen atoms to their substrates.<ref>{{cite journal | vauthors = Pollak N, Dölle C, Ziegler M | title = The power to reduce: pyridine nucleotides--small molecules with a multitude of functions | journal = The Biochemical Journal | volume = 402 | issue = 2 | pages = 205–18 | date = March 2007 | pmid = 17295611 | pmc = 1798440 | doi = 10.1042/BJ20061638 }}</ref> Nicotinamide adenine dinucleotide exists in two related forms in the cell, NADH and NADPH. The NAD<sup>+</sup>/NADH form is more important in catabolic reactions, while NADP<sup>+</sup>/NADPH is used in anabolic reactions.<ref>{{cite book| vauthors = Fatih Y |title=Advances in food biochemistry|publisher=CRC Press|year=2009|isbn=978-1-4200-0769-5|location=Boca Raton|pages=228|oclc=607553259}}</ref>

[[File:1GZX Haemoglobin.png|thumb|upright=1.35|right|The structure of iron-containing [[hemoglobin]]. The protein subunits are in red and blue, and the iron-containing [[heme]] groups in green. From {{PDB|1GZX}}.]]