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===Use of capital letters to designate the type of heme===
The practice of designating hemes with upper case letters was formalized in a footnote in a paper by Puustinen and Wikstrom,<ref>{{cite journal|pmid=2068092|pmc=52034|title=The heme groups of cytochrome o from Escherichia coli|author = Puustinen A, Wikström M.|journal = Proc. Natl. Acad. Sci. U.S.A.|volume = 88|issue=14|pages = 6122–6|year = 1991 |bibcode=1991PNAS...88.6122P|doi=10.1073/pnas.88.14.6122|doi-access=free}}</ref> which explains under which conditions a capital letter should be used: "we prefer the use of capital letters to describe the heme structure as isolated. Lowercase letters may then be freely used for cytochromes and enzymes, as well as to describe individual protein-bound heme groups (for example, cytochrome bc, and aa3 complexes, cytochrome b<sub>5</sub>, heme c<sub>1</sub> of the bc<sub>1</sub> complex, heme a<sub>3</sub> of the aa<sub>3</sub> complex, etc)." In other words, the chemical compound would be designated with a capital letter, but specific instances in structures with lowercase. Thus cytochrome oxidase, which has two A hemes (heme a and heme a<sub>3</sub>) in its structure, contains two moles of heme A per mole protein. Cytochrome bc<sub>1</sub>, with hemes b<sub>H</sub>, b<sub>L</sub>, and c<sub>1</sub>, contains heme B and heme C in a 2:1 ratio. The practice seems to have originated in a paper by Caughey and York in which the product of a new isolation procedure for the heme of cytochrome aa3 was designated heme A to differentiate it from previous preparations: "Our product is not identical in all respects with the heme a obtained in solution by other workers by the reduction of the hemin a as isolated previously (2). For this reason, we shall designate our product heme A until the apparent differences can be rationalized.".<ref>{{cite journal|title=Isolation and some properties of the green heme of cytochrome oxidase from beef heart muscle.|vauthors=Caughey WS, York JL|journal = J. Biol. Chem.|volume = 237|pages = 2414–6|year = 1962|issue=7|doi=10.1016/S0021-9258(19)63456-3|pmid=13877421|doi-access=free}}</ref> In a later paper,<ref>{{cite journal|title=Heme A of cytochrome c oxidase. Structure and properties: comparisons with hemes B, C, and S and derivatives|vauthors=Caughey WS, Smythe GA, O'Keeffe DH, Maskasky JE, Smith ML|journal = J. Biol. Chem.|volume = 250|issue = 19|pages = 7602–22|year = 1975|doi=10.1016/S0021-9258(19)40860-0|pmid=170266|doi-access=free}}</ref> Caughey's group uses capital letters for isolated heme B and C as well as A.
==Synthesis==
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