Editing Sulfur (section)

===Metalloproteins and inorganic cofactors===
Metalloproteins—in which the active site is a transition metal ion (or metal-sulfide cluster) often coordinated by sulfur atoms of cysteine residues<ref>{{Cite journal |last1=Pace |first1=Nicholas J. |last2=Weerapana |first2=Eranthie |date=2014-04-17 |title=Zinc-binding cysteines: diverse functions and structural motifs |journal=Biomolecules |volume=4 |issue=2 |pages=419–434 |doi=10.3390/biom4020419 |issn=2218-273X |pmc=4101490 |pmid=24970223|doi-access=free }}</ref>—are essential components of enzymes involved in electron transfer processes. Examples include [[plastocyanin]] (Cu<sup>2+</sup>) and [[nitrous-oxide reductase|nitrous oxide reductase]] (Cu–S). The function of these enzymes is dependent on the fact that the transition metal ion can undergo [[redox reaction]]s. Other examples include many zinc proteins,<ref>{{Cite journal |last1=Giles |first1=Niroshini M |last2=Watts |first2=Aaron B |last3=Giles |first3=Gregory I |last4=Fry |first4=Fiona H |last5=Littlechild |first5=Jennifer A |last6=Jacob |first6=Claus |date=2003-08-01 |title=Metal and Redox Modulation of Cysteine Protein Function |journal=Chemistry & Biology |language=en |volume=10 |issue=8 |pages=677–693 |doi=10.1016/S1074-5521(03)00174-1 |pmid=12954327 |issn=1074-5521|doi-access=free }}</ref> as well as [[iron–sulfur cluster]]s. Most pervasive are the [[ferrodoxin]]s, which serve as electron shuttles in cells. In bacteria, the important [[nitrogenase]] enzymes contain an Fe–Mo–S cluster and is a [[catalyst]] that performs the important function of [[nitrogen fixation]], converting atmospheric nitrogen to ammonia that can be used by microorganisms and plants to make proteins, DNA, RNA, alkaloids, and the other organic nitrogen compounds necessary for life.<ref>{{cite book|isbn= 978-0-935702-73-6 |first1= S. J.|last1= Lippard|first2= J. M.|last2= Berg|title= Principles of Bioinorganic Chemistry|publisher= University Science Books|date= 1994}}</ref>

Sulfur is also present in [[molybdenum cofactor]].<ref>{{Cite journal |last1=Schwarz |first1=Günter |last2=Mendel |first2=Ralf R. |date=2006 |title=Molybdenum cofactor biosynthesis and molybdenum enzymes |journal=Annual Review of Plant Biology |language=en |volume=57 |issue=1 |pages=623–647 |doi=10.1146/annurev.arplant.57.032905.105437 |pmid=16669776 |bibcode=2006AnRPB..57..623S |issn=1543-5008}}</ref>
:[[File:FdRedox.png|center|upright=3|Easiness of electron flow in a cluster provides catalytic effect of a respective enzyme.|thumb]]