Editing Period 4 element (section)

==Biological role==
Many period 4 elements find roles in controlling protein function as [[secondary messengers]], structural components, or enzyme [[Cofactor (biochemistry)|cofactors]]. A gradient of potassium is used by cells to maintain a [[membrane potential]] which enables [[Action potential|neurotransmitter firing]] and [[facilitated diffusion]] among other processes. Calcium is a common signaling molecule for proteins such as [[calmodulin]] and plays a critical role in triggering [[Muscle contraction#Excitation-contraction coupling|skeletal muscle contraction]] in vertebrates. Selenium is a component of the [[Non-proteinogenic amino acids|noncanonical]] [[amino acid]], [[selenocysteine]]; proteins which contain selenocysteine are known as [[selenoprotein]]s. [[:Category:Manganese enzymes|Manganese enzymes]] are utilized by both [[eukaryote]]s and [[prokaryote]]s, and may play a role in the virulence of some pathogenic bacteria.<ref>{{Cite journal|last1=Makhlynets|first1=Olga|last2=Boal|first2=Amie K.|last3=Rhodes|first3=Delacy V.|last4=Kitten|first4=Todd|last5=Rosenzweig|first5=Amy C.|last6=Stubbe|first6=JoAnne|date=2014-02-28|title=Streptococcus sanguinis class Ib ribonucleotide reductase: high activity with both iron and manganese cofactors and structural insights|journal=The Journal of Biological Chemistry|volume=289|issue=9|pages=6259–6272|doi=10.1074/jbc.M113.533554|issn=1083-351X|pmc=3937692|pmid=24381172|doi-access=free}}</ref> [[Vanabins]], also known as vanadium-associated proteins, are found in the blood cells of some species of [[Ascidiacea|sea squirts]]. The role of these proteins is disputed, although there is some speculation that they function as oxygen carriers. Zinc ions are used to stabilize the [[zinc finger]] [[Structural motif|milieu]] of many [[DNA-binding protein]]s.

Period 4 elements can also be found [[Coordination complex|complexed]] with organic [[small molecule]]s to form cofactors. The most famous example of this is [[heme]]: an iron-containing [[porphyrin]] compound responsible for the oxygen-carrying function of [[myoglobin]] and [[hemoglobin]] as well as the catalytic activity of [[Cytochrome P450|cytochrome enzymes]].<ref>{{Cite journal|last1=Caputo|first1=Gregory A.|last2=Vaden|first2=Timothy D.|last3=Calabro|first3=Anthony|last4=Lee|first4=Joshua Y.|last5=Kohn|first5=Eric M.|date=December 2018|title=Heme Dissociation from Myoglobin in the Presence of the Zwitterionic Detergent N,N-Dimethyl-N-Dodecylglycine Betaine: Effects of Ionic Liquids|journal=Biomolecules|language=en|volume=8|issue=4|pages=126|doi=10.3390/biom8040126|pmid=30380655|pmc=6315634|doi-access=free}}</ref> [[Hemocyanin]] replaces hemoglobin as the oxygen carrier of choice in the blood of certain invertebrates, including [[horseshoe crab]]s, [[tarantula]]s, and [[octopus]]es. [[Vitamin B12|Vitamin B<sub>12</sub>]] represents one of the few biochemical applications for cobalt.