Editing Protein complex (section)

{{short description|Type of stable macromolecular complex}}
[[Image:Kinesin_walking.gif|thumb|300px| [[Kinesin]] is a protein functioning as a molecular [[biological machine]]. It uses [[protein dynamics#Global flexibility: multiple domains|protein domain dynamics]] on [[Nanoscopic scale|nanoscale]]s]]

A '''protein complex''' or '''multiprotein complex''' is a group of two or more associated [[polypeptide chain]]s. Protein complexes are distinct from [[multidomain enzymes]], in which multiple [[active site|catalytic domains]] are found in a single polypeptide chain.<ref>{{Cite book  |vauthors=Price NC, Stevens L | title = Fundamentals of Enzymology: The Cell and Molecular Biology of Catalytic Proteins | date = 1999 | publisher = Oxford University Press | location = Oxford| isbn = 0-19-850229-X | edition = 3rd }}</ref>

Protein complexes are a form of [[protein quaternary structure|quaternary structure.]] [[Protein]]s in a protein complex are linked by [[non-covalent interactions|non-covalent]] [[protein–protein interaction]]s. These complexes are a cornerstone of many (if not most) biological processes. The cell is seen to be composed of modular supramolecular complexes, each of which performs an independent, discrete biological function.<ref name="pmid10591225">{{cite journal |vauthors=Hartwell LH, Hopfield JJ, Leibler S, Murray AW | title = From molecular to modular cell biology | journal = Nature | volume = 402 | issue = 6761 Suppl | pages = C47–52 |date=December 1999 | pmid = 10591225 | doi = 10.1038/35011540 | doi-access = free }}</ref>

Through proximity, the speed and selectivity of binding interactions between [[Enzyme|enzymatic]] complex and substrates can be vastly improved, leading to higher cellular efficiency. Many of the techniques used to enter cells and isolate proteins are inherently disruptive to such large complexes, complicating the task of determining the components of a complex.

Examples of protein complexes include the [[proteasome]] for molecular degradation and most [[RNA polymerase]]s. In stable complexes, large hydrophobic interfaces between proteins typically bury surface areas larger than 2500 square [[ångström|Å]]s.<ref name="pmid16524839">{{cite journal |vauthors=Pereira-Leal JB, Levy ED, Teichmann SA | title = The origins and evolution of functional modules: lessons from protein complexes | journal = Philos. Trans. R. Soc. Lond. B Biol. Sci. | volume = 361 | issue = 1467 | pages = 507–17 |date=March 2006 | pmid = 16524839 | pmc = 1609335 | doi = 10.1098/rstb.2005.1807 }}</ref>