Editing Protein structure (section)

==Domains, motifs, and folds in protein structure==
[[File:Domain Homology.png|thumb|right|Protein domains. The two shown protein structures share a common domain (maroon), the [[Pleckstrin homology domain|PH domain]], which is involved in [[phosphatidylinositol (3,4,5)-trisphosphate]] binding]]
Proteins are frequently described as consisting of several structural units. These units include domains, [[structural motif#In proteins|motifs]], and folds. Despite the fact that there are about 100,000 different proteins expressed in [[eukaryotic]] systems, there are many fewer different domains, structural motifs and folds.

===Structural domain===
A [[structural domain]] is an element of the protein's overall structure that is self-stabilizing and often [[protein folding|folds]] independently of the rest of the protein chain. Many domains are not unique to the protein products of one [[gene]] or one [[gene family]] but instead appear in a variety of proteins. Domains often are named and singled out because they figure prominently in the biological function of the protein they belong to; for example, the "[[calcium]]-binding domain of [[calmodulin]]". Because they are independently stable, domains can be "swapped" by [[genetic engineering]] between one protein and another to make [[chimera (protein)|chimera]] proteins. A conservative combination of several domains that occur in different proteins, such as [[protein tyrosine phosphatase]] domain and [[C2 domain]] pair, was called "a superdomain" that may evolve as a single unit.<ref>{{cite journal | vauthors = Haynie DT, Xue B | title = Superdomains in the protein structure hierarchy: The case of PTP-C2 | journal = Protein Science | volume = 24 | issue = 5 | pages = 874–882 | date = May 2015 | pmid = 25694109 | pmc = 4420535 | doi = 10.1002/pro.2664 }}</ref>

===Structural and sequence motifs===
The [[structural motif|structural]] and [[sequence motif]]s refer to short segments of protein three-dimensional structure or amino acid sequence that were found in a large number of different proteins

===Supersecondary structure===
Tertiary protein structures can have multiple secondary elements on the same polypeptide chain. The [[supersecondary structure]] refers to a specific combination of [[secondary structure]] elements, such as β-α-β units or a [[helix-turn-helix]] motif. Some of them may be also referred to as structural motifs.

===Protein fold===
A protein fold refers to the general protein architecture, like a [[helix bundle]], [[beta barrel|β-barrel]], [[Rossmann fold]] or different "folds" provided in the [[Structural Classification of Proteins database]].<ref name="Govinda rajan">{{cite journal | vauthors = Govindarajan S, Recabarren R, Goldstein RA | title = Estimating the total number of protein folds | journal = Proteins | volume = 35 | issue = 4 | pages = 408–414 | date = June 1999 | pmid = 10382668 | doi = 10.1002/(SICI)1097-0134(19990601)35:4<408::AID-PROT4>3.0.CO;2-A | url = http://www3.interscience.wiley.com/journal/65000323/abstract | url-status = dead | hdl-access = free | hdl = 2027.42/34969 | s2cid = 7147867 | archive-url = https://archive.today/20130105075413/http://www3.interscience.wiley.com/journal/65000323/abstract | archive-date = 5 January 2013 }}</ref> A related concept is [[protein topology]].