Editing Protein structure (section)

{{short description|Three-dimensional arrangement of atoms in an amino acid-chain molecule}}
{{Use dmy dates|date=June 2019}}
{{More citations needed|date=May 2018}}
{{Protein structure}}
'''Protein structure''' is the [[molecular geometry|three-dimensional arrangement of atom]]s in an [[amino acid]]-chain [[molecule]]. [[Protein]]s are [[polymer]]s{{snd}} specifically [[polypeptide]]s{{snd}} formed from sequences of [[amino acid]]s, which are the [[monomer]]s of the polymer.  A single amino acid monomer may also be called a ''residue'', which indicates a repeating unit of a polymer. Proteins form by amino acids undergoing [[condensation reaction]]s, in which the amino acids lose one [[water molecule]] per [[chemical reaction|reaction]] in order to attach to one another with a [[peptide bond]]. By convention, a chain under 30 amino acids is often identified as a [[peptide]], rather than a protein.<ref name="Stoker2015">{{cite book| vauthors = Stoker HS |title=Organic and Biological Chemistry|url=https://books.google.com/books?id=HRCdBQAAQBAJ&pg=PA371|date=1 January 2015|publisher=Cengage Learning|isbn=978-1-305-68645-8|page=371}}</ref> To be able to perform their biological function, proteins fold into one or more specific spatial conformations driven by a number of [[non-covalent interaction]]s, such as [[hydrogen bonding]], [[ionic interaction]]s, [[Van der Waals forces]], and [[hydrophobic]] packing. To understand the functions of proteins at a molecular level, it is often necessary to determine their [[Protein tertiary structure|three-dimensional structure]]. This is the topic of the scientific field of [[structural biology]], which employs techniques such as [[X-ray crystallography]], [[protein NMR|NMR spectroscopy]], [[Cryogenic electron microscopy|cryo-electron microscopy (cryo-EM)]] and [[dual polarisation interferometry]], to determine the structure of proteins.

Protein structures range in size from tens to several thousand amino acids.<ref name="Brocchieri2005">{{cite journal | vauthors = Brocchieri L, Karlin S | title = Protein length in eukaryotic and prokaryotic proteomes | journal = Nucleic Acids Research | volume = 33 | issue = 10 | pages = 3390–3400 | date = 2005-06-10 | pmid = 15951512 | pmc = 1150220 | doi = 10.1093/nar/gki615 }}</ref> By physical size, proteins are classified as [[nanoparticle]]s, between 1–100&nbsp;nm. Very large [[protein complexes]] can be formed from [[protein subunit]]s. For example, many thousands of [[actin]] molecules assemble into a [[microfilament]].

A protein usually undergoes [[Reversible process (thermodynamics)|reversible]] [[Conformational change|structural changes]] in performing its biological function. The alternative structures of the same protein are referred to as different [[conformational isomerism|conformations]], and transitions between them are called [[conformational change]]s.