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Most proteins [[protein folding|fold]] into unique 3D structures. The shape into which a protein naturally folds is known as its [[native conformation]]. Although many proteins can fold unassisted, simply through the chemical properties of their amino acids, others require the aid of molecular [[Chaperone (protein)|chaperones]] to fold into their native states. Biochemists often refer to four distinct aspects of a protein's structure:
* ''[[Primary structure]]'': the [[peptide sequence|amino acid sequence]]. A protein is a [[polyamide]].
* ''[[Secondary structure]]'': regularly repeating local structures stabilized by [[hydrogen bond]]s. The most common examples are the [[alpha helix|α-helix]], [[beta sheet|β-sheet]] and [[turn (biochemistry)|turns]]. Because secondary structures are local, many regions of different secondary structure can be present in the same protein molecule.
* ''[[Tertiary structure]]'': the overall shape of a single protein molecule; the spatial relationship of the secondary structures to one another. Tertiary structure is generally stabilized by nonlocal interactions, most commonly the formation of a [[hydrophobic core]], but also through [[Salt bridge (protein)|salt bridges]], hydrogen bonds, [[disulfide bond]]s, and even [[posttranslational modification|post-translational modification]]s. The term "tertiary structure" is often used as synonymous with the term ''fold''. The tertiary structure is what controls the basic function of the protein.
* ''[[Quaternary structure]]'': the structure formed by several protein molecules (polypeptide chains), usually called ''[[protein subunit]]s'' in this context, which function as a single [[protein complex]].
* ''[[Protein quinary structure|Quinary structure]]'': the signatures of protein surface that organize the crowded cellular interior. Quinary structure is dependent on transient, yet essential, macromolecular interactions that occur inside living cells.

Translations:Protein/32/en - Revision history

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