{{tag>structure}}
====== Loop =======

A protein loop is any stretch of non-regular polypeptide chain connecting
secondary structures.

__Short loops__ commonly have little structure and are often found joining two adjacent [[glossary:beta_strand | beta strands ]]. They are often referred to as reverse turns. 

__Long loops__ are often flexible and are able to adopt a number of different conformations, effectively making them invisible in x-ray crystal structures. These loops are likely to be involved in protein function and can switch from a 'open' conformation (frequently to allow access to an active site) to a 'closed' conformation (to protect reactive groups from bulk solvent).

===== References =====

{{pubmed>long:4010788}}
{{pubmed>long:8819975}}
{{pubmed>long:3775366}}