This motif is coined after the design of a Greek key which is shown in the diagram below. It is a beta hairpin that is folded over to make an antiparallel beta sheet. 

Grand Valley State University

Combinations of the secondary structure is a motif and is another way to classify the tertiary structure of proteins. 
Common motifs in proteins:
- Beta-alpha-beta – alpha helix connecting two parallel strands 
- Beta hairpin – antiparallel strands that are joined by reverse turns
- Alpha alpha – antiparallel alpha helices 
- Greek key – froms an antiparallel Beta sheet 



Protein Motif

Voet, D., Voet, J. G., & Pratt, C. W. (2013). Chapter 6: Proteins: Three-Dimensional Structure. In Fundamentals of biochemistry: Life at the molecular level (4th ed., pp. 127-175). Hoboken, NJ: Wiley. Retrieved January 05, 2021, from archive.org.

Fundamental of biochemistry: Life of the molecular level 4th edition: Chapter 6

This motif is the most common motif where an alpha helix connects two Beta sheet strands. Below is a cartoon image of a beta-alpha-beta motif. 

Beta-alpha-beta

A motif that contains antiparallel strands with reverse turns connecting them. 

Beta hairpin

This motif consists of two alpha helices that are directly connected to each other. 

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