Green Fluorescent Protein

GFP has 11 beta-strands in a barrel. All pairs except one are anti-parallel. (with an odd number it is not possible for all to be anti-parallel). The green color is due to the amino acids in the middle of the barrel, after cyclization and oxidation. That is one reason why it is so useful in many different species, it requires no extra factors. It does require some time to achieve oxidation.

Crystal structure of the Aequorea victoria green fluorescent protein.
Ormö M, Cubitt AB, Kallio K, Gross LA, Tsien RY, Remington SJ.
Science. 1996 Sep 6;273(5280):1392-5.

(PubMed)

pdb id (PDB)