Ribbon drawing of domain 2 of the enzyme glycogen phosphorylase, emphasizing the two sides (yellow vs orange) of the central twisted, 6-strand, parallel beta sheet, the flanking layers of alpha-helices (green) forming a classic doubly-wound "Rossman fold", and the unusual addition of an additional helical layer on each side (brown).  Hand-drawn by Jane Richardson around 1981.
<a href="//commons.wikimedia.org/wiki/User:Dcrjsr" title="User:Dcrjsr">Dcrjsr</a>
CC BY 3.0
<span class="int-own-work" lang="en">Own work</span>