Ribbon diagram of bovine pancreatic ribonuclease A enzyme (PDB file 7RSA), with beta strands as green arrows, alpha-helices as gold spirals, and the active-site histidine sidechains in blue.  From a kinemage display in Mage.
Dcrjsr
CC BY 3.0
<span class="int-own-work" lang="en">Own work</span>