molybdenum cofactor (Moco); hereby (an oxide of) molybdenum is complexed with molybdopterin
<a href="//commons.wikimedia.org/wiki/User:Yikrazuul" title="User:Yikrazuul">Yikrazuul</a>
<span class="int-own-work" lang="en">Own work</span>;  <a class="external mw-magiclink-pmid" rel="nofollow" href="//www.ncbi.nlm.nih.gov/pubmed/19675644?dopt=Abstract">PMID 19675644</a>