cofactors are ions or coenzymes necessary for enzyme activity. the fmn is located in a deep cleft (formed by the two polypeptide subunits), and held in place by extensive hydrogen-bond interactions with the protein. in this particular case, the fmn helps the enzyme to bind the substrates. in the absence of pyridoxal 5′-phosphate (plp), the active site of the enzyme is in an “open” conformation. once substrate binds and is converted to plp, the active site of the enzyme is in a partially “closed” conformation. specific amino acid residues can form hydrogen bonds with the plp, thus forming a lid that physically covers the active site, giving rise to the “closed” conformation.