the regulatory component ( mmob ) of 
 soluble methane monooxygenase 
 ( smmo ) has a unique n - terminal tail not found in regulatory proteins 
 of other bacterial multicomponent monooxygenases . 
 this n - terminal 
 tail is indispensable for proper function , yet its solution structure 
 and role in catalysis remain elusive . here , by using double electron 
 electron 
 resonance ( deer ) spectroscopy , we show that the oxidation state of 
 the hydroxylase component , mmoh , modulates the conformation of the 
 n - terminal tail in the mmoh2mmob complex , which in turn facilitates 
 catalysis . 
 the results reveal that the n - terminal tail switches from 
 a relaxed , flexible conformational state to an ordered state upon 
 mmoh reduction from the diiron(iii ) to the diiron(ii ) state . 
 this 
 observation suggests that some of the crystallographically observed 
 allosteric effects that result in the connection of substrate ingress 
 cavities in the mmoh2mmob complex may not occur in solution 
 in the diiron(iii ) state . 
 thus , o2 may not have easy access 
 to the active site until after reduction of the diiron center . 
 the 
 observed conformational change is also consistent with a higher binding 
 affinity of mmob to mmoh in the diiron(ii ) state , which may allow 
 mmob to displace more readily the reductase component ( mmor ) from 
 mmoh following reduction .