Enzymes differ from other catalysts in the exceptional substrate selectivity they exhibit.  However, the active sites of related enzymes are often very similar, even though different substrates are acted upon (for example in the superfamily of cytochrome P450s).  How does a given enzyme preferentially bind a particular substrate?  In a new paper appearing in the jounal Metallomics, Chemistry grad student Marina Dang and Profs. Susan Sondej Pochapsky and Thomas Pochapsky use nuclear magnetic resonance (NMR) to identify a helical structure remote from the active site of the enzyme cytochrome P450_cam that is responsive to changes in substrate.  They propose that this helix can adjust the position of residues that contact substrate in the enzyme active site, much like the spring that holds batteries in place against electrical contacts in a flashlight.