Unwinding reaction mechanisms for DNA helicases. (a) A monomeric helicase hydrolyzes ATP, causing conformational changes that couple unidirectional DNA translocation with destabilization of the duplex. Some of these enzymes have pinlike structures that function in strand separation. (b) A functional dimer model. A dimeric helicase has two identical subunits. These may alternate in binding to single- and double-stranded DNA or may translocate linearly along one strand of DNA (as shown here). In either case, the protein unwinds the DNA as it moves. (c) Hexameric helicases move along one DNA strand of a duplex DNA, separating the strands as it moves. The bound DNA strand is usually bound alternately by three pairs of dimers within the hexameric structure. The passing of the strand from one dimer to another is coupled to conformational changes driven by ATP hydrolysis. In the diagram, the red, green, and yellow subunit pairs represent the three states: one state has bound ATP, the second has bound ADP (immediately after hydrolysis), and the third has no bound nucleotide.