Evidence from oxygen exchange studies that the two heads of myosin are functionally different

Recent studies of oxygen exchange have shown that there are two normal pathways for the hydrolysis of MgATP by myosin in the presence of actin, each producing Pi at the same rate. These two apparent pathways for actin-activated hydrolysis differ greatly in the extent of oxygen exchange they support. This is revealed by an analysis of the distribution of [18O]Pi species produced by the hydrolysis of [gamma-18O]ATP. We have extended these studies to certain abnormal substrates, using Mn2+ in place of Mg2+, and dATP or ITP in place of ATP. The results, together with past findings, lead to the proposal that the two heads of myosin are functionally different. One of these (Head 1) is able to reversibly cleave bound MgATP and thereby support oxygen exchange while it is free of actin; the other (Head 2) cannot cleave bound MgATP at its active site while free of actin. However, in the presence of actin, both Head 1 and Head 2 cleave bound MgATP, support some oxygen exchange, and produce Pi at the same rapid actin-activated rate. Apparently, MgATP is positioned differently on the two heads when they are free of actin, with Mg2+ and the 6-amino group of ATP playing an important role in the orientation. This proposed difference between the heads could serve to make Head 1 react first with the actin filament, followed by Head 2. Thus, in muscle, the two heads on a myosin cross-bridge would interact with an actin filament and exert their pull in a fixed sequence.