J. Biol. Chem., Vol. 259, Issue 9, 5423-5429, May, 1984
Evidence from oxygen exchange studies that the two heads of myosin are functionally different
KK Shukla, HM Levy, F Ramirez and JF Marecek
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.
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