The Partial Structural Formula of the α-Chain of Human Hemoglobin

ured by incorporation of 0'8 from HsOia into Pi.These data demonstrate that all the exchange occurring during ATP hydrolysis can be accounted for by the exchange of Pi oxygens.Actomyosin in the presence of ATP or PP i, and myosin in the presence of ATP, are known to undergo structural modification (see for example (8) and ( 9)).Our findings show that such structural changes are accompanied by and might be directly correlated with enhanced exchange of P,i oxygens.Whether this occurs by interaction of the Pi from the medium at a site occupied by the Pi released from cleavage of ATP or at an entirely independent site is not known.The exchange could reflect reversible formation of a phosphoryl derivative.Particularly intriguing in this regard is the binding of Pi in presence of ATP recently studied by Gergely and Maruyama (10).As noted by Levy and Koshland(2), complete reversal of the hydrolysis of ATP does not occur.Under conditions as used herein with 40 mM Pi% and 50 mM adenosine diphosphate present, no detectable P3* (less than 0.5 '%) appeared in unhydrolyzed ATP after about 45 '% hydrolysis.In summary, skeletal muscle actomyosin and myosin in the presence of ATP catalyze an exchange of oxygens of Pi of the medium with water oxygens.This exchange accounts for the phosphate oxygen exchange observed during ATP hydrolysis.