Time resolved measurements show that phosphate release is the rate limiting step on myofibrillar ATPases.
The myofibril is a good model to study the ATPase of the muscle fibre. When myofibrillar ATPase reaction mixtures are quenched in acid, there is a burst of Pi formation, due to AM.ADP.Pi or Pi, as shown in the scheme: AM+ATP<-->A.M.ATP<-->AM.ADP.Pi<-->AM.ADP+Pi<-->AM+ADP. Therefore, in the steady state, either AM.ADP.Pi or AM.ADP or both predominate. To determine which, we studied the reaction using a Pi binding protein (from E. coli) labeled with a fluorophore such that it is specific and sensitive to free Pi [Brune, M. et al. (1994) Biochemistry 33, 8262-8271]. We show that the Pi bursts with myofibrillar ATPases (calcium-activated or not, or crosslinked) are due entirely to protein bound Pi. Thus, with myofibrillar ATPases the AM.ADP.Pi state predominates.
Lionne
C
C
INSERM U128, CNRS, Montpellier, France.
Brune
M
M
Webb
M R
MR
Travers
F
F
Barman
T
T
D000199:Actins / Q000378:metabolism; D000244:Adenosine Diphosphate / Q000378:metabolism; D000251:Adenosine Triphosphatases / Q000378:metabolism*; D000818:Animals; D002118:Calcium / Q000378:metabolism; D003432:Cross-Linking Reagents; D009126:Muscle Relaxation; D009210:Myofibrils / Q000201:enzymology / Q000378:metabolism*; D009218:Myosins / Q000378:metabolism; D010710:Phosphates / Q000378:metabolism*; D016658:Psoas Muscles; D011817:Rabbits; D013997:Time Factors
D000199:Actins; D003432:Cross-Linking Reagents; D010710:Phosphates; D000244:Adenosine Diphosphate; D000251:Adenosine Triphosphatases; D009218:Myosins; D002118:Calcium