A coenzyme and one of the most important compounds in the metabolism of all organisms, since it serves as a coupling agent between different enzymatic reactions. Adenosine triphosphate is adenosine diphosphate (ADP) with an additional phosphate group attached through a pyrophosphate linkage to the terminal phosphate group (see illustration). ATP is a powerful donor of
actinin a minor protein constituent of muscle, found to be concentrated in both the Z line and the I band. Two components of actinin have been identified: a-actinin, F-actin cross-linking protein, a dimer of ∼200 kDa with an action similar to that of actinogelin.; and b-actinin, a dimer of ∼70 kDa, similar in action to