Cardiac myosin isoforms from different species have unique enzymatic and mechanical properties.

The mammalian heart contains two cardiac myosin isoforms: beta-myosin heavy chain (MHC) is found predominantly in the ventricles of large mammals, and alpha-MHC is expressed in the atria. The sequence identity between these isoforms is approximately 93%, with nonidentical residues clustered in discrete, functionally important domains associated with actin binding and ATPase activity. It is well-established that rabbit alpha-cardiac myosin has a 2-fold greater unloaded shortening velocity than beta-cardiac myosin but a 2-fold lower average isometric force.