Final answer:
Smooth muscle shortening velocity increases with the fraction of MLC phosphorylated, which enhances the rate of ATP hydrolysis and leads to faster muscle contraction.
Step-by-step explanation:
Smooth muscle shortening velocity increases with the fraction of MLC (myosin light chain) phosphorylated. When calcium ions bind to calmodulin, it activates myosin kinase, which phosphorylates the myosin heads. Phosphorylation of the myosin heads allows them to attach to actin-binding sites and pull on the thin filaments, leading to muscle contraction. The increased phosphorylation of MLC enhances the rate of ATP hydrolysis by myosin heads, resulting in faster muscle shortening velocity.