ATP and Motility

At macroscopic and microscopic levels, ATP hydrolysis results in movements. The most familiar of these movements are those caused by muscle contraction. Muscle contraction is an example of the conversion of the phosphate bond (chemical) energy of ATP to mechanical energy. Vertebrate muscle is composed of two types of filaments, thick and thin. The protein myosin is the major component of the thick filaments, whereas actin and other proteins make up the thin filaments. The thick and the thin filaments are interdigitated. Muscle contraction is thought to take place by a sliding of the thin filaments relative to the thick filaments. Myosin has ATPase activity. The catalytic site in myosin is located on a part of the molecule (the head) that interacts with the actin filaments. ATP hydrolysis is thought to cause changes in the interactions of the myosin head with the actin filaments such that the head moves along the actin filament in one direction.

Muscle contraction is regulated by a Ca²⁺-binding protein in the thin filaments. Ca²⁺ is required for muscle contraction. During rest, the concentration of Ca²⁺ in muscle cells is kept low by the operation of two Ca²⁺-ATPases, one in the plasma membrane and the other in internalmembranes called the sarcoplasmic reticulum. The release of Ca²⁺ triggers muscle contraction, and its uptake into the lumen of the sarcoplasmic reticulum causes relaxation.