8.8 Exploring the biochemical and mechanical effects of intestinal  (Page 2/6)

Some of the mechanical components necessary for cellular contraction are actin thin filaments, myosin-II heavy filaments, intermediate filaments, and dense bodies and plaques [link] . Actin and myosin work in tandem in the mechanochemical transduction of force. Intermediate filaments and actin filaments function as cytoskeletal elements that provide structure to the cell as well as spread the force generated by actin and myosin around the entire cell membrane [link] , [link] . Intermediate filaments also function as connecting filaments that transmit force between dense bodies and dense plaques [link] , [link] .

Actin and myosin directly generate the force of contraction. When intracellular calcium concentrations rise, a globular protein protruding from the myosin-II heavy filament known as the myosin head becomes activated [link] , [link] . This head binds to an active site located along the length of the actin filament; multiple active sites exist along a single filament [link] . When ATP phosphorylates the myosin head, the structure undergoes a series of conformational changes that replicates a rowing motion, pulling the actin filament through the cytoplasm. This action is termed the powerstroke, which is estimated to move an actin filament 10 nm by exerting a force of 3-4 pN at the actin-myosin binding site [link] , [link] . Cycles of this powerstroke motion result in a continuous pattern of latching and releasing along these binding sites that pulls the actin through the cell. Actin, in turn, is attached to dense bodies or dense plaques, to which the force of contraction is applied [link] , [link] . Dense bodies (plaques) are nodal structures anchored in the cytoplasm (plasma membrane) and connected to contracting actin filaments [link] . When actin and myosin generate force, it is applied to these structures such that the SMC can undergo the conformational changes necessary for cellular contraction.

Actin and myosin powerstroking is regulated by a complex network of chemical reactions, as shown in Figure 1. Intracellular calcium ( $C{a}^{2+}$ ) is the primary chemical controlling the contraction-relaxation cycles [link] , [link] . Four $C{a}^{2+}$ ions bind to the cytoplasmic enzyme calmodulin, forming the calcium-calmodulin complex ( $C{a}_{4}C$ ). $C{a}_{4}C$ then binds to the enzyme myosin light-chain kinase ( $M_K$ ), activating its phosphorylation capabilities. This activated complex ( $C{a}_4{C}_{M_K}$ ) phosphorylates the regulatory myosin light-chain ( $ML{C}_{20}$ ) located at the base of the myosin head. Once activated, this head can bind with ATP to initiate the cross-bridge cycling necessary for contraction [link] , [link] . The myosin head's activation simultaneously is countered by the enzymatic activity of myosin light-chain phosphatase ( $M_L$ ) [link] . The degree to which the cell contracts depends on the relative quantities of $M_L$ and $C{a}_4{C}_{M_K}$ in the cell. When the cell relaxes from decreasing intracellular $\left[C{a}^{2+}\right]$ , $M_L$ deactivates $ML{C}_{20}$ at a greater rate than its antagonist pair can trigger activation, disabling the myosin head's ability to produce the powerstroke.

Phosphorylated myosin ( $M_p$ ) binds to an actin thin filament (A), forming the actomyosin complex ( $A_{M_p}$ ). $A_{M_p}$ catalyzes the breakdown of ATP into ADP and $P_i$ (inorganic phosphate), which leads to a conformational change in the myosin head. Once $P_i$ is released from $A_{M_p}$ , the myosin head pulls the actin filament through the cytoplasm in an action termed the powerstroke. This stroking motion is the moment of mechanochemical force generation, and the process continues cyclically during contraction.