Proteins, classes contractile

Certain proteins endow cells with unique capabilities for movement. Cell division, muscle contraction, and cell motility represent some of the ways in which cells execute motion. The contractile and motile proteins underlying these motions share a common property they are filamentous or polymerize to form filaments. Examples include actin and myosin, the filamentous proteins forming the contractile systems of cells, and tubulin, the major component of microtubules (the filaments involved in the mitotic spindle of cell division as well as in flagella and cilia). Another class of proteins involved in movement includes dynein and kinesin, so-called motor proteins that drive the movement of vesicles, granules, and organelles along microtubules serving as established cytoskeletal tracks. ... 

A class of motile systems completely different from and unrelated to the actin-myosin contractile systems is used in cellular structures as diverse as the mitotic spindle, protozoan and sperm flagella, and nerve axons. These systems are constructed from microtubules, very long, tubular structures built from a helical wrapping of the protein tubulin (Figure 8.19). There are two kinds of tubulin subunits, oi and each of molecular weight 55,000. They are present in equimolar quantities in the microtubule, which can be considered a helical array of ot-/i dimers. Alternatively, we can view the microtubule as consisting of 13 rows, or protofilaments, of alternating ot and subunits. Because the oi and b units are asymmetrical proteins, with a defined and reproducible orientation in the fiber, the microtubule has a definite sense of direction. 

The families of proteins that consume ATP while functioning as protein-based machines are called ATPases. Muscle contraction, just noted above, is a member of the family of linear (contractile) protein motors that also includes ATPases that walk along protein tubules and transport elements from one part of the cell to another. Another class of protein motors that uses ATP rotary and nonrotary ion pumps transports ions from one side to the other of the... 

The consilient mechanism was bom out of controlling the hydrophobic association-dissociation of elastic-contractile model proteins to achieve the possibility of some 18 classes of pairwise energy conversions (see Chapter 5, section 5.6). In the process a set of five Axioms became the phenomenology out of which the consilient mechanism arose. For the first time a common groundwork of explanation was able to perform the diverse energy conversions of biology. 

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