QUATERNARY STRUCTURES AND COMPLEX ENZYMES

P50

Microtubules (tubulin) dynein (+ to -) kinesin (- to +) Microfilaments (actin) myosin DNA helicases

Each step depends on flexibility (“rotation”) Each step hydrolyzes one ATP (--> ADP + Pi) Each step involves an exchange reaction Dyneins are often used by cells to generate large forces during, e.g., nuclear migration, mitotic spindle orientation, reorientation of the cytoskeleton during wound healing…. Dyneins are weak and erratic by themselves but very effective in groups. Kinesins are strong individually, but not stronger in groups. Cell, 152, 172 (2013), PNAS 109, 18447 (2012). (There is another motor protein, dynein, which moves along microtubules. Its 4-A crystal structure was recently reported (Science 331:1159, 3/4/11), and a theory for how it moves along microtubules has been advanced (Science 335:221, 1/13/12.) )

Conclusion: Flexibility in protein structures allows more complex functions Reversible O2 and CO2 binding Reversible protein-protein (e.g., kinesin-microtubule) binding Shows the importance of low G in protein shape changes