PS8KEY - Professor Hinck Bio 110 Fall problem set 1 A...

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Professor Hinck Bio 110, Fall problem set 1) A typical time course of polymerization of actin filaments from actin subunits is shown in the Figure below. A) Explain the properties of actin polymerization that account for each of the three phases of the polymerization curve? B) How would the curve change if you doubled the concentration of actin? Would the concentration of free actin at equilibrium be higher of lower than in the original experiment, or would it be the same in both? ANS . Phase A corresponds to a lag phase, during which actin monomers must assemble to form a nucleus for polymerization (thought to be a trimer of subunits). Formation of a nucleus (nucleation) is followed by rapid growth (phase B), as actin monomers are added to the ends of the growing filaments. At phase C, equilibrium is reached between the rate of addition of actin at the ends and its rate of release. Once equilibrium is reached, the concentration of free actin remains constant. B. If the starting concentration of actin were doubled, the lag phase would be shorter, the growth phase would be more rapid (steeper), and the mass of polymer at equilibrium would be twice as great. The concentration of free actin monomers at equilibrium— the critical concentration ( C c)—would be the same regardless of the initial actin concentration. 2) The growth rates at the plus and minus ends of actin filaments as a function of actin concentration are shown below in both a regular and expanded scale. These graphs present data similar to the data presented in lecture and in you book in Figure 16.14B. A) The data were gathered by measuring initial growth rates at each actin concentration. Similar data gathered for any Michaelis-Menten enzyme would generate a hyperbolic plot, instead of the linear plot shown here. Why does the growth rate of actin filaments continue to increase linearly with increasing actin
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Professor Hinck Bio 110, Fall problem set concentration, whereas an enzyme-catalyzed reaction reaches a plateau with increasing substrate concentration.? B) Figure B) above shows the filament growth rates at low actin concentration on an expanded scale. Imagine that you could add actin filaments to a solution of actin subunits at the concentration indicated by A, B, C, D and E. For each of these concentrations, decide whether the added actin filament would grow or shrink at its plus and minus ends. What is the critical concentration for the plus end? What is the critical concentration for the minus end? Would treadmilling occur at any of these concentrations? ANS: A. An enzyme-catalyzed reaction reaches a plateau when the enzyme becomes saturated with substrate. Beyond that point an increase in substrate concentration cannot increase the rate of the reaction, because the enzyme is already working at maximum capacity. In contrast, growth of an actin filament does not saturate. Each time a monomer is added to the filament, a new site for addition of the next monomer is created. Addition of new monomers occurs through productive collisions with the end of the filament. The number of
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