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Question 1

allosteric enzymes (3)

What it does+ has

Answer 1

• Enzymes that regulate the flux of biochemicals through metabolic pathways
• Has quaternary structure
• More than 1 active site

Question 2

Feedback inhibition (2)

Answer 2

• usually bear no structural resemblance to the substrate or the product of the enzyme they inhibit
• feedback inhibitors do not bind at the active site, but rather at a distinct regulatory site on the allosteric enzyme

Question 3

Allosteric enzymes display a — dependence of reaction velocity on substrate concentration in contrast to the —- curve seen with Michaelis–Menten enzymes.

Answer 3

• sigmoidal
• hyperbolic

Question 4

concerted model or the MWC model (5)

Answer 4

• allosteric enzymes have multiple active sites on different
  polypeptide chains.
• the enzyme can exist in two distinct conformations or states. One state, designated R for relaxed, is the active conformation, which catalyzes reactions. The other state, designated T for tense, is significantly less active.
• In the absence of substrate or signal molecules, R and T are in equilibrium, with T being the more stable state and thus the more common state
• all of the subunits or active sites of the enzyme must be in the same state; that is, all must be T or all must be R. No hybrids are allowed
• Finally, substrate (S) binds more readily to the R form of the enzyme than to the T form.

Question 5

explain the sigmoidal nature of allosteric kinetics

Answer 5

Consider a population of allosteric enzymes with each enzyme containing four active sites on four subunits. Because it is more stable, most of the enzymes will be in the T state. The binding of S to the T form is difficult; thus, there will be little activity at low substrate concentrations (FIGURE 7.10). However, if the substrate concentration is increased, eventually enough S will be present so that when a relaxed form of the enzyme spontaneously appears, S will bind to it. Because of the symmetry rule, if one S binds to the R form, all of the four potential active sites become trapped in the R form. Consequently, the next S to bind the enzyme will not have to unproductively collide with the many T forms, because R forms of the enzymes are accumulating owing to the binding of the initial S to the enzyme. The binding of substrate disrupts the equilibrium in favor of R. This behavior is called cooperativity and accounts for the sharp increase in of the velocity-versus-substrate concentration curve.

Question 6

threshold effect:

Answer 6

below a certain substrate concentration, there is little enzyme activity. However, after the threshold has been reached, enzyme activity increases rapidly

Question 7

The regulatory molecules alter the equilibrium between T and R forms. Explain:

Answer 7

A positive effector binds to the R form at a regulatory site,
distinct from the active site, and stabilizes this form, thus
increasing the concentration of R and making an R and S
interaction more likely. A negative effector binds to T and
stabilizes it; a negative effector increases the concentration of T and thereby decreases the likelihood of an R binding to an S.

Question 8

The positive effector —- the threshold concentration of substrate needed for activity, whereas the negative effector —- the threshold max concentration.

Answer 8

• lowers
• raises