Answers To Review Questions

1. E. All these statements about shape changes in proteins are true.

2. D. Note that this statement is equivalent to "kcat is the number of molecules of substrate converted to product per molecule of enzyme per unit time" but is not equivalent to the statement in answer A, which is false.

3. D. Because the subunits begin at low affinity but switch to the high-affinity form as oxygen binds, the curve for oxygen binding starts out shallow and then gets steeper. Answer E would produce exactly the opposite effect. Of the other answers: (A) is true, but irrelevant—myoglobin also contains heme prosthetic groups, but does not show cooperative behavior. (B) is false: both myoglobin and hemoglobin contain iron. It is chlorophyll, in plants, that uses magnesium. (C) is false: the oxygen affinities of all four subunits are similar and, in any case, a difference in affinity would not cause the binding curve to be S-shaped.

4. C. No catalyst, whether inorganic or biological, can alter the equilibrium constant of a reaction. (If one did, it would allow the generation of a perpetual motion machine.)

5. B. Concerning the other answers: (A) there are no enzymes that add H+, those groups that can be protonated (e.g., —NH2, histidine residues) accept H+ rapidly without the requirement for any catalyst. (C) Amino acids are not added to proteins once they have been synthesized on the ribosome, although processing of synthesized proteins can often involve the removal of one or more amino acids, as, for example, when the endoplasmic reticulum signal sequence is removed (page 223). (D) Those proteins that do bind oxygen, such as hemoglobin, do so spontaneously, without the need for a catalyst.

6. C. Both cofactors and prosthetic groups are nonprotein molecules that help a protein to perform its function, but cofactors are only loosely attached while prosthetic groups are attached tightly.

7. E. ATP is a substrate for phosphofructokinase, but it also acts at an independent regulatory site, reducing the activity of the enzyme.

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