Question

reactions that are important for the life of a cell rarely happen at all without a catalyst, but can be speeded up by as much as a factor of $10^{21}$ with an enzyme present. this is a million quadrillions - a number that is too large for our brains to comprehend. more typically, enzymes speed reactions by a factor of about 10 million and result in 500 reactions per second. this is still a rate that is almost impossible for the human mind to grasp. when you consider that some reactions occur almost continuously - 500 a second for every second you are alive - you are talking about a lot of enzyme - catalyzed reactions.
your genes encode the information required to make over 2700 different enzymes. consider what youve already learned about them:

1. they are each specific to one reaction;
2. they each contain an active site where reactants bind and where the bond - breaking and bond - making required for the reaction actually occur;
3. enzymes speed up reactions by bringing reactants together in an orientation that allows them to interact and then lowers the free energy present in the transition state;
4. although the enzyme is intimately involved in the reaction, it is the same molecule before the reaction starts and after it ends;
5. enzymes do not affect the amount of free energy in reactants or products.

based on information in your readings and logical predictions that you can make from that information, select all of the following statements that are correct.
if an amino acid in or near the active site changes, an enzymes ability to catalyze a reaction will likely change
enzymes get used up as they catalyze reactions, so they must be produced (dna→rna→proteins) continuously for reactions to continue.
enzymes make nonspontaneous (endergonic) reactions spontaneous (exergonic) by lowering the free energy of the products.
did you fully understand the concept covered in this section?

Explanation:

• First statement: The active site is crucial for enzyme - substrate interaction. A change in an amino acid in or near the active site can alter the shape of the active site. Since enzymes are specific to one reaction (as per the given information: "They are each specific to one reaction"), a change in the active - site structure (due to amino - acid change) is likely to change the enzyme's ability to catalyze the reaction.
• Second statement: The text clearly states "Although the enzyme is intimately involved in the reaction, it is the same molecule before the reaction starts and after it ends". So, enzymes are not used up during the reaction, and this statement is incorrect.
• Third statement: Enzymes lower the activation energy (by bringing reactants together in an orientation that allows them to interact and then lowering the free energy present in the transition state, as per the given information). They do not change the free energy of reactants or products (as per the given information: "Enzymes do not affect the amount of free energy in reactants or products"). A non - spontaneous (endergonic) reaction has a positive \(\Delta G\) (where \(\Delta G=G_{products}-G_{reactants}\)), and an enzyme cannot make it spontaneous (\(\Delta G < 0\)) because it does not change \(G_{products}\) or \(G_{reactants}\). This statement is incorrect.

Answer:

If an amino acid in or near the active site changes, an enzyme's ability to catalyze a reaction will likely change.