Ultimeaciax

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All of the following must change the rate of an enzyme-catalyzed rxn except:

A. changing pH
B. lower the temp
C. decreasing the concentration of sub
D. adding a noncompetitive inhibitor

I don't get their explanation why B is correct.
 
Dec 6, 2010
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The answer is B because lowering temperature will not make the reaction go faster. Higher temperature increases rate of reaction because there is high energy in the reaction making higher collision and higher chance of binding of enzyme to substrates creating more reactions. Thus lowering temperature will decrease the rate of reaction. Now if you have too high of temperature, your enzymes may degrade.
 

NYR56

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The answer is actually "C", I think you misread the answer key OP. Also, breakerz, they say change the rate, not increase, so your explanation is correct but you just misread the question.
 

indygobu

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I'd have to agree with NYR56. At points high above saturation, a relatively small decrease in [substrate] leads to no change in velocity.
 

indygobu

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Here's a better, more visual, explanation.

The question is pretty much asking which of the following does not necessarily always cause a change in rate of reaction.

The reason why it is is because if you assume that is very large compared to Km (so you have an amount of substrate greater than the saturation point of the enzyme) Km + drops to just because it is masking the value of Km. So you end up with V = Vmax/ which simplifies to V = Vmax, and it is a constant, with no variable change.
 
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Ultimeaciax

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Lowering the temp will not make the rxn go faster, but it will "change" (decrease) the rate of the rxn. So, either lowering the temp or increasing it would cause a change in the rxn rate.

Thanks for the answer breakerz0. I'm still confused.
 
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Ultimeaciax

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The answer is actually "C", I think you misread the answer key OP. Also, breakerz, they say change the rate, not increase, so your explanation is correct but you just misread the question.
I actually checked it many times and reread the answer explanation. The answer key says B.
 

NYR56

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My version has it as C, which I know is correct, so I guess you have a different edition. Read indygobu's post for a good explanation, but trust me, it's C. :)
 
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Ultimeaciax

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That's weird. I have the latest edition, 7th. How'd they go from a right to a wrong answer with an explanation?
 

NYR56

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I have the 7th edition too. Are you sure you're looking at the right answer? Page 259, #10. Explanation starts with "Decreasing the temperature always decreases the rate of any reaction..."
 

DJ303

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I have the 7th edition too. Are you sure you're looking at the right answer? Page 259, #10. Explanation starts with "Decreasing the temperature always decreases the rate of any reaction..."
I checked my book and the answer choice is C. But even without looking Ph Temp and inhibitors will definitely affect the rate of an enzymatic reaction (enzymes have optimal Ph and temps they function best at). Like what other posters above have nicely put substrate concentration affecting kinetics depends on whether the enzyme is saturated or not and since this is an EXCEPT question, this is the best answer.