Kaplan FL#1 Bio #21

[Jelly Bones]

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Hi all, I am not convinced that the answer kaplan provide for this Q makes sense/is correct.

Which of the following graphs best describes the relationship between reaction velocity and temperature in an enzyme catalyzed reaction?

From what I know, enzyme activity will increase reaction rate until a certain point (optimal temperature) but anything beyond that will reduce activity due to denaturation.

According to kaplan, Velocity vs. T is linear so A is correct, but I have never seen a graph like that for reaction velocity vs. temp of enzyme-catalyzed reactions.

 

[RH8448]

I see what you mean. I think the point of this question is that once denaturation occurs, the kcat (catalytic efficiency) decreases very fast. It's like breaking a hammer - once the hammer is broken it cannot be used anymore. Once the enzyme has denatured, the kcat decreases fast resulting in a fast dive in the curve described. When the temperature is really low, this doesn't denature the enzyme, therefore, the initial slope accelerates until the optimal temperature is reached. Generally, this can be assumed in this question, however, all enzymes are different and may have a more narrow range of optimal temperatures.

 

[Jelly Bones]

I'm sorry maybe I missed it but I don't see how that supports answer A. Both A and D show a drop in enzyme activity which comes as the enzyme gradually denatures.

There is no mammalian enzyme that will work with maximal activity over a range of temperatures as depicted in A, they are biological molecules that are evolved for specific environments. I have never seen a graph of enzyme activity vs. T showing a plateau of max activity, it's always a peak (google search will show this, also all biochem textbooks I have ever seen, and even kaplan's book!).

Graph D shows the characteristic enzyme rate vs. T relationship: increase in rate and then rapid drop after denaturation with a peak at the optimal temperature. Graph D still isn't perfect because they also don't indicate whether the axes start at 0 or not and this isn't something we should assume.

 

[RH8448]

Like I said I see what you mean. First of all it is a Kaplan exam so it would be clearer on the real exam. But, I said that the take away information that you need to take from this question is that once the enzyme is denatured, the kcat decrease faster than it did when the temperature was increasing. In other words you will have a parabolic curve that is "swayed" to the right. You will notice this in a google search as you said and I double checked this too. Answer D indicates a perfectly parabolic curve which is not what we are looking for. I would say this shape of graph is more relevant than the plateau. Also, you have to consider that this graph has no coordinates so we need to focus on info that we know like the slope. I hope that makes sense?