Membrane-bound Enzyme Activation

[lazypremed]

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I'm new here and am taking the thing about copyright infringement seriousIy, so sorry for the vagueness. Please let me know if a post like this is not acceptable under the copyright rules.

Anyhow, I recently encountered a practice question asking me to select evidence that refuted a specific hypothesis: that a membrane-bound enzyme is activated by a specific substrate. The hypothesis did not claim that this substrate was the only activator - just that it was AN activator.
The correct answer involved the evidence that the enzyme can be activated in the absence of said substrate. I did not choose this because I figured that the enzyme could simply be activated by a different substrate in addition to the one specified in the hypothesis.
The answer I chose was the evidence that the enzyme is only found in its activated state, thinking that therefore its activation would be independent of the hypothesized substrate. The explanation of the correct answer, however, states that it is possible that the unactivated form might be difficult to extract or that the activated form might simply predominate. Fine, but I still have an issue with the supposedly correct answer!
The only way that answer makes sense to me is if most membrane-bound enzymes have only one activator. Is this the case? Am I wrong in thinking it's quite possible for a membrane-bound enzyme to be activated by two entirely different substrates? Or am I missing something else?

Thanks in advance!

 

[jetsfan1234]

Hard to answer this without seeing the exact question...but here's my take on it

First, think about the function of a surface receptor. The whole point is to allow the cell to respond to the outside environment -- the receptor has an active site which binds some hormone in the extracellular environment, which triggers a specific chain of events. would it make sense for such a receptor to be "on" all the time? whats the point of having the surface receptor then? i think your answer is unlikely based on this logic.

Second -- yes, receptors are typically specific for ONE substrate, or maybe one very small group of similar substrates. If this GPCR is activated in the absence of the substrate in question, it probably means the substrate is not an activator.

 

[lazypremed]

Hard to answer this without seeing the exact question...but here's my take on it

First, think about the function of a surface receptor. The whole point is to allow the cell to respond to the outside environment -- the receptor has an active site which binds some hormone in the extracellular environment, which triggers a specific chain of events. would it make sense for such a receptor to be "on" all the time? whats the point of having the surface receptor then? i think your answer is unlikely based on this logic.

Second -- yes, receptors are typically specific for ONE substrate, or maybe one very small group of similar substrates. If this GPCR is activated in the absence of the substrate in question, it probably means the substrate is not an activator.

Thanks for the reply! I definitely understand your logic as to why my answer is incorrect. That was very helpful! To be a little more specific about the question (hoping I'm not breaking any rules), the enzyme in question is further downstream than a GPCR. In fact, the activating substrate would not be coming from outside the cell at all. Does the same thing still apply? While what you are saying rings true intuitively for GPCRs, I imagined it would be possible for a membrane-bound enzyme (not a GPCR) to have multiple different allosteric sites for different activators. Say, the activator in question was a G protein or something.

 

[deleted393595]

I'm new here and am taking the thing about copyright infringement seriousIy, so sorry for the vagueness. Please let me know if a post like this is not acceptable under the copyright rules.

Anyhow, I recently encountered a practice question asking me to select evidence that refuted a specific hypothesis: that a membrane-bound enzyme is activated by a specific substrate. The hypothesis did not claim that this substrate was the only activator - just that it was AN activator.
The correct answer involved the evidence that the enzyme can be activated in the absence of said substrate. I did not choose this because I figured that the enzyme could simply be activated by a different substrate in addition to the one specified in the hypothesis.
The answer I chose was the evidence that the enzyme is only found in its activated state, thinking that therefore its activation would be independent of the hypothesized substrate. The explanation of the correct answer, however, states that it is possible that the unactivated form might be difficult to extract or that the activated form might simply predominate. Fine, but I still have an issue with the supposedly correct answer!
The only way that answer makes sense to me is if most membrane-bound enzymes have only one activator. Is this the case? Am I wrong in thinking it's quite possible for a membrane-bound enzyme to be activated by two entirely different substrates? Or am I missing something else?

Thanks in advance!

Was this an AAMC question?

 

[lazypremed]

My friend sent it to me... not sure!