May 2, 2015
15
1
Status
Pre-Medical
I'm juggling three sources for my MCAT prep and ran into a discrepancy between the three on two topics dealing with enzyme inhibitors and Km Vmax.

1) EK says that for uncompetitive inhibitors Km and Vmax both decrease. Khan Academy says that Vmax decreases and Km stays the same. Who is correct?

2) BR and Khan only mention the "noncompetitive inhibitor category" and say that that Vmax decreases. EK agrees, BUT says that "noncompetitive inhibitors" are a subcategory of "Mixed inhibitors" and that a mixed inhibitor increases or decreases Km and decreases Vmax.

What do you think?
 

edgerock24

7+ Year Member
Jul 28, 2012
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Medical Student
I'm juggling three sources for my MCAT prep and ran into a discrepancy between the three on two topics dealing with enzyme inhibitors and Km Vmax.

1) EK says that for uncompetitive inhibitors Km and Vmax both decrease. Khan Academy says that Vmax decreases and Km stays the same. Who is correct?

2) BR and Khan only mention the "noncompetitive inhibitor category" and say that that Vmax decreases. EK agrees, BUT says that "noncompetitive inhibitors" are a subcategory of "Mixed inhibitors" and that a mixed inhibitor increases or decreases Km and decreases Vmax.

What do you think?
1) EK is correct.

2) Correct. Just know that Mixed Inhibitors always have Vmax's that decrease, but their Km's can either stay the same, increase, or decrease. This is because binding can occur at either the alloseteric site OR upon the E-S complex.
Thus, for non-competitive inhibitors binding occurs at the allosteric site, so Km stays the same and Vmax decreases.
 

Neuroplasticity

2+ Year Member
Jul 27, 2015
102
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Medical Student (Accepted)
1. I agree with EK too. Since an uncompetitive inhibitor binds to ES, you can think about it like le chatelier's principle in that the step E+S<->ES will more favor the products if you are taking ES away, which is like saying E has a higher affinity for S so Km is decreased.


2. A mixed inhibitor is an inhibitor that binds to E and ES. A noncompetitive inhibitor is just an inhibitor that binds to E and ES with the same affinity for both, so I'd say it is a type of mixed inhibitor. Thinking about it like le chatelier's principle again, a noncompetitive inhibitor 'takes the same amount' of E and ES away from the 'reaction', so neither the forward or reverse rates and affected, so Km stays the same

 
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