Lineweaver Burk plots

[Dochopeful13]

I am confused about the competitive inhibition plots. Can someone explain why the slope increased? I found the below answer but it still doesn't make sense. Why would km going up increase the y intercept? If this is the case then if Km goes down wouldn't Y go up, and the slope go down? This is not what happens in non competitive plots for example. Sorry for the long post but this topic is really confusing me.

If your enzyme of choice is busy interacting with the competitive inhibitor instead of with its normal substrate, then the normal substrate changes- the APPARENT Km increases. This means that the enzyme has a LOWER affinity for it. How does this affect our Lineweaver-Burk plot? If the Km goes UP, then the denominator of the y-intercept (-1/Km) increases. This means that the x-intercept has a smaller absolute value,and shifts to the RIGHT, closer to zero. However, your Vmax stays the same(even though it may be harder to reach with all that competitive inhibitor floating around). So the net result is a steeper slope of your line.

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[NextStepTutor_1]

Hi @Dochopeful13 -

Great question -- inhibition plots are a major topic of confusion. The first thing to make sure you understand about them—and all other graphs, for that matter—is which variables are on which axis. This is easy to skim over, but making a point to always notice this can help save you time and energy later. On a Lineweaver-Burk plot, the x-axis is 1/[substrate] and the y-axis is 1/v. Since Km is measured in units of [substrate], this means that the x-axis will tell us about Km. In fact, the x-intercept is -1/Km, and the y-intercept is 1/Vmax. The explanation that you cited seems to state otherwise, but you can check in any number of sources (even Wikipedia is not a bad first start) that the y-intercept is indeed 1/Vmax. (I googled that explanation, and earlier on the page they provide the correct info about the y-intercept, so it looks like there was just a typo in that explanation).

This sets us up to answer one of your questions: changing Km doesn't affect the y-intercept b/c the y-intercept is 1/Vmax. In terms of the slope, what happens is that increasing Km will push the x-intercept closer to zero while keeping the y-intercept (Vmax) the same, which will make the slope steeper.

Hope this clarifies things & best of luck!