Uncompetitive Inhibition Lineweaver Burk Plot

Nosotros've already covered a little scrap about the basics of enzyme kinetics, so now allow's movement on to discuss an important application of enzyme kinetics in the body (and in medicine)...

Enzyme inhibition

Enzymes are not always "on" and working; occasionally they are working on overdrive. Like all things in life, the key to good for you operation is balance. Many drugs piece of work to either cake or enhance enzymatic function. In 1934, Hans Lineweaver and Dean Burk took a look at the Michaelis-Menten equation and rearranged information technology into a overnice graphical class that'due south easy and intuitive to translate.

Of import formulas for enzyme inhibition

They started with the Michaelis-Menten equation:

Remember that Km reflects the enzyme's dissociation constant. A high Km ways weak bounden (the enzyme likes to dissociate from its substrate), and a low Km means strong binding (it doesn't like to dissociate from its substrate, meaning that information technology has a strong affinity for the substrate).

And then, they rearranged the variables so that nosotros could graph their relationship in a line, using our familiar y = mx + b equation.

Looking at these variables, we can see that the slope of the reaction curve is equal to Km/Vmax, and the y-intercept is equal to 1/Vmax. Something that's harder to know just by looking at this equation is that the x-intercept is equal to -1/Km.

Types of enzyme inhibition

Ordinarily, when nosotros are talking about manipulating enzymatic office, we are talking about inhibiting enzymes, and nosotros are doing so in a reversible way. This ways that the enzyme is manipulated in such a style that it can return to its normal function. Within reversible inhibition, we have competitive and noncompetitive (allosteric) inhibition.

1. Competitive inhibition: the added molecule competes with the enzyme's normal substrate for access to the enzyme'due south binding site.

By physically occupying the active binding site , the molecule blocks the enzyme's normal interaction with its substrate, thereby slowing the overall reaction velocity . Essentially, you lot are temporarily "knocking out" enzymes that could exist doing work for you, but can't because they're busying binding to a molecule that doesn't produce your final product of interest. Once the competitive inhibitor dissociates from the enzyme'due south binding site, that enzyme is free to interact with either its regular substrate or with some other molecule of the inhibitor.

Usually, competitive inhibitors are designed to have a higher affinity for the molecule than for the normal substrate , meaning that it "likes" the inhibitor more. Remember, whether the enzyme and the inhibitor crash-land into each other in the offset place is completely a function of hazard. That'south why y'all want to use competitive inhibitors in high volumes and so that in that location is a greater chance that they will crash-land into your enzyme's binding site and block the normal substrate from bounden to this binding site.

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 ways that the enzyme has a LOWER affinity for information technology. How does this affect our Lineweaver-Burk plot? If the Km goes UP, and then the denominator of the y-intercept (-1/Km) increases. This means that thex -intercept has a smaller absolute value, and shifts to the Correct, closer to zero. However, your Vmax stays the same (even though it may be harder to achieve with all that competitive inhibitor floating around). And so the net result is a steeper slope of your line.

2. Not-competitive (allosteric) inhibition: the added molecule does not bind in the enzyme'due south agile site, and therefore is not competing straight for that site with the enzyme's natural substrate.

Instead, the allosteric inhibitor is binding to another site on the enzyme that, when bound, changes the entire enzyme's shape JUST enough that its active site is no longer the correct shape to bind to its natural substrate. Therefore, in that location is no change in your Km because there is no modify in binding affinity at the agile site. However, fewer active sites are available, and so your reaction velocity decreases. When Vmax decreases, the y-intercept (1/Vmax) increases . The effect is that the slope of your line increase s, only your ten-intercept does not alter – the line looks like it pivots upward .

How will this appear on the MCAT?

These types of graphs may be included as function of a passage to show the effects of enzyme inhibitors in an experiment. Or, a question might ask you which graph represents the consequence of a reversible competitive inhibitor on enzyme kinetics. More importantly, understanding these graphs volition help y'all intuitively understand and call up concepts cardinal to enzyme inhibition.