Identifying Type of Inhibition - Biochemistry

Competitive inhibitors bind to the active site of the target enzyme. Km is the substrate concentration at which the reaction rate is at half Vmax. A competitive inhibitor can be outcompeted by adding additional substrate; thus Vmax is unaffected, since it can be accomplished with enough additional substrate. However, since we need to add additional substrate to compete with the inhibitor to get the reaction to the same rate, our Km increases.

This is an example of feedback inhibition, as feedback inhibition is a mechanism in which a molecule binds to an enzyme to decrease its activity. The mechanism is now balanced. Blocking an enzyme typically helps correct a metabolic imbalanace or assists in destruction of a pathogen. In this case the ATP binds to a site other than the protein's active site, and since it blocks PFK-1, a feedback inhibition has occurred.

The graph shown in the question stem is a Lineweaver-Burk plot, otherwise known as a double-reciprocal plot. In this plot, is plotted along the -axis and is plotted along the -axis. Furthermore, the -intercept in such a graph is equal to , and the -intercept is equal to .

From the graph shown in the question stem, we can see that there are two lines, each with different slopes. Each line corresponds to a certain concentration of inhibitor. (Note that one of the lines corresponds to no inhibitor, or a concentration of 0.)

It is evident that the two lines intersect each other along the -axis, right on the value. Consequently, we can conclude that the inhibitor in this case is not having any effect on the value of for the enzyme. Despite the different slopes for the two reactions, both of them have a common -intercept but differing -intercepts. This means that we can conclude the inhibitor in question must be competitive, since the result will be a rise in the value for the enzyme-catalyzed reaction, but will have no effect on the value.

This scenario is a classic example of allosteric inhibition. When cyanide binds to a site on cytochrome c oxidase other than the active site, cytochrome C oxidase becomes deactivated, stopping oxidative phosphorylation and causing cells to die since they cannot produce ATP anymore.

In competitive inhibition, the inhibitor binds the active site of the enzyme, competing with the substrate for this binding site. The of a competitively inhibited enzyme remains unchanged, but the increases. This means that a higher concentration of substrate is required to bring the reaction rate to . However, since this is competitive inhibition, and the maximum velocity is unchanged, we can overcome this increase in and achieve maximum velocity if we saturate the enzyme with substrate.

All types of inhibitors will induce a change in the of an enzyme except for competitive inhibitors. This is because competitive inhibitors have no effect on the enzyme-subtrate complex. The may still be reached, but by adding more substrate, since the is increased by a competitive inhibitor.

Competitive inhibition involves the substrate's access to the active site. In the case of competitive inhibition, the inhibitor blocks the substrate from the active site. As a result, the is unchanged, but the is increased. Recall that is the substrate concentration at which the reaction rate is . Additionally, the reaction rate will increase with increased concentration of competitive inhibitor and substrate, because they are competing for the active site, causing an increase in reaction rate.

Uncompetitive inhibitors can only bind the ES complex, whereas competitive and non-competitive inhibitors do not require the enzyme to be complexed with the substrate. , which describes the maximum reaction velocity of the enzyme, is decreased because the inhibitor slows the dissociation of the substrate from the enzyme, thereby slowing the rate at which the enzyme can interact with other substrate molecules.

This question is presenting us with a situation in which a chemical is being added to a mixture of enzyme and substrate, and its effects on the kinetic parameters of the reaction are observed. We're told that the and for this reaction both become reduced. We then are asked to identify which term best describes the added chemical.

To begin with, let's take note that all of the answer choices are some kind of inhibitor. Thus, we know that the chemical we're adding to the mixture is an inhibitor of some type. The challenge is in identifying which type of inhibition is happening. For this question, we'll need to have familiarity with each type of inhibition in order to identify the correct answer.

Let's start with what we know. Both the and the are being decreased. Right away, we can rule out competitive inhibition because the should remain the same.

We should also be able to rule out feedback inhibition right off the bat, as this kind of inhibition involves the products of a reaction putting a halt on the reaction that led to its production.

Next, we can also realize that two of the answer choices are so similar that they are actually saying nearly the same thing. Mixed inhibition is a case in which the inhibitor binds to the enzyme regardless of whether substrate is also bound to the enzyme. However, with mixed inhibition, the inhibitor shows greater affinity for either the free enzyme or the enzyme-substrate complex. In such a case, the for the reaction is expected to fall, but the can either increase or decrease.

Noncompetitive inhibition is a special type of mixed inhibition, in which the inhibitor binds both the free enzyme and the enzyme-substrate complex with equal affinity. In such a situation, the of the reaction will fall, but the will remain unchanged.

And finally, we look at uncompetitive inhibition, which is the correct answer. In this type of inhibition, the inhibitor binds only to the enzyme-substrate complex rather than the free enzyme. It does so by binding to an allosteric site, which is distinct from the active site to which substrate binds. Thus, there is no way to out-compete the inhibitor by adding more and more substrate, as can be done in competitive inhibition. The end result of this is that the becomes irrecoverably lowered. And since this value becomes less, the substrate concentration needed to obtain half of that reduced value (the ) also becomes decreased.

In feedback inhibition, the substances at the end of a reaction (in this case, CTP, the product) inhibit a previous reaction (in this case, the CTPase reaction). This tells the CTPase that a substantial amount of CTP is present, and to stop engaging in the reaction. The opposite, when a metabolic product facilitates further synthesis of that product, is known as positive feedback. Zymogens are inactive enzyme precursors - examples are pepsinogen and angiotensinogen. Cooperativity refers to the changes in binding affinity of an enzyme with multiple binding sites to its ligands. For example, hemoglobin has four oxygen binding domains; when one oxygen is bound, it facilitates the binding of the second, third, and fourth oxygens. Negative cooperativity is the opposite of this.

Enzymes bind to and stabilize transition states. So a molecule that resembles the transition state of a reaction will be able to bind to the enzyme for that reaction very readily and compete with the binding of the actual transition state. Therefore transition state analogs are competitive inhibitors.

The molecule in the question is classified as an enzyme inhibitor because it inhibits an enzymatic reaction. There are two types of inhibitors; competitive and noncompetitive inhibitors. Competitive inhibitors bind to the active site of the enzyme and prevent substrate from binding. They can be, however, dissociated with the addition of more substrates. This occurs because the substrates can dissociate the reversible bonds between inhibitor and enzyme and bind to active sites. Competitive inhibitors increase (or decrease the affinity of enzyme and substrate) but leave the unaltered. According to the information given in the question, we can conclude that the molecule is a competitive inhibitor.

Noncompetitive inhibitors bind irreversibly to an allosteric site of the enzyme and prevent substrate from binding to the active site. These types of inhibitors decrease the maximum reaction rate but leave the unaltered.

Competitive inhibitors bind to the active site of an enzyme via weak, intermolecular bonds (such as hydrogen bonds and hydrophobic interactions) that can be easily broken. This means that increasing substrate concentration will cause the weak bonds between competitive inhibitor and enzyme to break and, subsequently, open up the active site for the substrates. This is why competitive inhibition can be overcome by adding more substrates.

Noncompetitive inhibitors, on the other hand, bind to an allosteric site via strong covalent bonds. Once bound, noncompetitive inhibitors alter the shape of the active site, thereby making it harder for substrates to bind to enzyme. Increasing substrate concentration will not dissociate the strong, irreversible bonds between noncompetitive inhibitor and enzyme allosteric site.

Uncompetitive inhibition refers to an inhibitor that binds to the enzyme-substrate complex. This limits the amount of enzyme-substrate complexes that can be made into products, and so is decreased. It also decreases Km because the apparent affinity is increased due to the inability of the enzyme-substrate complexes to become unbound.

In this question, we're shown a line-weaver burk plot. One of the slopes represents the kinetic profile of an enzyme without inhibitor, while the other slope depicts an enzyme with inhibitor.

When looking at the graph, we notice that the two lines are parallel to one another. What this means is that the y-intercept is changing just as much as the x-intercept. This is a very valuable clue, because this lets us know that the of the reaction is decreasing just as much as that reaction's .

Since we know that both of these values are decreasing, we need to determine which type of inhibition has this characteristic. In competitive inhibition, the increases and the remains unchanged. Thus, this graph cannot be competitive inhibition.

Furthermore, this also cannot be noncompetitive inhibition. And, by extension, this cannot be mixed inhibition, which is just a special case of noncompetitive inhibition. In both of these forms of inhibition, the does indeed decrease. However, the value can either increase, decrease, or stay the same (in mixed inhibition). The value will not change by an amount equal to the change in .

The only other option left is uncompetitive inhibition. Indeed, in this type of inhibition, and are both decreased by the same degree.

A competitive inhibitor acts the increase the of a reaction, but does not alter the . This does not describe the inhibition on Enzyme A or Enzyme B. A noncompetitive inhibitor decreases the , but does not change the . This is the inhibition that is described on Enzyme A. An uncompetitive inhibitor decreases both the and the . This is the inhibition that is described on Enzyme B.

In this question, we're presented with a Lineweaver-Burk plot. In the plot, we are shown the parameters of a given enzyme both in the presence and in the absence of an inhibitor. We're asked to identify a true statement.

To begin with, we'll need to understand a few important points about enzyme inhibition. First, it's important to break inhibition up into its different types. Under the category of reversible inhibition, the inhibitor can bind in certain ways to the enzyme, and this will have an effect on the lineweaver-burk plot.

In competitive inhibition, the inhibitor binds only to the enzyme's active site. As a result, the substrate is unable to bind. In this scenario, the for the reaction will not change, but the will increase. In the plot shown in the question stem, this is not the case.

In uncompetitive inhibition, the inhibitor binds to an allosteric site on the enzyme only after substrate has bound. In other words, once the substrate has attached to the enzyme's active site, then the inhibitor will bind. Because the inhibitor can only bind to the enzyme-substrate complex, both the and of the reaction will decrease proportionately. In such a case, the plot will show two lines that are parallel to one another. This is not the case in the plot given to us in the question stem.

In mixed inhibition, the inhibitor is capable of binding to both the enzyme's active site as well as to the enzyme's allosteric site. Because of this, the of the reaction will always decrease, but the of the reaction can either decrease or increase, depending on whether the inhibitor has more affinity for one site over another. Based on the graph, we can see that this is not the case.

Finally, there is a special case of mixed inhibition called noncompetitive inhibition. In this case, the inhibitor binds to both the allosteric site and the active site with equal affinity. Because of this, the of the reaction will decrease, but the of the reaction will remain unchanged. As we can see in the plot shown to us in the question stem, this is the case because both lines intersect on the x-axis, meaning that they have the same value.

In this question, we're given a graph of initial reaction velocity as a function of substrate concentration. In addition, we're shown the course of the reaction both in the absence and in the presence of an inhibitor. We're asked to determine the type of inhibition that is occurring.

The most important thing to notice in this graph is that the maximal velocity for both is the same. In other words, adding the inhibitor has no effect on the maximum possible reaction rate. However, the maximal reaction rate will occur only at a higher substrate concentration when in the presence of inhibitor. Thus, we can classify this as competitive inhibition.

The correct answer is "pure noncompetitive inhibition." Noncompetitive inhibition, or mixed inhibition, is when the inhibitor binds to both the free enzyme and the enzyme-substrate complex, but may not bind equally to both. Competitive inhibitors bind to the free enzyme only at the enzyme’s substrate binding site, thus “competing” with the substrate for the binding site. Uncompetitive inhibitors do not bind the free enzyme but only to the enzyme-substrate complex.