Cofactors and coenzymes. Reversible, irreversible, competitive, and noncompetitive inhibitors. Allosteric enzymes. Feedback inhibition.
The cells of your body are capable of making many different enzymes, and at first you might think: great, let’s crank all of those enzymes up and metabolize as fast as possible! As it turns out, though, you really don’t want to produce and activate all of those enzymes at the same time, or in the same cell.
Needs and conditions vary from cell to cell and change in individual cells over time. For instance, stomach cells need different enzymes than fat storage cells, skin cells, blood cells, or nerve cells. Also, a digestive cell works much harder to process and break down nutrients during the time that follows a meal as compared with many hours after a meal. As these cellular demands and conditions changes, so do the amounts and functionality of different enzymes.
Because enzymes guide and regulate the metabolism of a cell, they tend to be carefully controlled. In this article, we’ll take a look at factors that can affect or control enzyme activity. These include pH and temperature (discussed in the active site article), as well as:
- Regulatory molecules. Enzyme activity may be turned "up" or "down" by activator and inhibitor molecules that bind specifically to the enzyme.
- Cofactors. Many enzymes are only active when bound to non-protein helper molecules known as cofactors.
- Compartmentalization. Storing enzymes in specific compartments can keep them from doing damage or provide the right conditions for activity.
- Feedback inhibition. Key metabolic enzymes are often inhibited by the end product of the pathway they control (feedback inhibition).
In the rest of this article, we'll examine these factors one at a time, seeing how each can affect enzyme activity.
Enzymes can be regulated by other molecules that either increase or reduce their activity. Molecules that increase the activity of an enzyme are called activators, while molecules that decrease the activity of an enzyme are called inhibitors.
There are many kinds of molecules that block or promote enzyme function, and that affect enzyme function by different routes.
Competitive vs. noncompetitive
In many well-studied cases, an activator or inhibitor's binding is reversible, meaning that the molecule doesn't permanently attach to the enzyme. Some important types of drugs act as reversible inhibitors. For example, the drug tipranivir, which is used to treat HIV, is a reversible inhibitor. It blocks activity of a viral enzyme that helps the virus make more copies of itself.
Reversible inhibitors are divided into groups based on their binding behavior. We won't discuss all of the types here, but we will look at two important groups: competitive and noncompetitive inhibitors.
- An inhibitor may bind to an enzyme and block binding of the substrate, for example, by attaching to the active site. This is called competitive inhibition, because the inhibitor “competes” with the substrate for the enzyme. That is, only the inhibitor or the substrate can be bound at a given moment.
- In noncompetitive inhibition, the inhibitor doesn't block the substrate from binding to the active site. Instead, it attaches at another site and blocks the enzyme from doing its job. This inhibition is said to be "noncompetitive" because the inhibitor and substrate can both be bound at the same time.
Diagram illustrating competitive and noncompetitive inhibition. The competitive inhibitor binds to the active site and prevents the substrate from binding there. The noncompetitive inhibitor binds to a different site on the enzyme; it doesn't block substrate binding, but it causes other changes in the enzyme so that it can no longer catalyze the reaction efficiently.
Competitive and non-competitive inhibitors can be told apart by how they affect an enzyme's activity at different substrate concentrations.
- If an inhibitor is competitive, it will decrease reaction rate when there's not much substrate, but can be "out-competed" by lots of substrate. That is, the enzyme can still reach its maximum reaction rate given enough substrate. In that case, almost all the active sites of almost all the enzyme molecules will be occupied by the substrate rather than the inhibitor.
- If an inhibitor is noncompetitive, the enzyme-catalyzed reaction will never reach its normal maximum rate even with a lot of substrate. This is because the enzyme molecules with the noncompetitive inhibitor bound are "poisoned" and can't do their job, regardless of how much substrate is available.
On a graph of reaction velocity (y-axis) at different substrate concentrations (x-axis), you can tell these two types of inhibitors apart by the shape of the curves:
This plot shows rate of reaction versus substrate concentration for an enzyme in the absence of inhibitor, and for enzyme in the presence of competitive and noncompetitive inhibitors. Both competitive and noncompetitive inhibitors slow the rate of reaction, but competitive inhibitors can be overcome by high concentrations of substrate, whereas noncompetitive inhibitors cannot.
Not familiar with this type of graph? No worries! The basics of enzyme kinetics graphs article has a step-by-step walkthrough.
Allosteric regulation, broadly speaking, is just any form of regulation where the regulatory molecule (an activator or inhibitor) binds to an enzyme someplace other than the active site. The place where the regulator binds is called the allosteric site.
The left part of this diagram shows allosteric inhibition. The allosteric inhibitor binds to an enzyme at a site other than the active site. The shape of the active site is altered so that the enzyme can no longer bind to its substrate.
The right part of this diagram shows allosteric activation. The allosteric activator binds to an enzyme at a site other than the active site. The shape of the active site is changed, allowing substrate to bind at a higher affinity.
Pretty much all cases of noncompetitive inhibition (along with some unique cases of competitive inhibition) are forms of allosteric regulation.
However, some enzymes that are allosterically regulated have a set of unique properties that set them apart. These enzymes, which include some of our key metabolic regulators, are often given the name of allosteric enzymes. Allosteric enzymes typically have multiple active sites located on different protein subunits. When an allosteric inhibitor binds to an enzyme, all active sites on the protein subunits are changed slightly so that they work less well.
There are also allosteric activators. Some allosteric activators bind to locations on an enzyme other than the active site, causing an increase in the function of the active site. Also, in a process called cooperativity, the substrate itself can serve as an allosteric activator: when it binds to one active site, the activity of the other active sites goes up. This is considered allosteric regulation because the substrate affects active sites far from its binding site.
Cofactors and coenzymes
Many enzymes don’t work optimally, or even at all, unless bound to other non-protein helper molecules called cofactors. These may be attached temporarily to the enzyme through ionic or hydrogen bonds, or permanently through stronger covalent bonds.Common cofactors include inorganic ions such as iron and magnesium . For example, the enzyme that builds DNA molecules, DNA polymerase, requires magnesium ions to function.
Coenzymes are a subset of cofactors that are organic (carbon-based) molecules. The most common sources of coenzymes are dietary vitamins. Some vitamins are precursors to coenzymes and others act directly as coenzymes. For example, vitamin C is a coenzyme for several enzymes that take part in building the protein collagen, a key part of connective tissue.
Chemical structure of vitamin C, which acts as a coenzyme for several enzymes.
Enzymes are often compartmentalized (stored in a specific part of the cell where they do their job) -- for instance, in a particular organelle. Compartmentalization means that enzymes needed for specific processes can be kept in the places where they act, ensuring they can find their substrates readily, don't damage the cell, and have the right microenvironment to work well.
For instance, digestive enzymes of the lysosome work best at a pH around , which is found in the acidic interior of the lysosome (but not in the cytosol, which has a pH of about ). Lysosomal enzymes have low activity at the pH of the cytosol, which may serve as "insurance" for the cell: even if a lysosome bursts and spills its enzymes, the enzymes will not begin digesting the cell, because they will no longer have the right pH to function.
Feedback inhibition of metabolic pathways
In the process of feedback inhibition, the end product of a metabolic pathway acts on the key enzyme regulating entry to that pathway, keeping more of the end product from being produced.
This may seem odd – why would a molecule want to turn off its own pathway? But it’s actually a clever way for the cell to make just the right amount of the product. When there’s little of the product, the enzyme will not be inhibited, and the pathway will go full steam ahead to replenish the supply. When there’s lots of the product sitting around, it will block the enzyme, preventing the production of new product until the existing supply has been used up.
Diagram illustrating feedback inhibition. The end product of a multi-step metabolic pathway binds to an allosteric site on the enzyme that catalyzes the committed step of the pathway, reducing the enzyme's activity. This regulation helps slow the pathway down when levels of the end product are already high (when more is not needed).
Typically, feedback inhibition acts at the first committed step of the pathway, meaning the first step that’s effectively irreversible. However, feedback inhibition can sometimes hit multiple points along a pathway as well, particularly if the pathway has lots of branch points. The pathway steps regulated by feedback inhibition are often catalyzed by allosteric enzymes.
For example, the energy carrier molecule ATP is an allosteric inhibitor of some of the enzymes involved in cellular respiration, a process that makes ATP to power cellular reactions. When there is lots of ATP, this feedback inhibition keeps more ATP from being made. This is useful because ATP is an unstable molecule. If too much ATP were made, much of it might go to waste, spontaneously breaking back down into its components (ADP and P).
ADP, on the other hand, serves as a positive allosteric regulator (an allosteric activator) for some of the same enzymes that are inhibited by ATP. For instance, ADP may act by binding to an enzyme and changing its shape so that it becomes more active.
Thanks to this pattern of regulation, when ADP levels are high compared to ATP levels, cellular respiration enzymes become very active and will make more ATP through cellular respiration.
Want to join the conversation?
- Allosteric regulation confuses me a lot. I don't really get it even after I watched the video on Khan Academy (MCAT) . Can anyone explain it to me briefly?(10 votes)
- I'll try an analogy — let me know if this helps.
Imagine that an enzyme is like tiny sculpture made from a wire twisted into a very complicated, but somewhat loose structure.
The substrate is another much smaller sculpture that fits into a gap in the first sculpture — let's say it fits perfectly.
Now think of hanging a weight off another part of the sculpture — the whole structure shifts a bit under the strain and now the substrate sculpture doesn't fit! In this situation the weight would be analogous to an allosteric inhibitor.
You could also imagine a similar scenario, but with the substrate fitting poorly until you added a weight — in this case the weight would be analogous to an allosteric activator.(57 votes)
- whats the difference between non competitive inhibition and allosteric regulation(involving inhibitor)? .its all so confusing(3 votes)
- Allosteric regulation and noncompetitive inhibitor bind to site other than active site but allosteric regulation change the conformation of enzyme and making the reaction less effective while the noncompetitive inhibitor, like mention in the reading just poison the enzyme so reaction does not take place at all.(11 votes)
- What is an allosteric activator?(2 votes)
- A molecule that attaches to the enzyme at a site (not the active site), changing the configuration of the enzyme, which allows the substrate to attach to the active site easier.(13 votes)
- In school, we conducted an experiment where a small piece of paper dipped in a liver solution was dropped into a test tube filled with hydrogen peroxide. After a few seconds, the liver juice coated paper rose to the the top. Why did it act in that way?(3 votes)
- This is because the liver cells contain enzymes called catalase which speed up the breaking down of hydrogen peroxide into water and oxygen. When the reaction happens, oxygen is released and it pushes the piece of paper up to the surface along with it. This reaction happens faster or slower depending on the concentration of the liver juice you soaked the paper with.(7 votes)
- Life is a process regulated by enzymes. What might be the sources of these enzymes? If particular enzyme is not available in person's cells, what sequence of events might result to produce it?(2 votes)
- Enzymes are encoded as genes in the DNA — these genes are then transcribed to produce RNA and (for most enzymes§) then translated† to make a protein that has a catalytic activity (i.e. is an enzyme).
Typically enzymes found within a cell are encoded by the DNA of that cell. However, multicellular organisms are colonized by many different microbes (these may be prokaryotes or other eukaryotes such as fungi) — these microbes often supply enzyme activities that aid the organism. The digestive system of animals is one example of this. In fact, most multicellular organisms depend on their microbiota for survival!
The process of regulating gene expression is highly complex, but there is KhanAcademy material on this is several places — I recommend starting here:
§Note: Some enzymes are make of RNA and these RNAs do not get translated.
†Note: Many proteins undergo post-translational modifications that are essential for them to function.(5 votes)
- if a allosteric inhibitor casues distortion of the enzyme's shape so that it cannot function, is it non-competitive or competitive?
(i looked up many resources ,such as princeton review AP bio,Barrons, internet, but they all weren't clear)(3 votes)
- Being allosteric lets you know that the inhibitor binds somewhere other than the active site, where the substrate binds. The location that the allosteric inhibitor binds is called the allosteric site.
Because it isn't "competing" for the same binding site as the substrate, you can tell that it is non-competitive as the substrate will still be able to bond at the active site.(2 votes)
- what would happen if our bodies do not have inhibitors?(3 votes)
- Good question
Than uncontrolled cell divisions, uncontrolled sugar breakdowns, uncontrolled and unlimited phosphorylations (until the moment of using up all resources) and ultimately leading to energy depletion and death.
It may cause a ruckus in the body and high dysregulation which would end up fatal.(2 votes)
- Once the competitive inhibitor is on the enzyme, doesn't it stay there? So it can't be outmuscled by substrate molecules since substrate molecules themselves have to move, but the inhibitor can just sit there.(1 vote)
- Both the inhibitor and the substrate do not stick on the enzyme for ever. The higher its affinity is the longer it stays. The inhibitor can be replaced by a higher substrate concentration. You need more substrate to get the 0,5-Vmax --> Km (substarte) increases. Vmax itself stays the same, once the substrate concentration is high enough to suppress all inhibitors, Vmax is determined by the enzyme concentration alone.(4 votes)
- How do you determine whether an enzyme's activity is enhanced or inhibited by regulatory cell binding?(2 votes)
- You follow up with the next steps. If reactions proceeds - it was activation, otherwise it must be some kind of inhibitor. :D(2 votes)
- I have a question about the graph. If the substrate concentration is decreasing, why does the graph show that substrate concentration increases as enzyme-reaction increases?
Doesn't the enzyme reaction rate represent the amount of product being created from the substrate? So as enemy reactions increase, product concentration increase, but substrate concentration decrease?(2 votes)
- Good question
Yes, it is true. The more the substrate, the more the enzymatic activity. Why? because an enzyme cannot exhibit catalytic activity on the thin air. It needs something to act on.
As for product. Why do you think that a substrate has to decrease in order of product to increase?
It is true that if you convert let's say one molecule into another, you removed that first molecule and initial concentration of substrate is decreased, but you can keep adding substrate and rate of reaction will increase as well.(1 vote)