This lesson will give a quick review of enzymes and enzyme function. The lesson will then briefly discuss non-competitive inhibition. It will explain how enzyme activity can be graphed with a focus on how the graph changes with non competitive inhibition. The lesson will also give examples of non-competitive inhibition.
Enzymes And Enzyme Function
A piece of bread left alone outside will, after a while, degrade and become part of the soil. If you eat the piece of bread, it will be broken down faster than if left alone. In eating the bread, you are acting as an enzyme. An enzyme speeds up chemical reactions that take place in a cell. Enzymes take substrates, like bread, and turn them into products, like the soil.
The area that enzymes bind substrates to is called the active site. When you eat a piece of bread, you chew it up in your mouth. Your mouth is akin to the active site where the chemical changes are taking place.
Many cellular functions rely on enzymatic activity. Therefore, it is important to regulate enzyme function. An out-of-control enzyme might form too many products that can adversely affect cellular function. If you continuously make brownies you will be left with no ingredients in your house for other desserts, and will have too many brownies to eat alone.
One way the cell can regulate enzyme function is through inhibition. By preventing the enzyme from turning substrate into product, cells can prevent a situation where there are too many products.
There are three main types of cellular inhibition: competitive, uncompetitive, and non-competitive. Since this lesson focuses on non-competitive inhibition, only non-competitive inhibition will be discussed.
Non-competitive inhibition is where an inhibitor binds an area other than the active site and changes the active site so that it can’t bind substrates. Let’s say you, substrate, were trapped with a hungry lion, an enzyme. If a lion tamer jumps on the lion’s back and muzzles the lion to prevent it from eating you, the lion-tamer would be acting as a non-competitive inhibitor. The muzzle represents a change in the active site; the lion can’t open its mouth to eat you. The enzyme’s active site is altered so it can’t bind substrate anymore.
Graphing Enzyme Activity
Enzyme function can be measured by looking at the substrate concentration, or the amount of substrate available, and how fast it is being changed into product. A Michaelis-Menten Plot can be made by plotting substrate concentration on the x-axis verses rate of product formation on the y-axis.
For example, a fisherman uses his hands, the enzyme, to bait a hook and can use 1 piece of bait, the substrate, to catch 1 fish, the product, in 4 minutes. The rate of product formation is 1 fish every 4 minutes. If 10 fishermen are on the docks, the number of fish caught will depend on the amount of bait available. If there are only 3 pieces of bait and 10 fishermen, the rate is only 3 fish every 4 minutes. By increasing the substrate concentration, we can increase the amount of product. With 10 pieces of bait we can catch 10 fish every 4 minutes.
Since there are only 10 fishermen the max amount of fish in 4 minutes is 10. At this point the enzymes are saturated. That is, all binding sites are full, all hands have bait. The maximum rate of fish production, represented by letter V, then is 10 fish every 4 minutes. This is known as Vmax, or maximum velocity.
Now, suppose we start handcuffing the fishermen. The hands themselves are not changed, but now the position, or conformation, of the hands is different, making them unable to pick up the bait. This is non-competitive inhibition. If 2 of the 10 fishermen are handcuffed, the maximum rate of fish production changes from 10 fish every 4 minutes to 8 fish every 4 minutes, even if you add more bait. Non-competitive inhibition then, decreases, or lowers, the Vmax and is unaffected by an increase in substrate concentration.
Enzyme activity can also be graphed by putting 1 over both variables, the substrate concentration and rate of product formation. It’s like changing 2 into ;- the reciprocal of 2 is 1/2, so this type of graph is known as a double-reciprocal plot. It is also more commonly known as a Lineweaver Burk Plot.
In a Lineweaver Burk Plot, regular enzyme activity is graphed as a straight line. Non-competitive inhibitors move this straight line and alter the y-intercept of the line. The x-intercept remains the same.
Non-Competitive Inhibition Examples
Scientists use this property of changing Vmax to identify non-competitive inhibitors. There are many examples of non-competitive inhibitors that play important roles in cellular function. Heavy metals like silver, mercury and lead can act as non-competitive inhibitors. Lead, for example, can block the enzyme that puts iron into hemoglobin precursors. Antibiotics can also act as non-competitive inhibitors. Doxycycline, for example, blocks collagenase and is used to treat gum disease.
There are also many non-competitive inhibitor drugs with various functions. Palmatine and berberine, isoquinoline derivatives, are bacterial neuraminidase inhibitors and potential agents to treat food poisoning. DHEA (dehydroepiandrosterone), 3-ATA (aminothioacridinone) and SGI-1027, a quinoline derivative, are drugs that target various enzymes and are being investigated for usage as anti-cancer agents. MPEP (2-methyl-6(phenylethynyl)pyridine) has a prospective use as treatment in Parkinson’s disease. Non-competitive inhibitors are found in our bodies as well. Manufactured ACE, angiotensin-converting enzyme, inhibitors are competitive inhibitors used in the treatment of some cardiovascular diseases. A non-competitive form of an ACE inhibitor can be found in human serum.
Enzymes bind substrates at an active site and increase rate of product formation. Enzyme activity can be graphed as a Michaelis-Menten Plot by plotting substrate concentration verses product formation. It can also be graphed as a double-reciprocal, or a Lineweaver-Burk plot. Enzymes can be regulated through competitive, uncompetitive and non-competitive inhibition. Non-competitive inhibitors bind to a site other than the active site, changing the enzyme conformation and preventing product formation. Their activity is not affected by substrate concentration. Vmax decreases with non-competitive inhibition and the y-intercept is changed on the Lineweaver-Burk plot. There are many non-competitive inhibitors including heavy metals, antibiotics and manufactured and natural drugs. Non-competitive inhibitors can cause disease or be used to alleviate disease.