Flashcards in Enzymes Deck (27):
Where do enzymes function?
Enzymes function inside cells (intracellular enzymes) and outside cells (extracellular enzymes)
What are enzymes?
-Enzymes are globular proteins which catalyse metabolic reactions
-They are soiled into a precise 3D shape with hydrophilic R groups on the outside of the molecule ensuring that they are soluble
-Lots of different amino acids, and active site and complex and all enzymes are globular because they can work (soluble)
What is the lock and key hypothesis?
1. An active site (lock) an enzyme
2. Two which the substrate (key) of the enzyme binds to
3. The shape of the active site allows the substrate to fit perfectly
4. The idea that the enzyme has a particular shape into which the substrate fits exactly is known as the lock and key hypothesis
-The substrate is held in place by temporary bonds which form between the substrate and some of the R groups of the enzyme's amino acids
-Forms the enzyme substrate complex
What are the features of an enzyme?
1. The enzyme is SPECIFIC for this substrate (but some are not very specific and some can process multiple substrates
2. Not used up in reaction
What is the induced fit hypothesis?
1. Enzyme molecules are more flexible that is suggested by a rigid lock and key
2. Same as lock and key but sometimes the substrate can change shape slightly as the substrate molecule enters the enzyme in order to ensure a perfect fit
3. This makes the catalysis even more efficient
How does an enzyme catalyse a reaction?
1. An enzyme may catalyse a reaction in which the substrate molecule is split into two or more molecules
2. It may catalyse the joining together of two molecules, as when making a dipeptide
-They increase the rate of reaction by lowering the activation energy of the reaction which they catalyse
-They do this by holding the substrate or substrates in such a way that their molecules can react more easily
-Reactions catalysed by enzymes will therefore take place rapidly at a much lower temperature than otherwise would
What are some examples of enzymes?
1. Lipase (hydrolase; hydrolysis)
2. Dehydrogenase (oxidoreductase; removal or electrons or hydrogen from a substrate)
3. Kinases (transferase; transfers functional groups)
How would you measure the rate of reaction?
1. Measure rate of the catalase-hydrogen peroxide reaction
-Measure the rate at which the starch disappears from the reaction mixture
2. Take samples from mixture at known times and a dying each small to some iodine in potassium iodide solution and starch will from a blue-black colour with this solution
3. Use colorimeter measure the intensity of the blue black colour obtained and use this as a measure of the amount of starch still remaining
4. If you do this over a period of time you can plot a curve of 'amount of starch remaining' against 'time'. You can then calculate the initial reaction rate (draw tangent at start?)
5. It is even easier to observe the course of this reaction if you mix starch, iodine in potassium iodide solution and amylase in a tube and take regular reading of the colour of the mixture in this one tube in a colorimeter
-However, this is not ideal because the iodine interest with the rate of the reaction and slows it down
How does the pH affect the rate of enzyme-catalysed reactions?
1. Most enzymes have an optimum pH, usually pH 7
2. If pH changes, and therefore the concentration of hydrogen ions (high in acid, low in alkali) changes
3. Hydrogen ions interact with the R groups of amino acids
4. This affects the ionic bonding between the groups which in turn affects the 3D arrangement of the enzyme molecule
5. The shape of the active site may change and therefore reduce the chances of the substrate molecule fitting into it
6. A pH which is very different from the optimum pH can cause denaturation of an enzyme
How does a buffer solution control the pH?
1. Buffer solutions each have a particular pH and maintain it even if the reaction taking place would otherwise cause pH to change
2. You add a measured volume of the buffer to your reacting mixture
How does temperature affect the rate of enzyme-catalysed reactions?
1. As temperature rises the the enzyme and substrate molecules move faster as they have more kinetic energy and so collisions:
-Happen more frequently causing substrate molecules to enter the active site more often
-They collide with more energy, and this makes it easier for bonds to be formed or broken so that the reaction can occur
What happens to the enzyme when the temperature is too much?
1. Above a certain temperature, the structure of the enzyme molecule vibrate so energetically that some of the bonds holdings he enzyme molecule in its precise shape begin to break (especially true of hydrogen bonds
2. The enzyme molecule begins to lose its shape and activity and so enzyme denatured (often irreversible)
-The temperature at which an enzyme catalyses a reaction at the maximum rate is called the optimum temperature
What is the effect of substrate concentration on the rate of enzyme-catalysed reactions?
1. As substrate concentration increase, the initial rate of reaction also increases, the more substrate molecules there are, the more often an enzyme-substrate complex is made
2. However if you continue to increase the substrate concentration whilst keeping the enzyme concentration constant, there comes a point where every enzyme active site is working continuously
3. The enzyme is working at its maximum possible rate, Vmax (V for velocity)
What is an enzyme inhibitor?
1. When another molecule binds to an enzyme's active site since it is very similar shape to the enzyme's substrate and so it inhibits the enzymes function
What is competitive inhibition?
1. If an inhibitor molecule binds only briefly to the site, there is competition between it and the substrate for the site
2. If there is much more of the substrate present than the inhibitor, substrate molecules can easily bind to the active site in the usual way, and so the enzymes function is unaffected
3. However if the concentration of the inhibitor RISES, or that of the substrate FALLS, it becomes less and less likely that the substrate will collide with a empty site
4. The enzyme's function is then inhibited
How can you reverse competitive inhibition?
Competitive inhibition is said to be reversible because it can be reversed by increasing the concentration of the substrate
What is an example of competitive inhibition?
-When a person has drunk ethylene glycol
1. Ethylene glycol is rapidly converted into body to oxalic acid, which can cause irreversible kidney damage
2. However the active site of the enzyme which converts ethylene glycol to oxalic acid will also accept ethanol
3. Therefore if the poisoned person is given a large dose of ethanol, the ethanol acts as a COMPETITIVE INHIBITOR, slowing down the action of the enzyme on ethylene glycol for long enough to allow the ethylene glycol to be excreted
What is non-competitive reversible inhibition?
1. When a molecules binds to another part of the enzyme rather than the active site
2. While the inhibitor is bound to the enzyme it can disrupt the normal arrangement of hydrogen bond and hydrophobic interactions holding the enzyme molecule in its 3D shape
3. The resulting distortion ripples across the molecule to the active site , making the enzyme unsuitable for the substrate
4. While the inhibitor is attached to the enzyme, the enzyme's function is blocked, NO MATTER HOW MUCH SUBSTRATE IS PRESENT
What is an example of non-competitive inhibition?
-In metabolic reactions must be very finely controlled and balanced so that no single enzyme can be allowed to 'run wild', constantly churning out more and more product
How can you control metabolic reactions?
1. Use the END PRODUCT of a chain of reactions as a non-competitive reversal inhibitor
2. As the enzyme converts substrate to product, it is slowed down because the end-product binds to another part of the enzyme and prevents more substrate binding
3. However the end-product can lose its attachment to the enzyme and go on to be used elsewhere, allowing the enzyme to reform into its active state
4. As product levels fall, the enzyme is able to top them up again (this is termed END-PRODUCT INHIBITION)
How would you investigate and explain the effect of immobilising an enzyme in alginate on its activity as compared with its activity when free in solution?
1. The enzyme is mixed with a solution of sodium alginate
2. Little droplets of this mixture are then added to a solution of calcium chloride
3. The sodium alginate and calcium chloride instantly react to from jelly, which turns each droplet into a little bead
4. The jelly bead contains the enzyme and the enzyme is held in the bead, or IMMOBILISED
What is the second part to investigating the effect of immobilising an enzyme in alginate on its activity as compared with its activity when free in solution?
1. These beads can be packed gently into column
2. A liquid containing the enzyme's substrate can be allowed to trickle steadily over them
3. As the substrate runs of the surface of the beads, the enzymes in the beads catalyse a reaction that converts the substrate into product
4. The product continues to trickle down the column, emerging from the bottom, where it can be collected and purified
How can immobilising enzymes be used commercially?
1. Enzyme lactase can be immobilised using alginate beads
2. Milk is then allowed to run through the column containing lactase beads
3. The lactase hydrolysis the lactose in the milk to glucose and galactose
4. The milk is therefore lactose-free and can be used to make lactose-free dairy products for people who cannot digest lactose
What are the advantages of using immobilising enzymes?
1. You can keep re-using the enzymes
2. The product is enzyme-free
3. Immobilised enzymes are more tolerant of temperature changes and pH changes than enzymes in solution
What is the Vmax?
-The theoretical maximum rate of the reaction it catalyses
-At Vmax all the enzyme molecules are bound to the substrate molecules (the enzyme is saturated with substrate)
How would you plot a graph to find out Vmax and Km?
1. Plot 1/substrate conc. on x axis and 1/velocity on y axis (double reciprocal plot)
2. 1/Vmax is the y-intercept, because this is where we know 1/[S] is zero (and therefore [S] is infinite)
3. -1/Km is the x intercept