Factors affecting enzymes Understand how enzymes act as catalysts to biological reactions and the factors that affect the rate of enzyme reactions.
Enzymes Enzymes are biological catalysts. They catalyse chemical reactions in biological systems. The are made of protein and have a globular structure. What is a catalyst? How do catalysts work?
Activation energy. “ For a reaction to occur, molecules must collide with the right amount of energy” This energy is needed to break the bonds in the reactants (substrate).
Recap • What does each of these get broken down to: • Maltose • Polypeptide • Cellulose • Lactose
Specific Shapes • Specific 3D shape and an ACTIVE SITE on its surface. • The molecule the enzyme acts on is called the SUBSTRATE. • The substrate & active site have COMPLEMENTARY SHAPES (fit together) and temporarily BIND together • SPECIFIC and catalyse/speed up only 1 reaction (or 1 group of reactions)
+ + STARCH AMYLASE MALTOSE AMYLASE SUBSTRATE + ENZYME PRODUCT + ENZYME The equation shows the breakdown of starch to _______, catalysed by the enzyme ______. _______ is the substrate for amylase. The substance produced at the end of the reaction is known as the product. ________ is specific to this reaction. The enzyme is not “used up” in the reaction but can be used again to react with other substrate molecules
Lock & Key Like all other molecules in a cell, enzymes constantly move about and collide with other molecules. If the enzyme binds with its substrate molecule they BIND forming an ENZYME –SUBSTRATE COMPLEX
E + S ES EP E + P This explanation of how enzymes catalyse reactions is called the lock & key mechanism. The same enzyme can also catalyse the reverse reaction
Why do enzymes lower the activation energy? • Scientists suggest different ways; • The enzyme-substrate complex might bring together substrate molecules • It might put the substrate under stress so bonds are broken more easily
Induced-Fit Hypothesis • Recent research suggests it is more likely that the active site and substrate are similar but only fully complementary when they bind. • As the substrate binds, the enzyme is INDUCED to change shape so that the molecules fit exactly
Quick test • Name the two models that can be used to describe the action of enzymes. • The active site of the enzyme is the place where the…. • Enzymes will speed up chemical reactions because they….
Factors affecting the rate of enzyme catalysed reactions. Temperature pH Substrate concentration Enzyme concentration Competitive inhibitors Non-competitive inhibitors How? Why? Graph
Optimum Conditions - 1 • An increase in temp. gives the enzyme & substrate more kinetic energy. • There is more chance of them colliding & forming E-S complexes • However, at high temps. the increase in kinetic energy causes the enzyme molecule to vibrate. • DENATURATION occurs as the vibrations cause the hydrogen bonds to break & the enzyme’s shape to change.
Optimum Conditions - 2 • Most enzymes have an optimum pH close to 7 which is the pH found in most cells. • Enzymes that work outside cells, extra-cellular enzymes, can have very different pH requirements. • pH can affect the shape of the active site. pH is a measure of the concentration of H+ ions & this can affect the hydrogen bondsholding the 3D structure together
Test Yourself B Rate A C Temperature 1.Why do most enzymes have optimum temperature between 40 & 50 oC? 2.Why does the rate of reaction increase between points A & B? 3. Explain why an enzyme stops working when it’s shape changes?
How did you do? 1. Above 50oC the enzyme has too much kinetic energy so hydrogen bonds vibrate & break. Below 40oC there is less kinetic energy so there are less collisions. 2. As the kinetic energy increases there are more collisions and more enzyme-substrate complexes are formed. Therefore the rate increases. 3. The shape of the enzyme’s active site is no longer complementary to the substrate so no E – S complexes form & no product is made.
Reaching a Maximum In addition to temperature & pH, the rate of an enzyme controlled reaction also depends on the amount of enzyme & the amount of substrate available. Increasing either factor increases the number of collisions. Q. Explain the initial rate increase as shown by the graph. A. The substrate concentration is limiting so not all active sites are occupied Eventually all active sites are occupied & the enzyme concentration has become the limiting factor.
Inhibition • INHIBITORS can prevent or slow the formation of enzyme-substrate complexes. • COMPETITIVE inhibitors are molecules that compete directly for the active site – they are similar shapes to the substrate. • NON-COMPETITIVE inhibitors are not similar to the substrate. T hey bind to a different area of the enzyme molecule, causing the shape of the molecule & therefore the shape of the active site to change. Heavy metals such as cadmium, lead, zinc & mercury can act in this way • END-POINT inhibition can take place in chemical processes that involve a series of enzyme-controlled reactions. When enough end-product has been made it acts as an inhibitor for the enzyme in the first step.
Imagine the essay… What factors can change the rate of an enzyme controlled reaction?