Lesson 4 Describe and explain the effect of

Lesson 4

� Describe and explain the effect of p. H on enzyme activity

� Think, pair, share �Effect of p. H on enzymes

p. H The acidity of a solution is measured by the concentration of hydrogen ions (H+) and is expressed in terms of p. H The p. H scale ranges from 0 to 14 Pure water has a p. H of 7. 0, which is the p. H of a neutral solution with equal numbers of H+ and OH- ions 0 1 2 3 4 5 6 7 INCREASING ACIDITY If an acid is added NEUTRAL to pure water, the hydrogen ion concentration increases, causing the solution to become acidic, which is measured as a lower p. H 8 9 10 11 12 13 14 INCREASING ALKALINITY If a base is added to pure water, the hydrogen ion concentration decreases and the hydroxyl ion (OH-) concentration increases. The solution becomes more basic (alkaline) and is measured as a higher p. H

Enzymes and p. H Each enzyme has its own optimum p. H where the rate of Each specific enzyme can only work reaction is maximum over a particular range of p. H The effects of p. H on the rate of enzyme controlled reactions display characteristically bell shaped curves B A C Enzyme A = amylase optimum p. H = 7. 2 Enzyme B = pepsin optimum p. H = 2. 0 Enzyme C = lipase optimum p. H = 9. 0 Changes in p. H can affect the ionic and hydrogen bonds responsible for the specific tertiary shape of enzymes. Extremes of p. H break these bonds and denature the enzyme

� Hydrogen ions (provided by acids) are attracted towards negatively charged ions, molecules or parts of molecules. � Hydrogen bonds and ionic forces mold the shape of the active site. � Excess hydrogen ions interfere with these bonds, changing the shape of active site. What will this do to the rate of reaction and why?

� Enzymes only work within a narrow p. H range. � Small changed in p. H either side of the optimum will only slow the rate of reaction. This is because the shape of the active site is disrupted, but not completely altered. � � � Extreme changes in p. H cause the active site’s shape to change permanently. The enzyme is denatured and can no longer catalyse the reaction.

1. 2. Explain why enzymes only work within narrow ranges of p. H. Changes in p. H result in excess H+ or OH- ions. These disrupt the bonds in the enzymes structure. This changes the shape of the active site. This means that an enzyme-substrate complex cannot form. The proteases pepsin and trypsin are both produced by cells in an inactive form. The acid in the stomach changes the enzymes into their active form. Suggest why these enzymes are first secreted in their inactive form. If the enzymes are active as soon as they are produced, they will digest proteins within the cells themselves.

1. (a) (i)Energy put in to get reaction started (Look for idea of getting started); 1 (ii) Curve showing energy levels at start and finish the same; and lowered activation energy; 2 (b) Benedict’s / Fehling’s reagent and heat; orange / red / brown / yellow / green; 2 (c)(i) Acid hydrolyses starch / breaks glycosidic bond; 1 (ii) Not specific / forms by–products / alters p. H / corrosive; 1 (d)(i) Molecules would have less (kinetic) energy; move slower; fewer collisions / fewer E–S complexes form; max 2 (ii) Change in p. H alters charge / shape; distorts active site / tertiary structure of enzyme / denatures enzyme; substrate will no longer fit active site; 3

�A buffer is something that resists changes in p. H. � In laboratory experiments, buffer solutions maintain the desired p. H for investigating enzyme action at a particular p. H. � They can also help to keep the p. H at a constant level as you investigate another factor. � There are chemicals in your blood that resist changes to p. H by donating or accepting hydrogen ions. Explain how this stops the p. H from changing.

� Low p. H = lots of H+ ions � H+ ions have a positive charge � Either extreme of H+ ion concentration can interfere with the hydrogen and ionic bonds holding the tertiary structure together. � If the p. H change affects the charge on the amino acids at the active site, then the properties of the active site change and the substrate can no longer bind � At high p. H values, the –COOH group will dissociate to become a charged –COO- group

� Discuss 5 things you have learnt this lesson with the person sitting next to you.

� Describe and explain the effect of p. H on enzyme activity