Enzymes and pH Lesson 4

Success Criteria Describe and explain the effect of pH on enzyme activity

Starter Think, pair, share Effect of pH on enzymes

pH INCREASING ACIDITY INCREASING ALKALINITY The acidity of a solution is measured by the concentration of hydrogen ions (H+) and is expressed in terms of pH The pH scale ranges from 0 to 14 Pure water has a pH of 7.0, which is the pH of a neutral solution with equal numbers of H+ and OH- ions 1 3 2 4 5 6 7 8 9 10 11 12 13 14 INCREASING ACIDITY NEUTRAL INCREASING ALKALINITY If an acid is added to pure water, the hydrogen ion concentration increases, causing the solution to become acidic, which is measured as a lower pH 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 pH

Enzymes and pH Each enzyme has its own optimum pH where the rate of reaction is maximum Each specific enzyme can only work over a particular range of pH The effects of pH on the rate of enzyme controlled reactions display characteristically bell shaped curves A B C Enzyme A = amylase optimum pH = 7.2 Enzyme B = pepsin optimum pH = 2.0 Enzyme C = lipase optimum pH = 9.0 Changes in pH can affect the ionic and hydrogen bonds responsible for the specific tertiary shape of enzymes. Extremes of pH break these bonds and denature the enzyme

What will this do to the rate of reaction and why? pH and Bonds 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 and ph Enzymes only work within a narrow pH range. Small changed in pH 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 pH cause the active site’s shape to change permanently. The enzyme is denatured and can no longer catalyse the reaction.

Questions Explain why enzymes only work within narrow ranges of pH. 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. Changes in pH 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. If the enzymes are active as soon as they are produced, they will digest proteins within the cells themselves.

Exam Question 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 pH / corrosive; 1 (d)(i) Molecules would have less (kinetic) energy; move slower; fewer collisions / fewer E–S complexes form; max 2 (ii) Change in pH alters charge / shape; distorts active site / tertiary structure of enzyme / denatures enzyme; substrate will no longer fit active site; 3 Could be set as homework.

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

Summary Low pH = 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 pH 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 pH values, the –COOH group will dissociate to become a charged –COO- group

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

Success Criteria Describe and explain the effect of pH on enzyme activity