pH and Enzymes

How can we make the glow sticks brighter/ dimmer?

Put these statements in order to explain why a high temperature reduces the rate of enzyme controlled reactions a.breaks bonds; b.active site changes, shape/AW; c.substrate will not fit/no enzyme-substrate complex formed; d.rate decreases/reaction stops; e.enzyme vibrates too much; f.changes, tertiary/3-D, structure/shape, of enzyme; g.enzyme denatured; h.enzyme increases in kinetic energy;

Buffers Something that resists changes in pH. You have some of these substances in your blood. They accept or donate hydrogen ions. Haemoglobin and some other proteins can also do this. They can be used to maintain desired pH’s during laboratory investigations

How do changes in pH affect enzymes? Hydrogen ions have a positive charge so are attracted to negatively charged ions, molecules or parts of molecules. Excess H + interferes with hydrogen bonds and ionic forces that hold together the tertiary structure of the enzyme. This causes the active site to change shape and when the substrate can no longer fit it is classed as denatured.

How do changes in pH affect enzymes? Increasing the concentration of H + also alters the charges on the active site of enzyme molecules as more protons cluster around negatively charged groups. This interferes with the binding of the substrate and therefore slow the rate of reaction.

pH ranges Enzymes work within a narrow range. Small changes either side of the optimum slow the rate of reaction. – However if the normal optimum pH is restored, the hydrogen bonds can reform and the active site shape is restored. At extremes of pH, the active site can be permanently changed, and thus denatured.

Example Blood needs to be kept between a range of 7.35 and 7.45 as there are many proteins found in the plasma. Changes can also cause vasodilation and vasoconstriction. Buffers e.g. hydrogencarbonate ions & potassium dihydrogenphosphate help to resist pH changes.

Different optimums… Intracellular – optimum pH is close to 7 Extracellular – optimum pH can differ.

Question Explain how the optimum pH for enzymes differs through the digestive system.

Answer Mouth – pH of 6.8: amylase works best and digests starch to maltose Stomach – pH between 1 and 2: amylase is denatured and pepsin is at optimum so digests protein into smaller peptide molecules. Small intestine – pH 7.8: Stomach acid is neutralised by bile and raises pH to slightly alkaline. Optimal pH for trypsin and enterokinase that further catalyse the digestion of peptides to amino acids.

Review Qs Casein is a protein found in milk powder. A student wants to investigate the effect of varying pH on the rate of casein breakdown by the enzyme trypsin. She intends to add trypsin to a series of solutions containing milk powder at different pHs and time how long it takes for each solution to turn from cloudy to translucent. a)Suggest how the student could vary the pH of the milk powder solutions.[1 mark] b)Give one variable that the student will need to control in this experiment.[1 mark] c)The student has read that the optimum pH for trypsin is pH8. Describe and explain what will happen to the rate of the reaction above this pH.[4 marks]

Answers a)Add a buffer solution of a different pH to each of the test tubes.[1 mark] b)The volume of milk powder used in each test tube/ the concentration of milk powder used in each test tube/ the volume of trypsin used/ the concentration of trypsin used/ temperature [1 mark] c)Above the optimum pH, the OH - ions in the alkaline solution; interferes with the ionic/hydrogen bonds; alters tertiary structure/ shape of active site; rate of reaction decreases; enzyme eventually denatures [4 marks]