A technician measures the initial rate of reaction of two digestive enzymes, salivary amylase and pepsin, across a range of pH values. The substrate concentration, enzyme concentration and temperature are kept the same throughout each enzyme's series of measurements. pH 1 2 3 5…

Answer: ((a)) The rate of reaction with salivary amylase rises as pH increases from 1 (rate 1) to a peak at pH 7 (rate 26), then falls sharply as pH increases further to 9 (rate 3). The pattern is a rise to an optimum followed by a fall, not a steady increase across the whole range. Salivary… ((b)) At pH 1 and pH 9, the hydrogen ion concentration is very different from that at pH 7 (much higher at pH 1, much lower at pH 9). This changes the ionisation of acidic and basic R-groups (side chains) of the amino acids making up the enzyme, including those in and around the activ… ((c)) Salivary amylase has an optimum pH of about 7 (a rate of 26 at pH 7, far higher than at any other pH tested), which matches the roughly neutral pH of saliva in the mouth, allowing it to work efficiently as soon as food is chewed and mixed with saliva. Pepsin, in contrast, has it…. Part (a): Describing the pattern for salivary amylase Reading the salivary amylase row across the table, the rate rises steadily from pH 1 (rate 1) through pH 2, 3 and 5, reaching a peak of 26 at pH 7. Above pH 7, the rate falls sharply, d…

AS & A Level Biology 9700: pH and Enzyme Activity in Digestion

pH	1	2	3	5	7	9
Rate with salivary amylase / arbitrary units	1	2	4	15	26	3
Rate with pepsin / arbitrary units	20	27	12	2	0	0

Rate with salivary amylase / arbitrary units

Rate with pepsin / arbitrary units

Part (a): Describing the pattern for salivary amylase

Reading the salivary amylase row across the table, the rate rises steadily from pH 1 (rate 1) through pH 2, 3 and 5, reaching a peak of 26 at pH 7. Above pH 7, the rate falls sharply, dropping to just 3 by pH 9.

So the pattern is a rise to a peak at pH 7, followed by a fall on either side. This is a bell-shaped curve, and pH 7 is the optimum pH for salivary amylase.

Part (b): Why the rate is lower away from the optimum pH

An enzyme’s tertiary structure, its precise three-dimensional folded shape, is held together partly by ionic bonds between oppositely charged R-groups (side chains) of amino acids, together with hydrogen bonds. The charge on many R-groups depends on the concentration of hydrogen ions (H+) in the surrounding solution, i.e. on pH.

At pH 1, the H+ concentration is very high; at pH 9, it is very low. Both are far from pH 7, so at both extremes the normal ionisation of R-groups (including those that form part of, or lie close to, the active site) is altered. This disrupts some of the ionic bonds (and hydrogen bonds) that maintain the tertiary structure, changing the precise shape of the active site. Once the active site is no longer as complementary to the substrate, fewer enzyme-substrate complexes can form each second, so the rate of reaction is much lower than at pH 7, where the enzyme’s shape (and the active site’s shape) is optimal.

Part (c): Matching optimum pH to environment

Salivary amylase reaches its highest rate at pH 7 (rate 26), matching the roughly neutral pH of saliva in the mouth. This means the enzyme works efficiently exactly where and when it is needed, as soon as starch-containing food is chewed and mixed with saliva.
Pepsin reaches its highest rate at pH 1-2 (rate 20 at pH 1, 27 at pH 2) and has essentially no activity by pH 7 (rate 0). This matches the very acidic environment produced by hydrochloric acid in gastric juice, allowing pepsin to digest proteins efficiently in the stomach.

Each enzyme’s optimum pH is therefore matched to the pH of the part of the digestive system in which it normally acts: an enzyme with pepsin’s optimum would be almost inactive in the near-neutral mouth, and an enzyme with amylase’s optimum would be almost inactive in the acidic stomach.

Final answers

(a) The rate rises to a peak of 26 at pH 7, then falls sharply to 3 by pH 9, a bell-shaped curve with an optimum pH of about 7.
(b) Extreme pH changes the ionisation of R-groups, disrupting ionic/hydrogen bonds that maintain tertiary structure and altering the active site’s shape so it is less complementary to the substrate.
(c) Salivary amylase’s optimum (pH 7) matches the near-neutral mouth; pepsin’s optimum (pH 1-2) matches the acidic stomach, each enzyme’s optimum pH suits where it normally functions.