DP Chemistry Questionbank

Amino acids act as buffers in solution. In aspartic acid, the side chain (R group) carboxyl has pK_a = 4.0. Determine the percentage of the side chain carboxyl that will be ionized (–COO^–) in a solution of aspartic acid with pH = 3.0. Use section 1 of the data booklet.

(i) Outline what is meant by product inhibition as it applies to hexokinase.

(ii) Product inhibition of hexokinase does not affect its K_m value. Using this information, deduce the type of binding site that the inhibitor attaches to.

(i) Estimate the K_m values of the two enzymes.

(ii) Suggest, with a reason, which enzyme will be more responsive to changes in the concentration of glucose in the blood.

Deduce the pH range in which glycine is an effective buffer in basic solution.

Enzymes are biological catalysts.

The data shows the effect of substrate concentration, [S], on the rate, v, of an enzyme-catalysed reaction.

Determine the value of the Michaelis constant (K_m) from the data. A graph is not required.

Outline the action of a non-competitive inhibitor on the enzyme-catalysed reaction.

Determine the value of the Michaelis constant, K_m, including units, from the graph.

Sketch a second graph on the same axes to show how the reaction rate varies when a competitive inhibitor is present.

Calculate the pH of a buffer solution which contains 0.700 mol dm^–3 of the zwitterion and 0.500 mol dm^–3 of the anionic form of alanine. 

Alanine pK_a = 9.87.

Outline the significance of the value of K_m.

Outline the significance of the value of the Michaelis constant, $K_{\mathrm{m}}$.

Determine the value of $V_{\max }$ and $K_{\mathrm{m}}$ in the absence and presence of the inhibitor.

Determine the value of the Michaelis constant, K_m, by annotating the graph.

The malonate ion acts as an inhibitor for the enzyme.

Suggest, on the molecular level, how the malonate ion is able to inhibit the enzyme.

Draw a curve on the graph above showing the effect of the presence of the malonate ion inhibitor on the rate of reaction.

Outline the significance of the value of the Michaelis constant, K_m.

Calculate the pH of a buffer system with a concentration of 1.25 × 10⁻³ mol dm⁻³ carbonic acid and 2.50 × 10⁻² mol dm⁻³ sodium hydrogen carbonate. Use section 1 of the data booklet.

pK_a (carbonic acid) = 6.36

Explain with reference to the binding site on the enzyme how a non-competitive inhibitor lowers the value of V_max.

UV-Vis spectroscopy can be used to determine the unknown concentration of a substance in a solution.

Calculate the concentration of an unknown sample of pepsin with an absorbance of 0.725 using section 1 of the data booklet.

Cell length = 1.00 cm

Molar absorptivity (extinction coefficient) of the sample = 49650 dm³ cm⁻¹ mol⁻¹

A different series of pepsin samples is used to develop a calibration curve.

Estimate the concentration of an unknown sample of pepsin with an absorbance of 0.30 from the graph.

Contrast the actions of non-competitive and competitive inhibitors of an enzyme and state their effects on the maximum rate of reaction, V_max, and the Michaelis–Menten constant, K_m.

A Michaelis–Menten plot for an enzyme-catalysed reaction is shown.

Sketch a curve to show the effect of a competitive inhibitor.

Suggest, based on the Michaelis–Menten plot, how a competitive inhibitor such as ethanol reduces the toxicity of methanol.

Enzymatic activity is studied in buffered aqueous solutions.

Calculate the ratio in which 0.1 mol dm⁻³ NaH₂PO₄ (aq) and 0.1 mol dm⁻³ Na₂HPO₄ (aq) should be mixed to obtain a buffer with pH = 6.10. Use section 1 of the data booklet.

pK_a (NaH₂PO₄) = 7.20

Outline which pK_a value should be used when calculating the pH of the solution, giving your reason.

State and explain how a competitive inhibitor affects the maximum rate, V_max, of an enzyme-catalyzed reaction.

Calculate the pH of the glutamine solution using section 1 of the data booklet.

Compare the effects of competitive and non-competitive inhibitors.

Calculate the ratio of [A⁻] : [HA] in a buffer of pH 6.0 given that pK_a for the acid is 4.83, using section 1 of the data booklet.