Question

An enzyme works well as a catalyst at a pH of 6.8. The pk of an essential catalytic residue is known to be 7.2 It is found that catalytic activity is significantly lower when pH= 8.1. What is likely the cause of the decrease in activity? Select all that apply. Select one or more: a. Glu or Asp must lose a proton to inactivate the protein, b. The protein has changed shape due to a change in charge. c. His may act as a proton donor in the active form of the enzyme. d. The protein has been degraded into its amino acids above pH 8.1.! NE

Answer 1

Introduction: Enzymes are biocatalysts. It increases the rate of a chemical reaction in biological systems without altering any change in the substrates or products. Examples are amylase and catalase. The catalytic activity is defined as a mole of catalyst that will accelerate a mole of substrate to product. pH is defined as the negative logarithm of hydrogen ion concentration. pKa determines the strength of acidity or basicity of a solution. The catalytic activity varies with changes in pH.

Answer: The cause of the decrease in activity was because of

a. Glu or Asp must lose a proton to inactivate the protein and

b. The protein has changed shape due to a change in charge

Explanation:

An enzyme has good activity at pH of 6.8 that has a pka value of 7.2. The catalytic activity was found to decrease when pH=8.1. The cause of the decrease in catalytic activity was because of

a. Glu or Asp must lose a proton to inactivate the protein

b. The protein has changed shape due to a change in charge

Several enzymes have optimum pH value of 7.0. Aspartic acid and glutamic acid act as proton donors having pKa value around 4.5. When the pH is greater than the pKa, the carboxylic acid side chains lose a proton to become negatively charged. Similarly when the pH is less than pKa, the aspartic and glutamic acid are uncharged.

Histidine has a pKa value of 6.0. The imidazole side chain of histidine is involved as a proton acceptor. When the pH is greater than pKa, it is uncharged whereas when the pH is less than pKa, the side chain accepts a proton to become positively charged.

Also, proteins undergo change in shape when it is exposed to various pH. This is because a change in pH alters the charge of the proteins. This causes a change in the shape of proteins which is called as denaturation of proteins. Hence option (a) and (b) are the correct choices.