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5 Cards in this Set
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Keeping the Sweet Taste of Corn The sweet taste of freshly picked corn (maize) is due to the high level of sugar in the kernels. Store-bought corn (several days after picking) is not as sweet, because about 50% of the free sugar is converted to starch within one day of picking. To preserve the sweetness of fresh corn, the husked ears can be immersed in boiling water for a few minutes (“blanched”) then cooled in cold water. Corn processed in this way and stored in a freezer maintains
its sweetness. What is the biochemical basis for this procedure? |
One of the simplest techniques for inactivating enzymes is heat denaturation. Freezing the corn lowers any remaining enzyme activity to an insignificant level.
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Rate Enhancement by Urease The enzyme urease enhances the rate of urea hydrolysis at pH 8.0
and 20 C by a factor of 1014. If a given quantity of urease can completely hydrolyze a given quantity of urea in 5.0 min at 20 C and pH 8.0, how long would it take for this amount of urea to be hydrolyzed under the same conditions in the absence of urease? Assume that both reactions take place in sterile systems so that bacteria cannot attack the urea. |
950 million years.
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Protection of an Enzyme against Denaturation by Heat When enzyme solutions are heated, there is a progressive loss of catalytic activity over time due to denaturation of the enzyme. A solution of the enzyme hexokinase incubated at 45 C lost 50% of its activity in 12 min, but when incubated at 45 C in the presence of a very large concentration of one of its substrates, it lost only 3% of its activity in 12 min. Suggest why thermal denaturation of hexokinase was retarded in the presence of one of its substrates.
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One possibility is that the ES complex is more stable than the free enzyme. This implies that the ground state for the ES complex is at a lower energy level than that for the free enzyme, thus increasing the height of the energy barrier to be crossed in passing from the native to the denatured or unfolded state.
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Effect of Enzymes on Reactions Which of the following effects would be brought about by any enzyme catalyzing the simple reaction
S k1 reversible k2 P where K'eq = [P]/[S]? (a) Decreased K'eq; (b) Increased k1; (c) Increased K'eq; (d) Increased G‡; (e) Decreased change G‡; (f) More negative change G' (g) Increased k2. |
(b), (e), (g). Enzymes do not change a reaction’s equilibrium constant and thus catalyze the reaction in both directions, making (b) and (g) correct. Enzymes increase the rate of a reaction by lowering the activation energy, hence (e) is correct.
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NADH and NAD are the reduced and oxidized forms, respectively, of the coenzyme NAD. Solutions of
NADH, but not NAD, absorb light at 340 nm. This property is used to determine the concentration of NADH in solution by measuring spectrophotometrically the amount of light absorbed at 340 nm by the solution. Explain how these properties of NADH can be used to design a quantitative assay for lactate dehydrogenase. |
The reaction rate can be measured by following the decrease in absorption at 340 nm (as NADH is converted to NAD) as the reaction proceeds. The researcher needs to obtain three pieces of information to develop a good quantitative assay for lactate dehydrogenase:
(i) Determine Km values. (ii) Measure the initial rate at several known concentrations of enzyme with saturating concentrations of NADH and pyruvate. (iii) Plot the initial rates as a function of [E]; the plot should be linear, with a slope that provides a measure of lactate dehydrogenase concentration. |
