Modern Experimental Biochemistry

Chemistry

Quiz 5 :

Kinetic Analysis of Tyrosinase

Quiz 5 :

Kinetic Analysis of Tyrosinase

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The table below gives initial rates of an enzyme-catalyzed reaction along with the corresponding substrate concentration. Use any graphical method to determine K M. and V max. img
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In this problem, we are asked to use a graphical representation to find the maximum reaction velocity ( V max ) and the Michaelis constant ( K M ).
We can calculate these values by plotting data for the initial reaction velocity ( v 0 ) versus the concentration of the substrate that gave us the resulting v 0 ( [S] ).
From raw date, when we plot v 0 versus v 0 / [S], we will get back a straight line. On this line, the v 0 -intercept will be V max , and the v 0 / [S]-intercept is V max / K M. In this way, we can find both terms from the graph.
Our data is as follows: img We first derive v 0 / [S] - our data table changes as follows: img We then graph v 0 versus v 0 / [S], receiving the following graph: img Thus, our V max will be 140, and our K M will be 4.0x10 -5

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img For each pair of wavelengths listed below, specify which one is higher in energy. (a) 1 nm (X-ray) or 10,000 nm (IR) (b) 280 nm (UV) or 360 nm (VIS) (c) 200,000 nm (microwave) or 800 nm (VIS)
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In this problem, we are asked to pick which wavelength in a pair has the higher energy:
A) 1 nm (X-ray) or 10,000 nm (IR)B) 280 nm (UV) or 360 nm (VIS)C) 200,000 nm (microwave) or 800 nm (VIS)As a rule, the smaller the wavelength, the higher the energy of the wave in question.
a)The x-ray is higher energy, as it has the smaller wavelength.
b)The UV wave is higher energy, as it has the smaller wavelength.
c)The visible light has higher energy, as it has a smaller wavelength.

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img Why must a cuvette with four translucent sides be used for fluorescence measurements?
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In this problem, we are asked to explain why a cuvette with four translucent sides must be used for fluorescence measurements.
The fluorescing light is measured at a right angle from the excitation light, therefore all sides of the cuvette must be translucent so recording can occur.

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One of the many straight-line modifications of the Michaelis-Menten equation is the Eadie-Hofstee equation: img Beginning with the Lineweaver-Burk equation, deliver the Eadie-Hofstee equation. Explain how it used to plot a straight line from experimental rate data. How are V max and K M calculated?
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img Several spectroscopic techniques were studied in this chapter. Which experimental techniques involve an actual measurement of radiation absorbed? (a) UV-VIS spectroscopy (b) NMR spectroscopy (c) MS (d) Fluorescence spectroscopy
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img What is the single structural characteristic that all of the fluorescent molecules in Figure 5.13 possess.
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Each of the compounds listed below is known to inhibit tyrosinase activity. img (a) Study the structure of each and try to predict whether the substance is a competitive or noncompetitive inhibitor of tyrosinase activity as measured by the dopachrome assay. Assume that the inhibition data were evaluated using a Lineweaver-Burk plot of l/ v 0 vs. l/[dopa]. (b) An alternative method for measuring tyrosinase activity is the use of an oxygen electrode to measure the rate of dioxygen utilization during phenol oxidation (see Equations E5.7 and E5.8). Predict whether each substance is a competitive or noncompetitive inhibitor of tyrosinase as measured by an oxygen electrode. Assume that a Lineweaver-Burk plot of l/ v 0 vs. 1/[O2 ] was used to evaluate the rate data.
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img Match the spectral region listed below with the appropriate molecular transition that occurs. The first problem is worked as an example. img
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What is the concentration (in mg/mL and µ g/mL) of tyrosinase in a solution that has an A 280 of 0.25?
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Compound X was tested as an inhibitor of the enzyme in Problem 2. Use the rate data in Problem 2 and the following inhibition data to evaluate compound X. Is it a competitive or noncompetitive inhibitor? Calculate K 1 for the inhibitor. img
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The enzyme concentration used in each assay in Problem 2 is 5 × 10 ?7 mole/liter. Calculate k 3 , the turnover number, in units of sec ?1.
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Show the mathematical steps required to derive the Lineweaver-Burk equation beginning with the Michaelis-Menlen equation.
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Study Figure E5.7, which displays the kinetic progress of an enzyme-catalyzed reaction. What time limit must be imposed on rate measurements taken using the fixed-time assay? Why?
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Using the data in Problems 2 and 3, calculate the specific activity of the enzyme in units/mg and katal/mg. Assume the enzyme has a molecular.weight of 55,000 and the reaction mixture for each assay is contained in a total volume of 1.00 mL.
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Study the 1 H spectrum of valine in Figure 5.14A and match each peak to the corresponding proton in the chemical structure. Explain any spin-spin coupling.
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img Why can you not use a glass cuvette for absorbance measurements in the UV spectral range?
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Predict what each of the following experimental changes will do to the initial rate of dopa oxidation catalyzed by tyrosinase. Answer each with "increase," "decrease," or "no change." (a) Increase the temperature of the reaction mixture from 25°C tO37°C. (b) Change the concentration of dopa from a value equal to 1 K M to a value equal tO2 K M. (c) Add a few drops of a 1 × 10 -3 M cysteine solution (d) Increase the reaction temperature from 37°C to 100JC. (e) Decrease the pH of the reaction mixture from pH 7.0 to pH = 1.0. (f) Change the concentration of tyrosinase from 1 µ g/assay tO2 µ g/assay.
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Study the 13 C spectrum of valine in Figure 5.14A. Identify the carbon atom that produces each peak in the spectrum.
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img In a laboratory experiment, you are asked to determine the molar con­centration of a solution of an unknown compound, X. The solution di­luted in half by water (1 mL of X and 1 mL of H2 O) has an absorbance at 425 nm of 0.8 and a molar extinction coefficient of 1.5 × 10 3 M ?1 cm ?1. What is the molar concentration of the original solution of X?
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Explain the differences between a spectrophotometer that uses a photo­tube for a detector and one that uses a photodiode array detector.
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