NH4 + inosine.
Home Kinetic constants for the purified adenosine deaminase will
be determined for two substrates and an inhibitor. Each team
will use their purified enzyme preparations for these studies.
Km data should be compared within a section to determine if
the native and recombinant enzymes have the same kinetic
properties.
Adenosine deaminase, ADA (Adenosine aminohydrolase, EC 3.5.4.4), reacts with adenosine or several adenine nucleoside analogs catalyzing the irreversible reaction:
adenosine + H2O NH4 + inosine.
The reaction scheme for this enzyme reduces to that of the most simple enzyme catalyzed reaction:
(Scheme 1).
E represents the enzyme, S is the substrate (adenosine), ES is the enzyme substrate complex, and P is the product. Because the reaction is performed in an aqueous solution, water concentration is saturating and does not contribute to the rate equation. This scheme is also valid for multisubstrate reactions when all the other substrates are held at saturating concentrations.
Measurements of the velocity of the reaction in the absence of products allow determination of kinetic parameters. These parameters aid in comparison of the properties of the enzyme from different species and sources. The kinetic constants are also useful for analyzing alternative substrates. The most universal parameters are known as the Michaelis-Menten constants.
maximum velocity: Vm = k3[Etotal]
"substrate affinity" constant: Km = (k2+k3) / k1
For multisubstrate and more complex binding schemes the Km is not as simple. In any case, the Km is defined experimentally as the substrate concentration required to give half maximal activity.
For Scheme 1 the rate equation is:
(2)
or inverted:
(3)
where
v= measured initial reaction velocity
Vm= maximum reaction rate for that enzyme concentration
[S] = substrate concentration
Km= Michaelis constant defined above
Because Vm is directly proportional to the enzyme concentration, a level of enzyme must be found that will produce a reaction rate that is conveniently measured. Careful choice of enzyme levels is also important to ensure that the level of substrate in the assay solution remains constant over the course of the measurement. It is wise to calculate the nmoles of substrate in the assay solution and determine that this amount does not vary by more than 10% during the course of the measured reaction. The first step of this experiment will determine a suitable level of adenosine deaminase to be used in the kinetic studies.
The assay for this enzyme is derived by looking at the UV absorption spectra of inosine and adenosine and determining the wavelength at which the difference in their absorbance is the greatest or most suitable (see "Absorbance Spectra" below). For our experiment, the wavelength of 235nm is chosen so that saturating levels of adenosine can be used. At this wavelength, an increasein absorbance is seen as adenosine is converted to inosine. The slope is opposite the 265nm assays that you used previously. The assays are completed using similar spectrophotometer instructions as for the previous assays.
In addition to adenosine, the deaminase can catalyze reactions with 2'-deoxyadenosine, 2'-fluoroadenosine, 2',6'-diaminopurine riboside, and 3'-deoxyadenosine and will dehalogenate several N6-haloadenosine compounds. Each substrate binds with different affinity and is converted to product at different rates resulting in different Km's and/or Vm's. In Part 2 of this experiment, the kinetic parameters of adenosine and 2'-deoxyadenosine will be compared as a measure of the influence of the sugar group of the substrate on the binding affinity and reactivity of the substrate.
An inhibitor of the adenosine deaminase reaction will be evaluated in Part 3 of this experiment. The type of inhibition, competitive, noncompetitive, or uncompetitive, will be determined and a Ki will be calculated.
Consider the following:
Place cuvettes containing phosphate buffer in the rear and front compartments of the Cary 118 spectrophotometer.
a) Record a baseline scan from 350nm to 220nm at a rate of 1nm/sec and chart speed of 20 sec/inch.
b) Reverse the scan direction and bring the pen back to the starting point. Do not adjust any settings on the instrument at this point.
c) Replace the phosphate buffer in the cuvette nearer to the front of the instrument with a nucleoside solution and repeat the scan using a different color ink pen in the recorder.
Repeat steps b) and c) with the other nucleoside.
0.05 M potassium phosphate, pH 7.4
adenosine deaminase (your most pure sample)
3 mM adenosine (in KH2PO4 buffer) NOTE: please take only 10-15 ml so there's enough for the entire class
3 mM 2'-deoxyadenosine (in KH2PO4 buffer)
3 mM N6-methyladenosine (in KH2PO4 buffer)
NOTE: if you want to save time do NOT print each graph; you cannot do anything else with the specs while printing.
I recommend either not printing at all OR printing just a few "sample" graphs that are representative of your study.
Note: Kinetic studies are difficult to perform successfully. It is imperative that measuring and pipetting be done with care and precision to accomplish the accuracy required for these studies.
Find the amount of enzyme to use in the kinetic assays by completing the following assays. Prepare 3 ml of assay solutions according to the following table and record the absorbance changes at 235nm. (The volumes of ADA given are typical but you may need to expand the range if your activity is dilute OR dilute your sample in ADA assay buffer if it's highly active. Use the assay results from Day 4 to get a rough estimate for an appropriate range for your sample and adjust the buffer volume if necessary.)
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It will be necessary to find a suitable volume of your stock
enzyme solution that produces a ΔA235/min
of 0.02-0.03 under conditions of saturating levels of substrate as
found in the above solutions. Construct a graph in your notebook
of ΔA/min versus amount of enzyme
(µl) used and determine the volume of enzyme to use for
the remaining kinetic measurements. You may round the calculated
volumes to a convenient volume (e.g., 10, 15, 20, etc. µl).
This graph should demonstrate that the reaction rate is linearly dependent on the level of enzyme at saturating levels of substrate. If your plot is not linear, see the instructor for further instructions.
To determine Km and Vm for a particular substrate, the rate
(or velocity) of the reaction is measured at various concentrations
of the substrate.
[The amount of enzyme and the total volume
of the assay solution must be held constant ±2% of
the total volume. Note that in Part I the total volume
of the assay solution varied by 35 µl. This change represents
less than 2% of the total solution and normal pipetting errors
are expected to be about 3% due to multiple additions. Therefore,
these small volume changes are within the experimental
error and will not make a significant contribution to the total
error.] One method to achieve varied concentration is to add
different volumes of a stock solution of the substrate and
adjust the volume of buffer to maintain the constant volume
of the reaction mixture. We will use this method for kinetic
determinations using adenosine and 2'-deoxyadenosine.
Construct a chart in your notebook of the volumes of each component added to the reaction mixture so that the total volume is 3 ml. Measure the reaction velocities with 0.005, 0.010, 0.020, 0.040, 0.080, 0.10, 0.30 ml of adenosine solution. Compare the reaction rates of the 0.1 ml and the 0.3 ml adenosine assays. If these rates are not within 10% of each other, try 0.35 ml of adenosine. A graph of velocity (nmol/min) versus [adenosine] (µM) should show that an asymptote is being approached. Repeat the assays using 2'-deoxyadenosine and plot the data on the SAME graph.
From the plot of velocity versus substrate concentration (µM) estimate the Km for adenosine to use in Part 3. (See Results examples at the end of this section.) BEFORE beginning Part 3, confirm with the Instructor or TA that your number is reasonable based on literature values for the enzyme.
N6-methyladenosine (also known as 6-methylaminopurine 9-ribofuranoside) is tested as an inhibitor of the adenosine deaminase reaction. The manner of inhibition (competitive, noncompetitive, or uncompetitive) with respect to adenosine and the inhibition constant (Ki) are determined. The effect of an inhibitor is measured over a range of non-saturating levels of substrate. The substrate is varied from 0.625 to 5 times its Km while constant levels of inhibitor are present in the assay solution. Run assays at five different concentrations of adenosine within this range then repeat in the presence of 2 different levels of inhibitor.
Make 10 to 20 ml of a solution of adenosine at approximately 30 times the Km determined in Part 2. Then make the following dilutions in phosphate buffer:
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Instead of adding different amounts of the adenosine solution to the assay solution as in Part 2, the inhibition studies will be done by adding the same volume (0.5 ml) of different concentrations. Either method of substrate addition is suitable for kinetic measurements but slightly more reproducible results are obtained by adding a constant volume.
Prepare 3.0 ml assay solutions from 0.5 ml of adenosine
solution plus appropriate volumes of enzyme and phosphate
buffer. Obtain reaction velocities for all 5 concentrations
of adenosine.
[NOTE: These results will be used to determine the
Km and Km-observed (Kmobs) / Km-apparent (Kmapp) of the
substrate for the inhibition studies. Do the Km and Vm compare
to those determined in Part 2? Does it matter if the
Vm's are the same as previously determined?]
Determine the level of N6-methyladenosine that causes 10 to 20% inhibition of the reaction rate (in nmol/min) of your most concentrated adenosine assay. This determination is accomplished by preparing an assay using the 1:1 dilution of adenosine and adding various volumes of inhibitor solution. After finding the appropriate level of inhibitor, include the inhibitor in the assay solution and measure the enzyme activity using each of the adenosine solutions. Keep the assay solution volume at 3.0 ml by changing the amount of phosphate buffer if necessary. Repeat the inhibition study using twice that amount of inhibitor.
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Copyright, Acknowledgements,
and Intended Use
Created by B. Beason (bbeason@rice.edu), Rice University, 16 June 1999
Updated 10 January 2015