1. 1
Tyrosinase Enzyme Kinetics Attached to L-Dopa and Inhibition Using
Sodium Benzoate
Alison O’Malley
Metropolitan State University of Denver
December 12, 2013
CHE: 4450
Lab Partners: Nellie Kaufman and Brett White

2. 2
Abstract
Tyrosinase enzymatic activity was studied from the mushroom Agaricus
bisporous to find the kinetic constants Km and Vmax using L-dopa as the substrate. The
inhibitor sodium benzoate at concentrations 0.1 mM and 0.2 mM was used to study the
effects of inhibition on Tyrosinase. The values of the Km and Vmax reflected mixed
inhibition as the numerical values both changed upon the addition of sodium benzoate.
With the kinetic constant values and the Line-Weaver Burke graph, it was determined
that sodium benzoate was a mixed inhibitor with an emphasis on uncompetitive
inhibition. Experimental conditions such as human errors in pipetting, moisture in the air,
cleanliness of equipment, and age of tyrosinase preparation influenced the data.
Introduction
Enzymes are biological macromolecule catalysts that play a very crucial part in
the body, speeding up chemical reactions that would otherwise take a very long time to
occur. They are characterized by two fundamental properties. First they increase the
rate of chemical reactions without themselves being consumed or permanently altered
by the reaction. Second, they increase reaction rates without altering the chemical
equilibrium between reactants and products (4). Enzymes accomplish this by altering
the substrates binding site to resemble the transition state. They bind to a substrate
through two most common configurations. The primary configuration is through induced
fit in which the confirmation of the enzyme and substrate are altered so it resembles the
transition state. The second way is through the lock and key mechanism in which the

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substrate fits perfectly into the active site of the enzyme. In cases where multiple
enzymes are involved in the catalysis, specific amino acid side chains in the active site
may react with the substrate and form bonds with reaction intermediates (1).
Tyrosinase is a multifunctional, glycosylated, and copper-containing oxidase,
which catalyzes the first two steps in mammalian melanogenesis (3). Inhibitors are
molecules that bind to enzymes and decrease their activity. Two types of inhibitors exist
with different results. One type is the inhibitors that are irreversible which initiate
permanent damage to the enzyme. The other types are reversible and encompass four
major categories: competitive, uncompetitive, non-competitive, and mixed competitive
inhibition (5). The first purpose of this study was to determine the kinetic constants Km
and Vmax using L-dopa as the substrate. The second portion was to examine the
effects of a chosen inhibitor, sodium benzoate on the enzymatic activity of Tyrosinase
and to determine the type of inhibition interaction of substrate and enzyme.
Materials and Methods
Enzyme kinetics of tyrosinase: The first step to this was to determine the optimal
enzyme concentration of tyrosinase. This was done by varying the tyrosinase solution
(preparation “A” ~100 units/ml), while keeping the 5 mM L-Dopa concentration constant.
The total amount of material was always the same with the addition of 0.1 potassium
phosphate buffer (pH 7.0) to bring the solution to 1.10 ml. The tyrosinase was stored on
ice during the entire experiment. The spectrophotometer (Vernier™ probes) was set to
470 nm. The phosphate buffer was used to calibrate the spectrophotometer. The

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phosphate buffer and L-dopa were pipetted directly into tubes according to the table 1.
The tubes were then vortexed. The following steps were rapidly performed: 10 ml of
enzyme solution from the first tube was poured into the first test tube, then poured into
the 1-cm cuvette, and the cuvette was put into the spectrophotometer. Timing was
immediately started and absorbance vs. time data was recorded every 2 minutes. Initial
rate was determined from the graph on the computer. The second part of this
experiment was to determine the Km and Vmax for L-dopa. In this part the enzyme
concentration was held constant whilst the amount of L-dopa was varied from
unsaturated-saturated. A table was set up similar to table 1. which also included 6
assays with 75 ul enzyme concentration and varying amounts of L-dopa from 0.15 to
5.0 mM, those amounts were 25, 50, 75, 100, 25 , and 1000 µl L-dopa. The remaining
volume was phosphate buffer to make a final volume of 1.10 ml. The buffer and L-dopa
was pipetted into a test tube and vortexed. This was repeated on all 6 test tubes and 6
enzyme assays were collected successively. The absorbances were collected at 2 min
and the ΔA/min for each level of L-dopa was charted.
Inhibition of tyrosinase: The previous experiment was performed twice with a 5 .0
mM sodium benzoate inhibitor added to each assay. The sodium benzoate was used in
two different concentrations for each trial, 0.1 M and 0.2 M. The inhibitor amount was
always the same during each trial. The optimal amount of enzyme determined in the
previous was determined by varying the 5.0 mM L-Dopa amount. The other variable
were held constant. The total amount of solution was also 1.10 ml with the addition of
the potassium phosphate buffer solution. Six different amounts of L-Dopa were
prepared as in table 3 & 4. This part of the experiment was performed the same way as

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previously done. For each assay, the absorbance vs. time was recorded every 2
minutes. The data was converted to a Hanes-Wolf plot for each assay. The kinetic
constants were determined from this plot. The data was then used to create a double
reciprocal Lineweaver-Burk plot. The inhibition type of sodium benzoate was
determined from this plot.
Results
Table 1: General Protocol for Tyrosinase Assay
Material (µl) 1 2 3 4 5
Phosphate
Buffer
90 80 50 25 0
L-Dopa 1000 1000 1000 1000 0
Tyrosinase 10 20 50 75 100
Table 2: Protocol for Tyrosinase Assay with enzyme concentration held constant
and amount of L-Dopa varied over a range from non-saturation to saturation
Material
(µl)
1 2 3 4 5 6
Buffer 1000 975 950 925 775 250
L-Dopa 25 50 75 100 250 1000
Tyrosinase 75 75 75 75 75 75
Substrate
conc. [S]
(µM)
114 227 341 455 1140 4550
Table 3: 0.1 M Sodium Benzoate in a 5 mM solution in 0.1 M phosphate buffer pH
7.0
Material
(µl)
1 2 3 4 5 6
Buffer 980 950 930 905 755 5
L-Dopa 25 50 75 100 250 1000
Tyrosinase 75 75 75 75 75 75
Inhibitor 22 22 22 22 22 22

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Table 4: 0.2M Sodium Benzoate in a 5 mM solution in 0.1 M phosphate buffer pH
7.0
Material
(µl)
1 2 3 4 5 6
Buffer 956 931 906 881 731 0
L-Dopa 25 50 75 100 250 1000
Tyrosinase 75 75 75 75 75 75
Inhibitor 44 44 44 44 44 44
Table 5: Kinetic constant values for Tyrosinase uninhibited and inhibited
Kinetic Constant
Tyrosinase
Uninhibited 0.1mM sodium
benzoate
0.2 mM sodium
benzoate
Km (µM) 490.35 293.25 259
Vmax (µM)/s 500 250 250
Figure 1: Spectral data of tyrosinase assay that was used to determine the optimal
enzyme concentration
R² = 0.9996
R² = 0.99946
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
0 20 40 60 80 100 120
Absorbance
Time (s)
Spectral Data of Tyrosinase Assay
Assay 1
Assay 2
Assay 3
Assay 4
Assay 5
Linear (Assay 3)
Linear (Assay 4)

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Figure 2: Spectral data, which was used to determine the Km and Vmax of uninhibited
tyrosinase assays
y = 2E+06x + 980.7
R² = 0.99041
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
0 0.0005 0.001 0.0015 0.002 0.0025 0.003 0.0035 0.004 0.0045 0.005
[S]/v (sec)
[S] (M)
Hanes-Woolf Plot
y = 4E+06x + 1037
R² = 0.99469
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
20000
0 0.0005 0.001 0.0015 0.002 0.0025 0.003 0.0035 0.004 0.0045 0.005
[S}/v (sec)
[S] (M)
Hanes-Woolf Plot

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Figure 3: Spectral data, which was used to determine the Km and Vmax of inhibited
Tyrosinase assays with 0.1 mM sodium benzoate inhibitor
Figure 4: Spectral data, which was used to determine the Km and Vmax of inhibited
tyrosinase assays with 0.2 mM sodium benzoate inhibitor
y = 4E+06x + 1037
R² = 0.99469
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
20000
0 0.0005 0.001 0.0015 0.002 0.0025 0.003 0.0035 0.004 0.0045 0.005
[S}/v (sec)
[S] (M)
Hanes-Woolf Plot

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Figure 5: Double reciprocal plot of uninhibited Tyrosinase and inhibited Tyrosinase by
sodium benzoate
The optimal enzyme concentration was determined to be 6.8 units/ml of
Tyrosinase based off fig. 1. From this figure the optimal amount of enzyme was
determined to be that used in assay 4 (table 1): 75 µl of ~100 units/ml were added to
each ~1.10 ml assay. Table 2-4 show the amounts of each substance used for
uninhibited tyrosinase and the two trials of inhibited tyrosinase by sodium benzoate. By
using the data showed in fig. 2 the kinetic constants Km and Vmax for uninhibited
tyrosinase were determined to be 490.35 µM and 500 µM/s. Figs.3 & 4 depicts the two
reactions with the different concentrations of inhibitor. The significance of fig.5 is that it
-6000000
-4000000
-2000000
0
2000000
4000000
6000000
8000000
10000000
12000000
14000000
-10000 -8000 -6000 -4000 -2000 0 2000 4000 6000 8000 10000
1/[v] (2/M)
1/[S](/M)
Lineweaver-Burke Plot
0.1mM
uninhibited
0.2mM
Linear (uninhibited)
Linear (0.2mM)

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illustrates the type of inhibitor sodium benzoate. Table 5 shows the determined kinetic
constants from each of three trials.
Discussion
In the first part of the experiment the optimal concentration of tyrosianse was
determined using uninhibited tyrsoinase assay of varying concentrations while keeping
L-Dopa constant. The data from spectrophotometer fig. 1 reveals that both assays 3
and 4 from table 2 delivered the most useful concentrations because the slopes were
steep and very linear. Of the two, assay 4 was chosen because it had a larger
concentration and subsequently gave a stronger spectrophotometer signal than assay
3.
With the combination of the previous data and the L-Dopa concentrations from
table 2 the kinetic constants for uninhibited tyrosinase were determined (Km= 400.35
µM; Vmax=500 µM/s). The literature values for uninhibited tyrosinase from the
mushroom Agaricus bisporous were: Km= 170 µM-1500 µM (6)(7) and Vmax 24.5 ± 1.0
mM/min - 60 mM/min ≈ 1470(± 60) - 3600 mM/s (6)(7)(8). The Vmax as a result of this
experiment did not fall within the literature range. However, the literature values varied
greatly and therefore the values obtained in this experiment were likely valid results.
Due to the fact these wide ranges exist it is evident that this experiment depends less
on enzyme and substrate and more on experimental conditions. These experimental
conditions include human error in pipetting, moisture in the lab as well as lab
temperature, and age of the tyrosinase preparation

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The data from the Lineweaver Burke plot fig.5 contradicts the numerical data of the
kinetic constants table 5. At first the plot from fig.5 looks like it is reflecting mixed or non-
competitive inhibition. However, upon evaluation of the numerical changes of the kinetic
constants from table 5 (Vmax and Km change) in relation to the plot, the complete
assessment indicated mixed inhibition. The plot from fig. 5 does not indicate classic
uncompetitive inhibition, which would necessitate the uninhibited line and both the
inhibited lines be parallel. However, only the line representing the 0.2 mM concentration
of sodium benzoate and the uninhibited line demonstrate uncompetitive inhibition. In
conclusion, the inhibition of tyrosinase by sodium benzoate is most likely mixed with a
probable emphasis on uncompetitive. The experimental conditions strongly influenced
the outcome of the Km and Vmax. This pertinent information could direct future
experiments to produce more accurate results.

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References
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Journal of Molecular Science, 10(6), 2440-2475 DOI:
http://dx.doi.org/10.3390%2Fijms10062440
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3) Chung, K., Jeong, H., Jang, E., Choi, Y., Kim, D., Kim, S., Lee, K., Lee, H.,Chun, P.,
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Retrieved on December 6, 2013, from http://www.ncbi.nlm.nih.gov/books/NBK9921/
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http://www.sigmaaldrich.com/Graphics/COfAInfo/SigmaSAPQM/SPEC/T3/T3824/T3824
BULK_SIGMA_.pdf

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7) Selinheimo, E., Gasparetti, C., Mattinen, M., et al. (2009) Enzyme Microb.
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