Enzymatic assay
The enzymatic monophenolase activity was determined by
monitoring the change in UV absorbance at 305 nm (dopachrome
formation (Rodríguez-López et al., 1992)) (Fig. 1D). When measuring the activity of ‘‘active forms’’ of TYR, 10 lL of enzyme solution
were added to 1 mL reaction mixture containing 35 mM potassium
phosphate buffer, pH 6.5, and 0.033 mM tyrosine as substrate. One
unit [U] monophenolase activity was defined as the increase of
1 mAU per minute at a path length of 1 cm at 25 C. When measuring activity of the latent forms of the enzyme, the buffer system
(for better solubility of SDS) was changed to 10 mM sodium phosphate buffer, pH 6.5, containing 1.3 mM SDS. Latency was calculated using the following equation.
latency ½% ¼ 1 activityðSDSÞ½U mL1
activityðþSDSÞ½U mL1
! 100
Analytical polyacrylamide gel electrophoresis (PAGE)
Analyses by denaturating SDS–PAGE under reducing (2-
mercaptoethanol) and non-reducing conditions (data for nonreducing not shown), respectively, were established as described
elsewhere using a total polyacrylamide concentrations of 12% (Laemmli, 1970). Reduced samples were reacted with 3-iodopropionamide for alkylation of Cysteines. Sample load onto the gel was
about 2 lg. Gels were stained with Coomassie Brilliant Blue. The target protein (L-TYR, 62 kDa) as well as the faint bands (15 kDa) in
the A-TYR lane were cut out and used for protein identification.
Imaging of the gels was done with the Gel Doc™ XR of Bio Rad
(Fig. 2).
Protein identification and sequence analysis
The gel piece covering the single bands obtained by denaturing
SDS–PAGE (reduced and non-reduced) and containing approximately 2 lg of protein were used. Proteomic analyses were carried
out after tryptic digestion by use of nanoLC–ESI-MS/MS with a
LTQ-Orbitrap mass spectrometer. In addition samples were also
measured by Proteom Factory AG (www.proteomefactory.com).
Protein identification was based on the Mascot Search software
and the NCBInr 110509 database as well as Peaks Studio 6.0 search
software and UniProt/SwissProt database, respectively. Peptide
mass tolerance was 5 ppm and fragment mass tolerance 0.5 Da.
Variable modifications allowed were oxidation of methionine
and, in the cases of reduced samples, also propionamidation of cysteines. For detailed information about the used method- and
search software-parameters see the Supporting information.
Determination of the molecular mass
The mass spectrum of the intact protein was measured using a
nanoESI-QTOF mass spectrometer (maXis 4G UHR-TOF, Bruker)
with a mass resolving power of about 40,000 in the used m/z –
range and a mass accuracy of better than 5 ppm (confirmed by
standard proteins). Prior to MS measurements, the purified L-TYR
solution was ultra filtrated by centrifugation (10,000 rpm) and
the buffer system was changed to 5 mM ammonium acetate pH
5.5 in order to reduce salt concentration to a minimum. Afterward
acetonitrile (ACN, MS grade) and formic acid were added to a final
concentration of 25% (v/v) ACN and 0.05% (v/v) formic acid. Sample
introduction (1 lL sample volume contains about 0.3 lg of protein,
taken from 20 lL total sample volume) was done by using a nanospray robotic device (Nanomate, Advion Biosciences).
Comp