A chymotrypsin from the P. californiensis digestive gland was
purified to homogeneity as observed after electrophoresis, this
enzyme has a molecular mass of 35.7 kDa (Fig. 1A, lane 1); when
the shrimp chymotrypsin was reduced with DTT and boiled, the
protein migrated with a mass band of 27 kDa (Fig. 1A, lane 2).
Three different clones were found; they were composed of
770 bp that coded for 256 amino acids. Clone Ch_20 (GenBank
ID: JX889717) was used in further comparisons because the insert
was confirmed from the analysis of many plasmids. Variations of
the other two isoforms were semi-conserved; the deduced chymotrypsinogen
was identified as Chymo_P.cal. The deduced amino
acid sequence was confirmed by the N-terminal (seven residues)
and mass spectrometry of the purified shrimp chymotrypsin (79
residues in six peptides covering 40% of the deduced sequence;
Fig. 2, dotted boxes). Results from mass spectrometry could not
differentiate between the nucleotide sequences that were
obtained. When the molecular mass was derived from the deduced
primary structure, the result was 23.72 kDa. Differences between
theoretical weight (23.7 kDa) and mobility in electrophoresis
(27 kDa) could be related to post-traductional modification, such
as glycosylation predicted by the public NetOGlyc software
(Shirai, 2008) at residues Thr135 and Thr137 (Fig. 2, residues double
underlined). The molecular weight difference with unreduced
pure shrimp chymotrypsin (35.7 kDa) could be explained to conformation
changes between folded and unfolded proteins. Bovine
chymotrypsin has a basic pI of 8.7, whereas shrimp chymotrypsin,
like most crustacean serine proteases, has an exceptionally acidic
pI of 3.6 (Table 1). Only one protein was obtained by the isoelectric
point, which supports the idea that a unique isoform was purified.
Although pure shrimp chymotrypsin was stored at 20 C in 50 ll
samples to avoid thawing and refreezing, two proteins appeared
after five months (Fig. 1C, lane 1), probably as a result of autohydrolysis
or conformations changes. However, both proteins had
proteolytic activity (Fig. 4, insert).