Hydrolases in Polymer Chemistry: Part III: Synthesis and Limited Surface Hydrolysis of Polyesters and Other Polymers - Enzymatic Polymerisation - page 117

higher dyebath exhaustion for reactive and acid dyes. PA surface hydrolysis was
demonstrated by using FTIR analysis based on changes in the 3300-3500, 1533,
1657 and 1000-1300 cm − 1 areas [ 19 ]. Based on DSC studies, no changes in glass
transition and melting points were observed upon limited enzyme hydrolysis. Other
authors have quantified the release of monomeric and oligomeric reaction products
resulting during hydrolysis of PA with a protease Bacillus subtilis . Again, treat-
ment with this enzyme led to increased hydrophilicity and enhanced binding of
reactive dyes [ 22 ]. Hydrolysis of PA monofilaments with aspartic protease Protease
M, cysteine protease Bromelain and metallo-protease Corolase N was confirmed by
titration of generated carboxylic end-groups [ 21 ] .
Apart from proteases, cutinases and lipases were demonstrated to hydrolyse PA.
Hydrolysis of PA with a lipase (not further specified) was confirmed by using FTIR
analysis and dye-binding assays [ 32 ]. Several reports have assessed the potential of
cutinases for PA functionalisation, and a fungal cutinase from F. solani was suc-
cessfully genetically engineered towards higher activity on a PA oligomer and PA
[ 23 - 25 ].
Production of amidases capable of hydrolysing PA was recently reported both
for fungi and bacteria. An amidase from the fungus Beauveria brongniartii in-
creased the hydrophilicity of PA6, resulting in a reduction of the drop dissipation
time from 60 to 7 s after 60 min incubation, while the surface tension

increased
after 3 min of enzyme treatment from 46.1 to 67.4 mNm [ 33 ]. This 55 kDa amidase
hydrolysed both aliphatic and aromatic amines but did not show protease activity.
Similarly, a bacterial amidase from Nocardia farcinica with PA hydrolase activity
did not show protease activity [ 20 ] . Again, considerable hydrophilicity increases
of PA were measured, based on rising height and tensiometry measurements after
treatment with this enzyme. The N. farcinica polyamidase belongs to the amidase
signature family and, consequently, hydrolysed various small amides and esters in-
cluding p -nitroacetanilide and p -nitrophenylbutyrate [ 34 ] .
Unlike serine proteases, lipases and esterases, which are all characterised by
the catalytic triad Ser-His-Asp, the catalytic reaction of this polyamidase involves
the Ser-Ser-Lys triad [ 35 , 36 ] . Interestingly, individual representatives of the ami-
dase signature family enzymes show very distinct substrate specificities, which
could be due to binding of the substrate by residues outside the signature se-
quence [ 35 , 36 ]. Surprisingly, several closely related amidases (based on amino acid
sequences) within the amidase signature family did not hydrolyse PA but were re-
ported to degrade cyclic nylon oligomers [ 37 , 38 ] (Fig. 1 ) . For PA oligomer hydrol-
ysis by Arthrobacter sp. KI72 and Pseudomonas sp. NK87, a 6-aminohexanoate-
cyclic-dimer hydrolase (EI), a 6-aminohexanoate-dimer hydrolase (EII) and an
endo -type 6-aminohexanoate-oligomer hydrolase (EIII) have been described [ 27 ] .
EIII hydrolyses the cyclic tetramer and dimer as well as linear oligomers endo -wise
[ 37 ]. Like the N. farcinica polyamidase, the cyclic dimer hydrolases belonged to the
amidase signature family whereas the linear dimer hydrolase (EII) activity evolved
in an esterase with

σ

-lactamase folds. In contrast, the endo -acting EIII showed the
lowest homology to the N. farcinica polyamidase [ 20 , 27 ] .

β