Enzyme Technology
Production of amino acids
Another early application of an immobilised
enzyme was the use of the aminoacylase from Aspergillus oryzae to resolve
racemic mixtures of amino acids.
[5.4]
Chemically synthesised racemic N-acyl-DL-amino acids are hydrolysed
at pH 8.5 to give the free L-amino acids plus the
unhydrolysed N-acyl-D-amino acids. These products are easily separated by
differential crystallisation and the N-acyl-D-amino acids racemised
chemically (or enzymically) and reprocessed. The enzyme is immobilised by
adsorption to anion exchange resins (e.g., DEAE-Sephadex) and has an operational
half-life of about 65 days at 50°C in PBRs with residence times of about 30
min. The reactors may be re-activated in situ by simply adding more enzyme. The
immobilised enzyme has proved a more economical process than the use of free
enzyme mainly due to the more efficient use of the substrate and reductions in
the cost of enzyme and labour.
Novel and natural L-amino acids can be
produced by the chemical conversion of aldehydes through DL-amino nitrites to
racemic DL-hydantoins (reaction scheme [5.5]) followed by enzymic hydrolysis
with hydantoinase and a carbamoylase (reaction scheme [5.6]) at pH 8.5. Both
enzymes may be obtained from Arthrobacter species.
D -Amino acids are important constituents in
antibiotics and insecticides. They may be produced in a manner similar to the L-amino acids but using hydantoinases of differing specificity. The
Pseudomonas
striata enzyme is specific for D-hydantoins, allowing their specific hydrolysis
to D-carbamoyl amino acids which can be converted to the D-amino acids by
chemical treatment with nitrous acid. They remaining L-hydantoin may be simply
racemised by base and the process repeated.
[5.5]
[5.6]
L -Aspartic acid is widely used in the food
and pharmaceutical industries and is needed for the production of the low
-calorific sweetener aspartame. It may be produced from fumaric acid by the use
of the aspartate ammonia-lyase (aspartase) from Escherichia coli.
aspartate ammonia-lyase
−OOCCH=CHCOO− + NH4+
−OOCCH2CH(NH3+)COO−
[5.7]
fumaric
acid
L-aspartic
acid
A crude immobilised aspartate ammonia-lyase
(50000 U g−1) may be prepared by entrapping Escherichia coli cells in a
k -carageenan
gel crosslinked with glutaraldehyde and hexamethylenediamine. The process is
operated in a PBR at pH 8.5 using ammonium fumarate as the substrate, with a
reported operational half-life of 680 days at 37°C.
Urocanic acid is a sun-screening agent which
may be produced from L-histidine by the histidine ammonia -lyase (histidase)
from Achromobacter liquidum (see reaction scheme [1.4]). The organism
cannot be used directly as it has urocanate hydratase activity, which removes
the urocanic acid. However, a brief heat treatment (70°C, 30 min) inactivates
this unwanted activity but has little effect on the histidine ammonia-lyase. A
crude immobilised-enzyme preparation consisting of heat -treated cells
entrapped in a polyacrylamide gel has been used to effect this conversion,
showing a half-life of 180 days at 37°C.
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This page was established in 2004 and last updated by Martin
Chaplin
on
20 July, 2015
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