Abstract
In order to examine the structure–activity relationship and the substrate specificity of human
d-amino acid oxidase (h.DAO), a single amino acid mutation had been established as proline-219-luecine (P-219-L). The gene encoding mutant h.DAO has been cloned and expressed in
Escherichia coli BL21 (DE3). It was observed that the host cell was negatively affected by the expressed mutant h.DAO, resulting in a remarkable decrease in the cell growth and consequently the amount of the produced enzyme. To overcome this problem, we investigated several factors that may affect the cell growth rate and the mutant h.DAO production such as optimization of the glucose concentration as a main carbon source and the yeast extract concentration as a main nitrogen source, optimization of dissolved oxygen (DO%) concentration and the addition of benzyl alcohol (BA, which can artificially induce a strong heat shock response at low temperature), to enhance the production of natively folded soluble fraction of the recombinant protein. These parameters were tested on both shake flask level and fed-batch bioreactor level. The Western blot analysis and the enzyme activity assay indicated the higher level of the mutant expression towards enhancement of the conditions by using our designed approach.
The specific activity (which was used as an indicator for the level of the desired protein produced
=
U/mg protein) and the OD
600
nm
of the host cells (which was used as an indicator for the cell growth), reached to be 0.061
U/mg protein and 3.44, respectively upon using fed-batch culture system containing the optimized medium composition (15
g/l glucose and 5
g/l yeast extract). While upon using the shake flask level, these values were 0.032 and 1.1, respectively. Enhancement of the cell growth and the enzyme production was noticed after DO% optimization upon using 500
rpm agitation speed and 1.8
v.v.m. (volume volume minute) aeration. The specific activity for the mutant enzyme and the OD
600
nm
of the host cells reached to be 0.14
U/mg protein and 7.1, respectively. Finally upon using the optimized culture composition (15
g/l glucose and 5
g/l yeast extract), optimized DO% (using 500
rpm agitation speed and 1.8
v.v.m.) and 0.1
mM BA at the fed-batch bioreactor level, the specific activity and the OD
600
nm
of the host cells increased significantly to be 0.21
U/mg protein and 11.3, respectively at 24
h culture. These results indicate the importance of our approaches to overproducing mutant h.DAO in soluble form in
E. coli.