This study expressed a mutant form of the Acinetobacter baumannii β-lactamase ADC-7 in E. coli to analyze its function. Plasmid DNA was purified and verified to contain the correct mutant sequence. The mutant enzyme was expressed in E. coli and purified using nickel affinity chromatography. Kinetic assays showed the mutant had lower catalytic activity and efficiency than wild-type ADC-7, indicating the arginine to glutamic acid mutation decreased ADC-7 function by altering substrate binding. This verifies the important role of this amino acid position in ADC-7.

1. Recombinant expression & biochemical analysis of a mutant
ADC-7 β-lactamase in E. coli
Abstract
The overuse of antibiotics has resulted in the emergence of deadly, multi-drug resistant
bacteria. Infections caused by Acinetobacter baumannii are currently at a serious threat
level for hospital and community settings. This bacteria utilizes a class C β-lactamase
called ADC-7 to digest carbapenem antibiotics. For this study, a mutant form of ADC-7
was expressed in BL21(DE3) E. coli using a pET28a plasmid and T7 expression
system. Agarose gel electrophoresis was used to verify the plasmid structure was
correct by running the restriction digest and PCR products. Results from the
chromatogram showed arginine 148 (R148E) mutated to glutamic acid. The mutant
enzyme was purified using nickel affinity chromatography, and expression was verified
by SDS-PAGE and western blot. Kinetics of the wild-type and mutant were compared
via a CENTA assay. The kcat and kcat /KM of the mutant was lower than the wild-type
enzyme, indicating the mutation decreased the function of ADC-7. This also verifies
the role R148E plays in binding to the substrate, CENTA, and may be a useful target for
chemotherapy in Acinetobacter baumannii infections.
Introduction
Acinetobacter baumannii produces ADC-7, a class C β-lactamase enzyme responsible
for resistance against β-lactam antibiotics such as carbapenems and cephalosporins. As
this is a deadly adaptation, it is crucial to understand the biochemical functions of
ADC-7. For this study, a mutant form of ADC-7 was expressed in E. coli and purified
using nickel chromatography.
GOALS:
• Verify the correct plasmid was obtained from site-directed metagenesis/plasmid
purification by restriction enzyme digest and polymerase chain reaction (PCR)
• Transform competent BL21(DE3) E. coli cells with the plasmid to express a mutant
form of ADC-7
• Purify mutant ADC-7 and compare the kinetics with wild-type ADC-7
Results
Discussion and conclusion
References and Acknowledgements
Jeannie Kane
Department of Chemistry
Grand Valley State University, Allendale, MI 49401
Figure 2. Chromatograms of wild-type ADC-7 (left) and mutant
ADC-7 (right). Results from the chromatogram revealed arginine 148
(R148E) replaced with a glutamic acid residue.
Figure 3. Results for DNA analysis. (A) DNA gel for mutant ADC-7 PCR product
and digest of the pET28a-ADC-7 plasmid with the restriction enzymes AvaI and
BgIll. Based on the fragment results in table 2, the gel verified the plasmid structure
was correct. The number of base pairs for the PCR product was also very close to the
expected at 1055 base pairs. (B) Table 1 displays the migration distances for the base
pair standards and log(bp) that were used to construct a standard curve. (C) Plot of
log(bp) vs migration distance to obtain the standard curve equation y = −0.151x +
4.7445. This was used to calculate the number of base pairs in each fragment on the
gel.
Table 2. Calculated number of base pairs for fragments
from restriction enzyme digests and PCR product.
Figure 1. Graphic map of the pET28a-ADC-7 plasmid with AvaI and
BgIll restriction sites. The pET28a plasmid with T7 expression system was
used to express mutated ADC-7 in BL21(DE3) E. coli cells. Important
features include the T7 promotor, lacO, lacI, ribosome binding site (RBS),
cut sites for AvaI and BgIll, kanamycin resistance gene (KanR), ColE1
origin, and the mutated ADC-7 DNA fragment.
Sample calculation for number of base pairs of
fragment 1 for AvaI:
10(−0.151(6.7))+4.7445 = 5405 base pairs
0 .0 0 .1 0 .2 0 .3
0
2 0
4 0
6 0
8 0
1 0 0
[C E N T A ],1in 1m M
v
o
1
(
µ
M
/m
in
)
Table 3. Comparison of kinetics for the
mutant and wild-type ADC-7.
Figure 6. Kinetic assays for the mutant and wild-
type ADC-7. Mutant and wild-type ADC-7 kinetics
were determined via the CENTA assay. The wild-type
had a higher Vmax compared to the mutant.
Figure 5. Results for protein purification. (A) Samples in the SDS-PAGE lanes from left to right:
Precision Plus Protein Dual Color Standards, Noninduced, 15 min IPTG, 30 min IPTG, 45 min IPTG, 1
hr IPTG, 2 hr IPTG (diluted CFE lysate), Ni2+ column load eluent, pooled fractions ADC-7 mutant,
pooled fractions ADC-7 (1/2 amount). The molecular weight of mutant ADC-7 was determined to be
37.3 kDa from the standard equation y = 0.0784x + 1.4305. (B) The western blot confirms the presence
of the His-tag on the protein throughout the purification. Samples in lanes from left to right: Precision
Plus Protein Dual Color Standards from Biorad, Noninduced, 15 min IPTG, 30 min, 45 min, 1 hr, 2 hr
(diluted CFE – lysate), Ni2+ column load eluent, Ni2+ column wash eluent, pooled fractions (ADC-7
mutant), ½ amount loaded in lane 9.
(A) (B)
(C)
Pemberton,*O.*A.,*Noor,*R.*E.,*Kumar*M*V,*V.,*Sanishvili,*R.,*Kemp,*M.*T.,*Kearns,*F.*L.,*.*.*.*Chen,*Y.*(2020).*Mechanism*of*proton*transfer*in*class*A*βI
lactamase*catalysis*and*inhibition*by*avibactam. Proceedings+of+the+National+Academy+of+Sciences+of+the+United+States+of+America, 117(11),*5818I5825.*
doi:10.1073/pnas.1922203117
Magdeldin,*S.*(Ed.).*(2012). Affinity+chromatography.*BoD–Books*on*Demand.
Sample calculation for kcat of mutant
ADC-7:
kcat=
./01
[3456/3]
=
8.98
:;
<
9.9==8>;
= ?9. @ <AB
Figure 4. Diagram of nickel in
affinity chromatography column
binding to a poly-histidine tag.
Mutant ADC-7 containing a poly-
histidine tag was was purified in one
step using nickel affinity
chromatography.
(A) (B)
C
HC
NH3
+
O-
O
C
HC
NH3
+
O-
O
NH
NH2
NH2
+
O-
O
• The mutation from a basic amino acid side chain to an
acidic amino acid side chain decreased the function of
ADC-7
• Wild-type ADC-7 had a higher kcat than the mutant
(85.6 s-1 vs 60.8 s-1)
• Wild-type ADC-7 also had a higher catalytic efficiency
(607 s-1/µM vs 259 s-1/µM in the mutant)
• R148E plays a role in substrate binding
Figure 7. Arginine (top) vs glutamic
acid (bottom) altered ADC-7 function.
Glutamic acid is shorter in length and is
acidic compared to arginine which has a
longer chain and is basic. The difference
in these properties altered ADC-7 activity.