Affiliation:
1. Instituto de Biomedicina de Valencia (IBV-CSIC), Valencia, Spain
Abstract
ABSTRACT
To help clarify the control of arginine synthesis in
Thermotoga maritima
, the putative gene (
argB
) for
N
-acetyl-
l
-glutamate kinase (NAGK) from this microorganism was cloned and overexpressed, and the resulting protein was purified and shown to be a highly thermostable and specific NAGK that is potently and selectively inhibited by arginine. Therefore, NAGK is in
T. maritima
the feedback control point of arginine synthesis, a process that in this organism involves acetyl group recycling and appears not to involve classical acetylglutamate synthase. The inhibition of NAGK by arginine was found to be pH independent and to depend sigmoidally on the concentration of arginine, with a Hill coefficient (
N
) of ∼4, and the 50% inhibitory arginine concentration (I
0.5
) was shown to increase with temperature, approaching above 65°C the I
0.50
observed at 37°C with the mesophilic NAGK of
Pseudomonas aeruginosa
(the best-studied arginine-inhibitable NAGK). At 75°C, the inhibition by arginine of
T. maritima
NAGK was due to a large increase in the
K
m
for acetylglutamate triggered by the inhibitor, but at 37°C arginine also substantially decreased the
V
max
of the enzyme. The NAGKs of
T. maritima
and
P. aeruginosa
behaved in gel filtration as hexamers, justifying the sigmoidicity and high Hill coefficient of arginine inhibition, and arginine or the substrates failed to disaggregate these enzymes. In contrast,
Escherichia coli
NAGK is not inhibited by arginine and is dimeric, and thus the hexameric architecture may be an important determinant of arginine sensitivity. Potential thermostability determinants of
T. maritima
NAGK are also discussed.
Publisher
American Society for Microbiology
Subject
Molecular Biology,Microbiology
Cited by
47 articles.
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