The de novo biosynthesis of arginine in microorganisms and plants is accomplished via several enzymatic steps. The enzyme N-acetyl glutamate kinase (NAGK) catalyzes the phosphorylation of the gamma-COO- group of N-acetyl-L-glutamate (NAG) by adenosine triphosphate (ATP) which is the second rate limiting step in arginine biosynthesis pathway. Here we report the crystal structure of putative N-acetyl glutamate kinase (NAGK) from Therm us thermophilus HB8 (TtNAGK) determined at 1.92 angstrom resolution. The structural analysis of TtNAGK suggests that the dimeric quaternary state of the enzyme and arginine insensitive nature are similar to mesophilic Escherichia coli NAGK. These features are significantly different from its thermophilic homolog Thermatoga maritima NAGK which is hexameric and arginine-sensitive. TtNAGK is devoid of its substrates but contains two sulfates at the active site. Very interestingly the active site of the enzyme adopts a conformation which is not completely open or closed and likely represents an intermediate stage in the catalytic cycle unlike its structural homologs, which all exist either in the open or closed conformation. Engineering arginine biosynthesis pathway enzymes for the production of L-arginine is an important industrial application. The structural comparison of TtNAGK with EcNAGK revealed the structural basis of thermostability of TtNAGK and this information could be very useful to generate mutants of NAGK with increased overall stability. (C) 2012 Elsevier Inc. All rights reserved.