Abstract
Many organisms have genes to protect themselves from toxic conditions such as high ethanol and/or ammonia concentrations. When a high ethanol condition is induced to Zymomonas mobilis ZM4, a representative ethanologenic organism, this bacterium overexpresses several genes to overcome this ethanol stress. Among them, we characterized a gene product annotated as an arginase (zmARG) from Z. mobilis ZM4. Even though all of the arginase-determining sequence motifs are not strictly conserved in zmARG, this enzyme converts L-arginine to urea and L-ornithine in the presence of a divalent manganese ion. The revealed high-resolution crystal structure of zmARG shows that it has a typical globular α/β arginase fold with a protruded C-terminal helix. Two zinc ions reside in the active site, where one metal ion is penta-coordinated and the other has six ligands, discerning this zmARG from the reported arginases with two hexa-liganded metal ions. zmARG forms a dimeric structure in solution as well as in the crystalline state. The dimeric assembly of zmARG is formed mainly by interaction formed between the C-terminal α-helix of one molecule and the α/β hydrolase fold of another molecule. The presented findings demonstrate the first reported dimeric arginase formed by the C-terminal tail and has two metal ions coordinated by different number of ligands.
Highlights
IntroductionL-ornithine is further converted into citrulline through the catalytic activity of transcarbamoylase in the urea cycle ( known as the ornithine cycle) that removes and excrete the highly toxic ammonia outside the organism
Arginine catabolism generates two metabolic intermediates, L-ornithine and urea
We selected a gene annotated as an arginase and two referenced genes of a pyruvate dehydrogenase E1 component beta subunit and a CRISPR-associated protein Csy3 family and made their expression constructs for E. coli
Summary
L-ornithine is further converted into citrulline through the catalytic activity of transcarbamoylase in the urea cycle ( known as the ornithine cycle) that removes and excrete the highly toxic ammonia outside the organism. A deficiency of the arginase activity in human results in Argininemia, a disease caused by accumulation of arginine and ammonia in the blood, resulting in the development of neurological problems and other symptoms (Wong et al, 1993). Known arginases commonly have a typical α/β hydrolase fold and a conserved two metal ion-binding sequence (Kanyo et al, 1996). They are speculated to be an ancestor enzyme of histone deacetylases and polyamine deacetylases (Hai et al, 2017)
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