|
type |
Journal Article |
authors |
Gu, W.; Zhao, G.; Eddy, C.; Jensen, R. A. |
title |
Imidazole acetol phosphate aminotransferase in Zymomonas mobilis: molecular genetic, biochemical, and evolutionary analyses |
journal |
J Bacteriol |
Activity |
2.6.1.9 |
Family |
2.6.1.9 |
sel |
selected |
ui |
7883715 |
year |
(1995) |
volume |
177 |
number |
6 |
pages |
1576-84 |
| |
keywords |
Amino Acid Sequence |
abstract |
hisH encodes imidazole acetol phosphate (IAP) aminotransferase in Zymomonas mobilis and is located immediately upstream of tyrC, a gene which codes for cyclohexadienyl dehydrogenase. A plasmid containing hisH was able to complement an Escherichia coli histidine auxotroph which lacked the homologous aminotransferase. DNA sequencing of hisH revealed an open reading frame of 1,110 bp, encoding a protein of 40,631 Da. The cloned hisH product was purified from E. coli and estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis to have a molecular mass of 40,000 Da. Since the native enzyme had a molecular mass of 85,000 Da as determined by gel filtration, the active enzyme species must be a homodimer. The purified enzyme was able to transaminate aromatic amino acids and histidine in addition to histidinol phosphate. The existence of a single protein having broad substrate specificity was consistent with the constant ratio of activities obtained with different substrates following a variety of physical treatments (such as freeze-thaw, temperature inactivation, and manipulation of pyridoxal 5'-phosphate content). The purified enzyme did not require addition of pyridoxal 5'-phosphate, but dependence upon this cofactor was demonstrated following resolution of the enzyme and cofactor by hydroxylamine treatment. Kinetic data showed the classic ping-pong mechanism expected for aminotransferases. Km values of 0.17, 3.39, and 43.48 mM for histidinol phosphate, tyrosine, and phenylalanine were obtained. The gene structure around hisH-tyrC suggested an operon organization. The hisH-tyrC cluster in Z. mobilis is reminiscent of the hisH-tyrA component of a complex operon in Bacillus subtilis, which includes the tryptophan operon and aroE.(ABSTRACT TRUNCATED AT 250 WORDS) |
last changed |
2009/07/07 13:54 |
|