Acetolactate synthase
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(Bacterial acetolactate synthase)-like
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| Identifiers | |
| Symbol | ILVBL |
| Entrez | 10994 |
| HUGO | 6041 |
| OMIM | 605770 |
| RefSeq | NM_176826 |
| UniProt | Q99651 |
| Other data | |
| EC number | 2.2.1.6 |
| Locus | Chr. 19 p13.1 |
The acetolactate synthase (ALS) enzyme (also known as acetohydroxy acid synthase, or AHAS) is the first step in the synthesis of the branched-chain amino acids (valine, leucine, and isoleucine).
Inhibitors of ALS are used as herbicides that slowly starve affected plants of these amino acids, which eventually leads to inhibition of DNA synthesis. They affect grasses and dicots alike. The ALS inhibitor family includes sulfonylureas (SUs), imidazolinones (IMIs), triazolopyrimidines (TPs), pyrimidinyl oxybenzoates (POBs), and sulfonylamino carbonyl triazolinones (SCTs).
[edit] Regulation
Acetolactate synthase consists of three pairs of subunits. Each pair includes a large subunit, which is thought to be responsible for catalysis, and a small subunit for feedback inhibition. Each subunit pair is located on its own operon. Together, these operons code for several enzymes involved in branched-chain amino acid biosynthesis. Regulation is different for each operon.
The ilvGMEDA operon (encoding ALS II, branched-chain-amino-acid transaminase, dihydroxy-acid dehydratase, and threonine ammonia-lyase) is regulated by feedback inhibition in the form of transcriptional attenuation. That is, transcription is reduced in the presence of the pathway's end products, the branched-chain amino acids.
The ilvBNC operon, which encodes ALS I and a ketol-acid reductoisomerase, is similarly regulated, but is specific to isoleucine and leucine; valine does not affect it directly.
Both the ilvGMEDA and ilvBNC operons are derepressed during shortages of the branched-chain amino acids by the same mechanism that represses them. Both of these operons as well as the third, ilvIH, are regulated by leucine-responsive protein (Lrp).
[edit] External links
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