Histidine ammonia-lyase (EC 4.3.1.3, histidase, histidinase) is an
enzyme
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...
that in humans is encoded by the ''HAL''
gene
In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...
.
It converts
histidine
Histidine (symbol His or H) is an essential amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated –NH3+ form under biological conditions), a carboxylic acid group (which is in the ...
into
ammonia
Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous ...
and
urocanic acid. Its systematic name is
L-histidine ammonia-lyase (urocanate-forming).
Function
Histidine ammonia-lyase is a cytosolic enzyme catalyzing the first reaction in histidine catabolism, the nonoxidative
deamination
Deamination is the removal of an amino group from a molecule. Enzymes that catalyse this reaction are called deaminases.
In the human body, deamination takes place primarily in the liver, however it can also occur in the kidney. In situations of ...
of
L-histidine to ''trans''-urocanic acid.
The reaction is catalyzed by
3,5-dihydro-5-methyldiene-4''H''-imidazol-4-one, an electrophilic co-factor which is formed autocatalytically by cyclization of the protein backbone of the enzyme.
Pathology
Mutations in the gene for histidase are associated with
histidinemia and
urocanic aciduria.
References
Further reading
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External links
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EC 4.3.1
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