Comparative Analysis of
β-Carotene Hydroxylase
Genes for Astaxanthin Biosynthesis
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Abstract
Astaxanthin (3,3′-dihydroxy-4,4′-diketo-β-carotene)
(<b>1</b>) is a carotenoid of significant commercial value due
to its superior antioxidant potential, application as a component
of animal feeds, and ongoing research that links its application to
the treatment and prevention of human pathologies. The high commercial
cost of <b>1</b> is also based upon its complex synthesis. Chemical
synthesis has been demonstrated, but produces a mixture of stereoisomers
with limited applications. Production from biological sources is limited
to natural producers with complex culture requirements. The biosynthetic
pathway for <b>1</b> is well studied; however, questions remain
that prevent optimized production in heterologous systems. Presented
is a direct comparison of 12 β-carotene (<b>2</b>) hydroxylases
derived from archaea, bacteria, cyanobacteria, and plants. Expression
in <i>Escherichia coli</i> enables a comparison of catalytic
activity with respect to zeaxanthin (<b>3</b>) and <b>1</b> biosynthesis. The most suitable β-carotene hydroxylases were
subsequently expressed from an efficient dual expression vector, enabling <b>1</b> biosynthesis at levels up to 84% of total carotenoids. This
supports efficient <b>1</b> biosynthesis by balanced expression
of β-carotene ketolase and β-carotene hydroxylase genes.
Moreover, our work suggests that the most efficient route for astaxanthin
biosynthesis proceeds by hydroxylation of β-carotene to zeaxanthin,
followed by ketolation