In vitro characterization of astaxanthin biosynthetic enzymes.

Escherichia coli strains expressing the marine bacteria (Agrobacterium aurantiacum and Alcaligenes sp. strain PC-1) astaxanthin biosynthetic genes (crtZ and W), Haematococcus pluvialis bkt, and Erwinia uredovora crtZ genes were used for in vitro characterization of the respective enzymes. Specific enzyme assays indicated that all of the enzymes are bifunctional, in that the CrtZ enzymes formed zeaxanthin from beta-carotene via beta-cryptoxanthin, as well as astaxanthin from canthaxanthin via phoenicoxanthin (adonirubin). The BKT/CrtW enzymes synthesized canthaxanthin via echinenone from beta-carotene and 4-ketozeaxanthin (adonixanthin) with trace amounts of astaxanthin from zeaxanthin. Comparison of maximum catalytic activities as well as selectivity experiments carried out in the presence of both utilizable substrates indicated that the CrtZ enzymes from marine bacteria converted canthaxanthin to astaxanthin preferentially, whereas the Erwinia CrtZ possessed a favorability to the formation of zeaxanthin from beta-carotene. The CrtW/BKT enzymes were not so defined in their substrate preference, responding readily to fluctuations in substrate levels. Other properties obtained indicated that the enzymes were strictly oxygen-requiring; and a cofactor mixture of 2-oxoglutarate, ascorbic acid, and Fe2+ was beneficial to activity. Based on enzymological data, a predicted pathway for astaxanthin biosynthesis is described, and it is proposed that CrtZ-like enzymes be termed carotenoid 3, (3')-beta-ionone ring hydroxylase and CrtW/BKT carotenoid 4, (4')-beta-ionone ring oxygenase.