Review

Conversion of β-carotene into astaxanthin: Two separate enzymes or a bifunctional hydroxylase-ketolase protein?

Juan F Martín^1,2 , Eduardo Gudiña^1,4 and José L Barredo³

¹  Institute of Biotechnology of León (INBIOTEC), Science Park, Av. Real 1, 24006, León, Spain

²  University of León, Campus de Vegazana s/n, 24071, León, Spain

³  R&D Biology, Antibióticos S.A., Avenida de Antibióticos, 59-61, 24009, León, Spain

⁴  Lab. de Ciência e Tecnologia Alimentar. Departamento de Engenharia Biológica, Universidade do Minho, Campus de Gualtar, 4710-057, Braga, Portugal

author email corresponding author email

Microbial Cell Factories 2008, 7:3doi:10.1186/1475-2859-7-3

Published:	20 February 2008

Abstract

Astaxanthin is a xanthophyll of great interest in animal nutrition and human health. The market prospect in the nutraceutics industries for this health-protective molecule is very promising. Astaxanthin is synthesized by several bacteria, algae and plants from β-carotene by the sequential action of two enzymes: a β-carotene, 3,3'-hydroxylase that introduces an hydroxyl group at the 3 (and 3') positions of each of the two β-ionone rings of β-carotene, and a β-carotene ketolase that introduces keto groups at carbons 4 and 4' of the β-ionone rings. Astaxanthin is also produced by the yeast-like basidiomycete Xanthophyllomyces dendrorhous. A gene crtS involved in the conversion of β-carotene to astaxanthin has been cloned simultaneously by two research groups. Complementation studies of X. dendrorhous mutants and expression analysis in Mucor circinelloides reveals that the CrtS enzyme is a β-carotene hydroxylase of the P-450 monooxygenase family that converts β-carotene to the hydroxylated derivatives β-cryptoxanthin and zeaxanthin, but it does not form astaxanthin or the ketolated intermediates in this fungus. A bifunctional β-carotene hydroxylase-ketolase activity has been proposed for the CrtS protein. The evidence for and against this hypothesis is analyzed in detail in this review.