The substrate specificity of β,β-carotene 15,15′-monooxygenase

The synthesis of several substrate analogues of the enzyme beta,beta -carotene 15,15'-monooxygenase is reported. The substrate specificity of enriched enzyme fractions isolated from chicken intestinal mucosa was investigated. Regarding substrate binding/cleavage, these experiments demonstrate that i) any deviation from the 'rod-like' beta,beta -carotene structure is not tolerated, ii) one 'natural', unsubstituted beta -ionone ring is required, iii) the position and presence of the Me groups attached to the polyene chain is significant. These results suggest a hydrophobic barrel-like substrate binding site in which the protein's amino acid residues through interaction with the Me groups, direct the central C=C bond in binding distance to the active site's metal-oxo center, supporting the unique regiospecificity of cleavage to retinal (provitamin A).