Retinal biosynthesis in Eubacteria: In vitro characterization of a novel carotenoid oxygenase from Synechocystis sp. PCC 6803

Retinal and its derivatives represent essential compounds in many biological systems. In animals, they are synthesized through a symmetrical cleavage of β-carotene catalysed by a monooxygenase. Here, we demonstrate that the open reading frame sII1541 from the cyanobacterium Synechocystis sp. PCC 6803 encodes the first eubacterial, retinal synthesizing enzyme (Diox1) thus far reported. In contrast to enzymes from animals, Diox1 converts β-apo-carotenals instead of β-carotene into retinal in vitro. The identity of the enzymatic product was proven by HPLC, GC-MS and in a biological test. Investigations, of the stereospecifity showed that Diox1 cleaved only the all-trans form of β-apo-8′-carotenal, yielding all-tans-retinal. However, Diox1 exhibited wide substrate specificity with respect to chain-lengths and functional end-groups. Although with divergent K_m and V_max values, the enzyme converted β-apo-carotenals, (3R)-3-OH-β-apo-carotenals as well as apo-lycopenals into retinal, (3R)-3-hydroxy-retinal and acycloretinal respectively. In addition, the alcohols of these substrates were cleaved to yield the corresponding retinal derivatives.