Seeing things as they really are: The enzyme catalyzing the central cleavage of β‐carotene (1) to retinal (2) is not, as previously thought, a dioxygenase. Incubation of the substrate analogue α‐carotene in the presence of highly enriched 17O2 and H218O revealed a monooxygenase mechanism.

Ein kleiner Paradigmenwechsel: Das Enzym, das die zentrale Spaltung von β‐Carotin 1 zu Retinal 2 katalysiert, ist entgegen der bisherigen Annahme keine Dioxygenase. Die Ergebnisse der Inkubation des Substratanalogons α‐Carotin in Gegenwart von stark angereichertem 17O2 und H218O belegen einen Monooxygenase‐Mechanismus.

Porphobilinogen synthase condenses two molecules of 5-aminolevulinate in an asymmetric way. This unusual transformation requires a selective recognition and differentiation between the substrates ending up in the A site or in the P site of porphobilinogen synthase. Studies of inhibitors based on the key intermediate first postulated by Jordan allowed differentiation of the two recognition sites. The P site, whose structure is known from X-ray crystallographic studies, tolerates ester functions well. The A site interacts very strongly with nitro groups, but is not very tolerant to ester functions. This differentiation is a central factor in the asymmetric handling of the two identical substrates. Finally, it could be shown that the keto group of the substrate bound at the A site is not only essential for the recognition, but that an increase in electrophilicity of the carbon atom also increases the inhibition potency considerably. This has important consequences for the recognition process at the A site, whose exact structure is not yet known.

Porphobilinogen synthase condenses two molecules of 5-aminolevulinate in an asymmetric way. This unusual transformation requires a selective recognition and differentiation between the substrates ending up in the A site or in the P site of porphobilinogen synthase. Studies of inhibitors based on the key intermediate first postulated by Jordan allowed differentiation of the two recognition sites. The P site, whose structure is known from X-ray crystallographic studies, tolerates ester functions well. The A site interacts very strongly with nitro groups, but is not very tolerant to ester functions. This differentiation is a central factor in the asymmetric handling of the two identical substrates. Finally, it could be shown that the keto group of the substrate bound at the A site is not only essential for the recognition, but that an increase in electrophilicity of the carbon atom also increases the inhibition potency considerably. This has important consequences for the recognition process at the A site, whose exact structure is not yet known.