Mechanism and stereochemistry of the porphobilinogen deaminase and protoporphyrinogen IX oxidase reactions: stereospecific manipulation of hydrogen atoms at the four methylene bridges during the biosynthesis of haem.

Abstract

[2,11S-3H2;2,11-14C2] Porphobilinogen (PBG) with an enantiomeric excess greater than 81 % was prepared in a coupled enzyme system from [2RS-3H2;2-14C2]glycine. Incorporation of this chiral PBG into haem using a broken cell enzyme system from chicken blood showed retention of only one tritium per haem molecule. Degradation of the haem showed that tritium had been incorporated specifically into the C-10 position of haem. Control experiments with [11RS-3H2;2,11-14C2]-PBG showed an equal distribution of tritium at all four meso positions of haem. The implications of this result are discussed in terms of the mechanism of two enzymes in haem biosynthesis: porphobilinogen deaminase (hydroxymethylbilane synthase) and protoporphyrinogen IX oxidase.