PubMed · 10543118
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T Yamaguchi, A Sugimoto. 1999. [Biliverdin].. https://pubmed.ncbi.nlm.nih.gov/10543118/
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The origin of the unusual regioselectivity of heme oxygenation, i.e. the oxidation of heme to delta-biliverdin (70%) and beta-biliverdin (30%), that is exhibited by heme oxygenase from Pseudomonas aeruginosa (pa-HO) has been studied by (1)H NMR, (13)C NMR, and resonance Raman spectroscopies. Whereas resonance Raman indicates that the heme-iron ligation in pa-HO is homologous to that observed in previously studied alpha-hydroxylating heme oxygenases, the NMR spectroscopic studies suggest that the heme in this enzyme is seated in a manner that is distinct from that observed for all other alpha-hydroxylating heme oxygenase enzymes for which a structure is known. In pa-HO, the heme is rotated in-plane approximately 110 degrees, so the delta-meso-carbon of the major orientational isomer is located within the HO-fold in the place where the alpha-hydroxylating enzymes typically place the alpha-meso-carbon. The unusual heme seating displayed by pa-HO places the heme propionates so that these groups point in the direction of the solvent-exposed heme edge and appears to originate in large part from the absence of stabilizing interactions between the polypeptide and the heme propionates, which are typically found in alpha-hydroxylating heme oxygenase enzymes. These interactions typically involve Lys-16 and Tyr-112, in Neisseriae meningitidis HO, and Lys-16 and Tyr-134, in human and rat HO-1. The corresponding residues in pa-HO are Asn-19 and Phe-117, respectively. In agreement with this hypothesis, we found that the Asn-19 Lys/Phe-117 Tyr double mutant of pa-HO exists as a mixture of molecules exhibiting two distinct heme seatings; one seating is identical to that exhibited by wild-type pa-HO, whereas the alternative seating is very similar to that typical of alpha-hydroxylating heme oxygenase enzymes and is related to the wild-type seating by approximately 110 degrees in-plane rotation of the heme. Furthermore, each of these heme seatings in the pa-HO double mutant gives rise to a subset of two heme isomeric orientations that are related to each other by 180 degrees rotation about the alpha-gamma-meso-axis. The coexistence of these molecules in solution, in the proportions suggested by the corresponding area under the peaks in the (1)H NMR spectrum, explains the unusual regioselectivity of heme oxygenation observed with the double mutant, which we found produces alpha- (55%), delta- (35%), and beta-biliverdin (10%). Alpha-biliverdin is obtained by oxidation of the heme seated similar to that of alpha-hydroxylating enzymes, whereas beta- and delta-biliverdin are formed from the oxidation of heme seated as in wild-type pa-HO.
To begin the functional dissection of light signal transduction pathways of maize (Zea mays), we have identified and characterized the light-sensing mutant elm1 (elongated mesocotyl1). Seedlings homozygous for elm1 are pale green, show pronounced elongation of the mesocotyl, and fail to de-etiolate under red or far-red light. Etiolated elm1 mutants contain no spectrally active phytochrome and do not deplete levels of phytochrome A after red-light treatment. High-performance liquid chromatography analyses show that elm1 mutants are unable to convert biliverdin IX alpha to 3Z-phytochromobilin, preventing synthesis of the phytochrome chromophore. Despite the impairment of the phytochrome photoreceptors, elm1 mutants can be grown to maturity in the field. Mature plants retain aspects of the seedling phenotype and flower earlier than wild-type plants under long days. Thus, the elm1 mutant of maize provides the first direct evidence for phytochrome-mediated modulation of flowering time in this agronomically important species.
(2R)- and (2S)-phytochromobilin dimethyl ester have been prepared in enantiomerically pure form, specifically (13)C-labeled at C(10) or C(15).