Katsumi Iida

Mechanism of the ring contraction process in vitamin B12 biosynthesis by the anaerobe Propionibacterium shermanii under aerobic conditions.

The mechanism of the ring contraction process during vitamin B12 biosynthesis by the anaerobe Propionibacterium shermanii was investigated under both aerobic and anaerobic conditions by means of feeding experiments with δ‐amino[1‐13C]levulinic acid (a biosynthetic intermediate of tetrapyrrole) and δ‐amino[1‐13C,1,1,4‐18O3]levulinic acid in combination with 13C‐NMR spectroscopy. We showed that the characteristic mechanism of the ring contraction process (the generation of precorrin‐3x from formation of the γ‐lactone from the ring A acetate group at C1 and hydroxylation at C20 by molecular oxygen catalyzed by CobG, and the migration of ring D by cleavage of the carbon–oxygen bond at C1 of precorrin‐3x) in the aerobe Pseudomonas denitrificans was not seen in P. shermanii under aerobic conditions, and the mechanism of the ring contraction process in P. shermanii was the same irrespective of the presence or absence of oxygen.

CARBON SOURCE DEPENDENCE OF THE RATIO OF δ-AMINOLEVULINIC ACID BIOSYNTHESIS VIA THE C5 AND SHEMIN PATHWAYS IN EUGLENA GRACILIS (EUGLENOPHYCEAE)(1).

The ratio of two biosynthetic pathways was estimated, the C5 and Shemin pathways, to δ‐aminolevulinic acid (ALA, a biosynthetic intermediate of tetrapyrrole) from the 13C‐enrichment ratios (13C‐ER) at the carbon atoms of chl a (after conversion to methyl pheophorbide a) biosynthesized by Euglena gracilis G. A. Klebs when l‐[3‐13C]alanine was used as a carbon source. On the basis of these estimations, we confirmed that ALA was efficiently biosynthesized via both the C5 and Shemin pathways in the plastids of E. gracilis, and we determined that the ratio of ALA biosynthesis via the Shemin pathway was increased in the ratio of 14%–67%, compared with that in our previous d‐[1‐13C]glucose feeding experiment ( Iida et al. 2002 ). This carbon source dependence of the contributions of the two biosynthetic pathways might be related to activation of gluconeogenesis by the amino acid substrate. The methoxy carbon of the methoxycarbonyl group at C‐132 of chl a was labeled with the 13C‐carbon of l‐[methyl‐13C]methionine derived from l‐[3‐13C]alanine via [2‐13C]acetyl coenzyme A (CoA), through the atypical tricarboxylic acid (TCA) cycle, gluconeogenesis, and l‐[3‐13C]serine. The phytyl moiety of chl a was also labeled on C‐P2, C‐P31, C‐P4, C‐P6, C‐P71, C‐P8, C‐P10, C‐P111, C‐P12, C‐P14, C‐P151, and C‐P16 from 13C‐isoprene (2‐[1,2‐methyl,3‐13C3]methyl‐1,3‐butadiene) generated from l‐[3‐13C]alanine via [2‐13C]acetyl CoA.

Investigation of the biosynthesis of acetyl-CoA and oxaloacetic acid from pyruvic acid and the quantitative evaluation of incorporated 13C-labeled l-alanine in Arthrobacter hyalinus

Studies on the contribution to acetyl-CoA and oxaloacetic acid from the pyruvic acid transformation from l-alanine in Arthrobacter hyalinus were conducted by means of feeding experiments with l-[1-13C]alanine and l-[3-13C]alanine, followed by an analysis of the labeling patterns of coproporphyrinogen III using 13C NMR spectroscopy. The results demonstrated that l-alanine was transformed via pyruvic acid to both acetyl-CoA and oxaloacetic acid. Additionally, the quantitative analysis indicated that pyruvic acid was transformed to acetyl-CoA and oxaloacetic acid in the ratio of 1:0.8.

Metabolic pathways leading from amino acids to vitamin B12 in Propionibacterium shermanii, and the sources of the seven methyl carbons

The metabolic pathways leading from l‐[2‐13C]aspartic acid, [2‐13C]glycine and l‐[methyl‐13C]methionine to vitamin B12 were investigated, focusing on the biosynthetic pathways leading to the aminopropanol moiety of vitamin B12 and on the role of the Shemin pathway leading to δ‐aminolevulinic acid (a biosynthetic intermediate of tetrapyrrole), by means of feeding experiments with Propionibacterium shermanii in combination with 13C‐NMR spectroscopy. The 13C‐methylene carbons of l‐[2‐13C]aspartic acid, which is transformed to [2‐13C]glycine via l‐[2‐13C]threonine, and [2‐13C]glycine added to the culture medium served mainly to enrich the seven methyl carbons of the corrin ring through C‐methylation by S‐adenosyl‐l‐[methyl‐13C]methionine derived from catabolically generated l‐[methyl‐13C]methionine in the presence of tetrahydrofolic acid. The results indicate that the catabolism of these amino acids predominates over pathways leading to (2R)‐1‐amino‐2‐propanol or δ‐aminolevulinic acid in P. shermanii. Feeding of l‐[methyl‐13C]methionine efficiently enriched all seven methyl carbons. In the cases of [2‐13C]glycine and l‐[methyl‐13C]methionine, the 13C‐enrichment ratio of the methyl carbon at C‐25 (the site of the first C‐methylation) was less than those of the other six methyl carbons, probably due to the influence of endogenous d‐glucose in P. shermanii. The almost identical 13C‐enrichment ratios of the other six methyl carbons indicated that these C‐methylations during vitamin B12 biosynthesis were completed before the amino acids were completely consumed. However, in the case of l‐[2‐13C]aspartic acid, the 13C‐enrichment ratios of five methyl carbons were low and similar, whereas the last two sites of C‐methylation (C‐53 and C‐35) were not labeled, presumably because of complete consumption of the smaller amount of added label. The ratios of 13C‐incorporation into the seven methyl carbons are influenced by the conditions of amino acid feeding experiments in a manner that is dependent upon the order of C‐methylation in the corrin ring of vitamin B12.