Summary auto-generated
This study examined how Escherichia coli synthesizes methionine from homocysteine, focusing on the roles of vitamin B6 and one-carbon donors. Using washed bacterial suspensions from three auxotrophic strains, researchers found that vitamin B6 (pyridoxal or related compounds) was essential when serine served as the methyl donor for methionine synthesis. However, when glycine was the one-carbon source, the vitamin B6 requirement differed markedly depending on prior growth conditions. Organisms grown in the presence of glycine could synthesize methionine from glycine and homocysteine without added pyridoxal, suggesting an alternative, more efficient pathway. Using radioactive glycine, researchers demonstrated that the carbon from glycine's C-2 position appeared in methionine's methyl group at four times the activity found in the free serine pool, indicating that free serine is not an intermediate in glycine utilization. Cobalamin was required for maximal methionine synthesis with both serine and glycine donors. These findings suggest that growth on glycine induces a pyridoxal-independent mechanism for using glycine as a one-carbon donor, distinct from the serine-dependent pathway.
Key findings
- Vitamin B6 (pyridoxal) is essential for methionine synthesis when serine is the one-carbon donor, but becomes optional when glycine is used if organisms were grown in glycine-containing medium
- Growth in glycine-containing medium induces an alternative, more efficient mechanism for glycine utilization that is independent of or requires much lower concentrations of pyridoxal
- Isotopic labeling shows that glycine's C-2 carbon incorporates into methionine's methyl group without first appearing in a free serine pool, indicating serine is not an intermediate
- Cobalamin is required for maximal methionine synthesis regardless of whether serine or glycine serves as the one-carbon donor
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Abstract
SUMMARY: Washed suspensions of two strains of Escherichia coli which require vitamin B6 for growth form methionine in a reaction mixture containing homo-cysteine, serine, glucose, p-aminobenzoic acid and cobalamin only when pyridoxal or another member of the vitamin B6 group (except pyridoxamine phosphate) is also added.
Serine is replaced by glycine as donor of the required one-carbon unit, but the activity of glycine is markedly increased when the organism is harvested from a medium containing glycine. Pyridoxal is not required for the utilization of glycine by such organisms, but is essential when glycine has not been present during growth. It is concluded that growth on glycine induces the formation of an alternative, and more efficient, mechanism for using glycine as one-carbon donor which is independent of pyridoxal. This mechanism was studied in suspensions of an auxotroph requiring serine or glycine for growth by isotopic technique; C-2 of glycine is incorporated into the niethyl group of methionine at four times the activity at which it appears in a serine pool. Free serine is therefore not an intermediate in the utilization of glycine as a one-carbon donor. Cobalamin is required for maximal activity of both glycine and serine in the reaction.