Proton exchange on carbons 2 and 3 of serine during their conversion into methyl groups of methionine and thymine in Escherichia coli.

The conversion of the C-2 and C-3 carbons of serine and their attached protons into the methyl groups of methionine and thymine was studied in vivo with exponentially growing cultures of Escherichia coli. This was accomplished by measuring the distribution of deuterium incorporated into the methyl groups of thymine and methionine when cells were grown in the presence of deuterated serines. The results show that both the C-2 and C-3 carbons of serine are converted into the methyl groups of these compounds. The extent of incorporation of deuterium from both the C-2 and C-3 carbons of serine, however, was found to be different for the thymine and methionine methyl groups. In addition, partial exchange of carbon-bonded deuteriums was observed to occur from both of these carbon sources. Evidence is presented that demonstrates that the exchange of the C-3 deuterium most likely results from the reversible conversion of 5,10-methylene-H4-folate to 5,10-methenyl-H4folate catalyzed by 5,10-methylene-H4folate dehydrogenase. The specific enzymatic site for the exchange of the C-2 protons was not identified but was confirmed by the presence of glycine in the cell containing less than the expected abundance of a single deuterium. A possible explanation for the difference in the deuterium incorporation into these two methyl groups is discussed.