Synthesis of the Aspartic and Glutamic Families of Amino Acids in Escherichia Coli

Abstract

Read before the Academy April 30, 1953; communicated by M. A. Tuve, June 29, 1953 In the previous paper' the synthesis of aspartic and glutamic acids was shown to be a chief consequence of the operation of the Krebs cycle in Escherichia coli. The present paper will describe the biosynthesis of a number of other amino acids for which either aspartic or glutamic acid serves as the parent substance. The isotopic competition method2-4 among carbon compounds has been used extensively in these studies. Where only one carbon source, such as glucose or acetate supplies all the carbon needs of the cell, all the cellular carbon becomes radioactive. If, in addition, an unlabeled source of carbon is made available to the cell, it is often found to be selectively utilized for specific biosyntheses. In this way the labeled and unlabeled compounds may be said to compete for a place in the biologically syn-thesized chemicals of the cell. It is desirable in most studies to label the compound of a competing group which is the least widely used by the cell, for then even very minor ways of utilization are readily observed. How-ever, it is not always possible to obtain metabolites in the labeled form: in these cases a labeled metabolite such as Cl4-glucose* opens the way to study of the non-radioactive metabolites by observing their competitive effects. The use of various C14-labeled compounds in this way has revealed pathways of synthesis for proline and arginine of the glutamic family, and for diaminopimelic acid, lysine, threonine, isoleucine, and methionine of the aspartic family. These amino acids and their parent substances, aspartic and glutamic acids, contain over half the carbon of the bacterial protein. Thus, the pathways which lead to them number among the important assembly lines for protein synthesis. The pathways found are in essential accord with those based upon studies with biochemical mutants or cell-free enzyme systems. Materials and Methods.-The C14-labeled compounds, assay techniques,