Amino Acids Act as Negative Regulators of Their Own Synthesis

Amino Acids Act as Negative Regulators of Their Own Synthesis 

Metabolite flow to tryptophan is controlled by inhibition of anthranilate synthase by tryptophan (see fig. 

This is consistent with the principle that the end product inhibits the first enzyme which is unique to the pathway. This is a general phenomenon in amino acid biosynthetic pathways to ensure against the buildup of excess end product which is always a waste and oftentimes can cause problems. 

In many cases the amino acid pathway branches so that two or more amino acids are formed. Aspartate is the precursor of four other amino acids found in proteins Isoleucine, threonine, methionine, and lysine (see fig. 21.2). The first step in this overall pathway entails the conversion of aspartate to /3-aspartyl-phosphate by aspartokinase. One might imagine that all four of the amino acid end products of this pathway would act together to inhibit this enzyme. However, in E. coli a different solution has been found. In this bacterium there are three aspartokinases which appear to be parts of different multienzyme complexes leading to threonine and leucine for aspartokinase I, methionine for aspartokinase II and lysine for aspartokinase III. As might be expected threonine and isoleucine inhibit aspartokinase I, 

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