The rate of synthesis of amino acids depends mainly on the amounts of the biosynthetic enzymes and on their activities. We now consider the control of enzymatic activity. The regulation of enzyme synthesis will be discussed in Section 15.

As we have seen in every metabolic pathway that we have studied so far, the first irreversible reaction, or the committed step, is usually an important regulatory site. The final product of the pathway (Z) often inhibits the enzyme that catalyzes the committed step (A → B).

This kind of control is essential for the conservation of building blocks and metabolic energy. Consider the biosynthesis of serine. The committed step in this pathway is the oxidation of 3-phosphoglycerate, catalyzed by the enzyme 3-phosphoglycerate dehydrogenase. The E. coli enzyme is a tetramer of four identical subunits, each comprising a catalytic domain and a serine-binding regulatory domain (Figure 31.10). The binding of serine to a regulatory site reduces the value of V_max for the enzyme; an enzyme bound to four molecules of serine is essentially inactive. Thus, if serine is abundant in the cell, the enzyme activity is inhibited, and so 3-phosphoglycerate, a key building block that can be used for other processes, is not wasted.

The regulation of branched pathways is more complicated because the concentration of two products must be accounted for. In fact, several intricate feedback mechanisms have been found in branched biosynthetic pathways.

Feedback inhibition and activationTwo pathways with a common initial step may each be inhibited by its own product and activated by the product of the other pathway. Consider, for example, the biosynthesis of the amino acids valine, leucine, and isoleucine (Figure 31.11). A common intermediate, hydroxyethyl thiamine pyrophosphate (hydroxyethyl-TPP), initiates the pathways leading to all three of these amino acids. Hydroxyethyl-TPP reacts with α-ketobutyrate in the initial step of the pathway leading to the synthesis of isoleucine. Hydroxyethyl-TPP can also react with pyruvate in the committed step for the pathways leading to valine and leucine. Thus, the relative concentrations of α-ketobutyrate and pyruvate determine how much isoleucine is produced compared with valine and leucine. How are these competing reactions regulated so that equal amounts of isoleucine, valine, and leucine are synthesized? Threonine deaminase, the PLP enzyme that catalyzes the formation of α-ketobutyrate, is allosterically inhibited by isoleucine (Figure 31.11). This enzyme is also allosterically activated by valine. Thus, this enzyme is inhibited by the end product of the pathway that it initiates and is activated by the end product of a competitive pathway. This mechanism balances the amounts of different amino acids that are synthesized.

Enzyme multiplicityThe committed step can be catalyzed by two or more enzymes with different regulatory properties, a strategy referred to as enzyme multiplicity (Figure 31.12). For example, the phosphorylation of aspartate is the committed step in the biosynthesis of threonine, methionine, and lysine. Three distinct aspartokinases catalyze this reaction in E. coli. Although the mechanisms of catalysis are essentially identical, their activities are regulated differently: one enzyme is not subject to feedback inhibition, another is inhibited by threonine, and the third is inhibited by lysine.

Cumulative feedback inhibitionA common step is partly inhibited by each of the final products, acting independently, in cumulative feedback inhibition. The regulation of glutamine synthetase in E. coli is a striking example of this inhibition. Recall that glutamine is synthesized from glutamate, NH₄⁺, and ATP. Glutamine synthetase regulates the flow of nitrogen and hence plays a key role in controlling bacterial metabolism. The amide group of glutamine is a source of nitrogen in the biosyntheses of a variety of compounds, such as tryptophan, histidine, carbamoyl phosphate, glucosamine 6-phosphate, cytidine triphosphate, and adenosine monophosphate. Glutamine synthetase is cumulatively inhibited by each of these final products of glutamine metabolism, as well as by alanine and glycine. In cumulative inhibition, each inhibitor can reduce the activity of the enzyme, even when other inhibitors are exerting their own maximal inhibition. The enzymatic activity of glutamine synthetase is switched off almost completely only when all final products are bound to the enzyme.