Acetolactate Synthase

There are two major forms of acetolactate synthase (IUB recommended name, EC 4.1.3.18). One form is on the biosynthetic pathway for branched-chain amino acids, while the other form is devoted to the production of acetoin. ^{1 , 2} The former enzyme from bacteria, ^{3 - 6} yeast, ^{7 - 8} and higher plants ^{9 , 10} is a flavoprotein with a requirement for FAD, while the latter enzyme isolated from Aerobacter aerogenes is not a flavoprotein. ² Acetolactate synthase that participates in the biosynthesis of branched-chain amino acids has also commonly been referred to as acetohydroxyacid synthase. An argument ¹¹ has been put forward that the latter name is better suited to the FAD-requiring enzyme (rather than the IUB recommended name), as it catalyzes two reactions involving α-keto acids, the condensation of two molecules of pyruvate to form acetolactate and CO₂ (specific to leucine and valine biosynthesis), and the condensation of pyruvate and α-ketobutyrate to form a-aceto-a-hydroxybutyrate and CO₂ (specific to isoleucine biosynthesis), while the FAD-independent enzyme catalyzes only the former reaction to an appreciable extent (Figure 1). ^{12 , 13} However, the ability of the FAD-requiring enzyme to catalyze the latter reaction varies for different isozymes in enteric bacteria (Escherichia coli and Salmonella typhimurium). ¹³ Although there are many other differences between the Aerobacter aerogenes enzyme and some of the enzymes from enteric bacteria, yeast, or higher plants, the one feature that clearly delineates the enzymes that are essential to the biosynthesis of branched-chain amino acids from the acetoin biosynthetic enzyme is that the former enzymes are flavoproteins.