Aspartokinase of Rhodopseudomonas spheroides: REGULATION OF ENZYME ACTIVITY BY ASPARTATE β-SEMIALDEHYDE

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Abstract 1. The enzyme aspartokinase has been purified about 240-fold from extracts of the photosynthetic bacterium Rhodopseudomonas spheroides. An absolute requirement for adenosine triphosphate and a divalent cation for enzyme activity has been shown. No other nucleoside phosphates can serve as the phosphate donor; Mn++ can replace Mg++. Potassium ion stimulates the phosphorylation reaction by increasing the maximal velocity of the reaction and by facilitating the binding of an additional ATP molecule per molecule of enzyme. 2. None of the end product amino acids of the aspartic family, namely, lysine, methionine, threonine, or isoleucine, or any combination of these has any regulatory effect on the enzyme activity. However, the activity is strongly inhibited by aspartate β-semialdehyde, a key intermediate for the synthesis of the amino acids noted. The inhibition is competitive with respect to both aspartate and ATP. It is concluded that feedback inhibition by aspartate β-semialdehyde of R. spheroides aspartokinase provides an effective mechanism in regulating biosynthesis of these four important amino acids. 3. Results of heating experiments show that several end product amino acids, particularly lysine and threonine, protect the enzyme against heat inactivation, indicating binding sites for these amino acids on the enzyme molecule. It is suggested that, in contrast to other bacterial aspartokinases, these amino acids although bound on the enzyme, have lost the ability to interact with the catalytic sites, and thus are unable to modify enzyme activity.