Studies in valine biosynthesis. X. The acetolactate synthase from Rhodopseudomonas spheroides.

The first committed enzyme in valine biosynthesis, acetolactate synthase, in the photosynthetic bacterium, Rhodopseudomonas spheroides, required added pyruvate (apparent Km--4.5 mM), Mg2+ (Km--1.01 mM), diphosphothiamine (Km--29.6 micrometer), flavin adenine dinucleotide, and a buffer pH of 7.2--7.4 for enzymatic activity. The synthase was affected by L-valine, an end-product inhibitor, in a competitive manner. The presence of acetolactate synthase, along with other earlier observed enzymes, completes the identification of the valine biosynthetic pathway in this photo-organotroph.