Regulation of Lysine and Threonine Synthesis.

Human and monogastric animals cannot synthesize 10 out of the 20 amino acids and therefore need to obtain these from their diet. Among the essential amino acids, lysine and threonine are considered to be exceedingly important in that they are the most limiting essential amino acids in cereal grains, which represent the largest source of food worldwide (Bright and Shewry, 1983). Because of the nutritional importance of lysine and threonine, the regulation of their metabolism has been studied extensively at the biochemical, genetic, and, more recently, molecular levels. Like many bacterial species, higher plants synthesize lysine and threonine from aspartate using two different branches of the aspartate family pathway, as shown schematically in Figure 1. The enzymes involved in lysine and threonine synthesis have been reviewed in detail (Bryan, 1980). In this review, I discuss the complex biochemical, cellular, developmental, physiological, and environmental controls of the synthesis of lysine and threonine. I also focus on specific enzymes that play major regulatory roles in the synthesis of these amino acids.