Evolution of lysine and arginine biosynthesis revealed by substrate specificity of lysine biosynthetic enzymes in Thermus thermophilus.

Metabolic pathways are considered to originate from broad-specificity ancestors that later diverged into specialized routes. Thermus thermophilus possesses an unusual amino group carrier protein (AmCP)-mediated lysine biosynthetic pathway alongside a canonical arginine biosynthetic pathway. Although each route is considered specific to its cognate amino acid, several lysine biosynthetic enzymes have been shown to accept arginine intermediates. We herein investigated [LysW]-aminoadipate kinase (LysZ; EC:2.7.2.17) and [LysW]-L-2-aminoadipate 6-phosphate reductase (LysY; EC:1.2.1.103), which catalyze the second and third steps, respectively, in the conversion of α-aminoadipate (AAA) to lysine using amino group carrier protein LysW (AmCP), to define their specificity and evolutionary origin. To examine the potential promiscuity, we engineered LysX variants capable of synthesizing LysW-Glu, an artificial LysW-bound analogue that mimics an arginine pathway intermediate. LysZ exhibited activity for LysW-Glu that was approximately 60% of the original activity for LysW-AAA. The activity of LysY for LysW-Glu phosphate was estimated to be approximately 15-20% of that observed with LysW-AAA phosphate. The present study revealed that both enzymes can also act on an arginine biosynthetic intermediate, but with distinct degrees of efficiency. Phylogenetic reconstructions further suggested that an AmCP-mediated biosynthetic pathway represents a primitive route for the synthesis of lysine and arginine in a primordial cell. More generally, the results obtained herein will contribute to a more detailed understanding of the evolutionary strategies employed by nature to specialize and expand metabolic pathways and adjust enzyme promiscuity.