31. Another important contribution of Horowitz was his 1945 proposal on the backward evolution of biosynthetic pathways . 32. Tabtoxin biosynthesis branches off from the lysine biosynthetic pathway before the formation of diaminopimelate ( DAP ). 33. Other biosynthetic pathways likely exist, but the enzymes catalyzing these reactions have not yet been identified. 34. There are two distinct lysine biosynthetic pathways : the diaminopimelic acid pathway and the ?-aminoadipate pathway. 35. Biosynthetic pathways are often highly regulated such that building-blocks are synthesized only when supplies are low.36. FPPS, a geranyltranstransferase, catalyzes reactions ( shown with red arrows ) central to many biosynthetic pathways . 37. In prokaryotes, biosynthetic pathways exist for the tetracyclic steroid framework ( e . g . in hopanoid framework. 38. These two sites catalyze the last two steps of the de novo uridine monophosphate ( UMP ) biosynthetic pathway . 39. HMG CoA reductase occurs early in the biosynthetic pathway and is among the first committed steps to cholesterol formulation. 40. Because all amino acids except glycine are chiral, biosynthetic pathways must generate the correct isomer with high fidelity.
