FIG. 15. Biosynthetic pathways of isoleucine, valine, and leucine, showing sites of gene action (22, 136. 426, 549, 727. 854, 1112). Isoleucine and valine are synthesized along parallel pathways catalyzed by common enzymes. The leucine precursors alpha- and beta-isopropylmalate were formerly called beta-OH-beta-carboxyisocaproate and alpha-OH-beta-carboxyisocaproate, respectively.