Tocopherol biosynthetic pathway in higher plants. Tocopherols consist of a polar chromanol ring and a lipophilic prenyl chain derived from homogentisic acid and phytyl diphosphate. The shikimate pathway produces the homogentisic acid, whereas the 2-C-methyl-d-erythritol-4-phosphate (MEP) pathway produces phytyl diphosphate. Phytyl transferase (HPT) catalyzes the reaction of phytyl diphosphate addition to homogentisic acid, producing the common precursor of the tocopherol biosynthetic pathway, methyl-6-phytyl-1,4-benzoquinone (MPBQ). MPBQ-methyltransferase (MPBQ-MT) adds a methyl alkyl to MPBQ, to produce 2,3-dimethyl-6-phytyl-plastoquinol (DMPBQ). MPBQ and DMPBQ are cyclized by tocopherol cyclase (TC) to form δ-tocopherol and γ-tocopherol, respectively. The last step of tocopherol biosynthesis is methylation of δ-tocopherol and γ-tocopherol, which produces β-tocopherol and α-tocopherol, respectively. These reactions are catalyzed by γ-tocopherol methyltransferase (γ-TMT).